SANDY BEACH MACROFAUNA COMMUNITIES ON THE NORTH SHORE OF
PRINCE EDWARD ISLAND: THE INFLUENCE OF COAST TYPE AND
MACROPHYTE WRACK
A Thesis
Submitted to the Graduate Faculty
In Partial Fulfilment of the Requirements
for the Degree of Master of Science
Department of Biology
Faculty of Science
University of Prince Edward Island
Mitchell R. MacMillan
Charlottetown, Prince Edward Island
January 2012
©2012. M.R. MacMillan
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ABSTRACT
Sandy beaches on the north shore of Prince Edward Island (PEI) are
associated with three main shoreline types: sand dunes, glacial till bluffs and
sandstone cliffs. Standard snapshot sampling techniques were used to determine
the influence of physical variability of beaches associated with these three
shorelines on invertebrate macrofauna communities. There was no significant
difference in morphodynamics between sandy beaches associated with the three
shoreline types in terms of 1/slope, sediment grain size or the Beach Deposit
Index. However, erosion rates were significantly greater at shorelines associated
with till bluffs and sand dunes. Significant differences were also found between
macrofauna communities associated with sandstone cliffs and those associated
with sand dunes and till bluffs. The former communities were characterized by
low densities of the polychaete Scolelepis squamata, and the amphipod
Platorchestia platensis. In contrast, the latter communities were characterized by
much higher polychaete densities and by the amphipods Haustorius canadensis
and Americorchestia megalophthalma. Significant positive relationships were
found between the rate of coastal erosion and macrofauna abundance and
species richness. However, there were no significant relationships between the
measures of beach morphodynamics and the biological descriptors. The results
of this study suggest that macrofauna communities are linked to shoreline type,
which show distinctive levels of coastal erosion. Areas experiencing higher levels
of erosion and sediment redistribution seem clearly favourable to suspensionfeeding macrofauna, including Scolelepis squamata, which comprised over 95%
of total macrofauna abundance at these sites.
Sandy beaches lack attached plants and therefore macroalgae and
seagrasses that become stranded ashore (wrack) represent an important source
of food and shelter. The standing crop of wrack and its influences on macrofauna
communities were therefore assessed on the three shoreline types. Wrack was
primarily composed of two macrophytes: eelgrass (Zostera marina) and a
species of rockweed (Fucus serratus). Wrack cover was slightly, although not
significantly, greater on beaches associated with sandstone cliffs, which also
showed higher patch densities and wrack water contents. At beaches associated
with sand dunes, macrofauna abundances were significantly greater in patches
of wrack versus nearby bare sediments. In experimental wrack manipulations,
significant positive relationships were found between macrofauna abundance
and wrack wet mass, dry mass and water content. Macrofauna consistently
preferred rockweed over eelgrass, regardless of whether the wrack was fresh or
aged. Nutritional quality (in terms of the concentration of proteins, lipids and
carbohydrates) and feeding rates by talitrid amphipods were also significantly
greater for rockweed than for eelgrass tissues. These results suggest that
nutritional quality in addition to a few physical factors contribute significantly to
the structure of the supralittoral macrofauna in the study area. Overall, the two
studies included here provide baseline macroinvertebrate community information
for sandy beaches on the north shore of PEI. Such information is relevant for the
management of these coastal ecosystems, and the vertebrate and invertebrate
communities that they support.
ACKNOWLEDGEMENTS
First and foremost I would like to thank Dr. Pedro Quijon for allowing me
the opportunity to study under his supervision at UPEI. His incredible support,
guidance and encouragement was invaluable. I am also grateful to my
supervisory committee members Dr. Donna Giberson and Dr. Darren Bardati for
their insight and feedback throughout the duration of this study, and Dr. Tim
Rawlings, the external examiner at my thesis defence.
I would like to thank Christina Pater, Veronique Dufour, Bradley
MacMillan, Megan Tesch, Kyle Knysh, Tyler Wheeler, Cassandra Mellish,
Jessica Willis and Marianne Parent for their assistance in the lab and/or field. For
their guidance in biochemistry techniques, I wish to thank Dr. Bourlaye Fofana,
Dr. Kaushik Ghose, David Main and Guru Selvaraj. I also wish to thank Tim
Barret for his advice regarding statistical analysis. I wish to thank the UPEI
Biology Department for the use of facilities, equipment and vehicles, as well as
Pat Doyle and Gilbert Blatch for technical assistance, and also Parks Canada for
access to Prince Edward Island National Park. I would like to thank my fellow lab
mates Kevin Sorochan, Tyler Pickering and Melanie Rossong for their support
and camaraderie. Finally I would like to thank all my friends and family members
for their support and encouragement over the last two years.
I am grateful for the financial support I have received through scholarships
at UPEI. This research was supported by a grant from Environment Canada
through UPEI’s Climate Change Research Program. Additional support came
from a NSERC Discovery grant to Dr. Pedro A. Quijon.
TABLE OF CONTENTS
Page No.
Title Page.................................................................................................................... i
Conditions for the use of the thesis........................................................................... ii
Permission to Use Graduate Thesis.........................................................................iii
Certification of Thesis Work..................................................................................... iv
Abstract...................................................................................................................... v
Acknowledgements..................................................................................................vii
Table of Contents....................................................................................................viii
List of figures.............................................................................................................. x
List of tables............................................................................................................. xiii
CHAPTER 1. INTRODUCTION AND OBJECTIVES...............................................1
1.1. Introduction.........................................................................................................2
1.2. Study objectives..................................................................................................7
1.3. References....................................................................
8
CHAPTER 2. LITERATURE REVIEW....................................................................11
2.1. Control of sandy beach invertebrate communities by the physical
environment............................................................................................................. 13
2.1.1. Main hypotheses explaining sandy beach species richness....................19
2.2. Influence of Wrack on Macrofauna Assemblages.......................................... 22
2.2.1. Wrack as Habitat for Macrofauna.............................................................. 23
2.2.2. Wrack as a Food Source for Macrofauna..................................................28
29
2.2.3. Succession of Macrofauna species in Wrack.................................
2.3. Main geomorphological features of PEI coasts.............................................. 31
2.3.1. Sand Dunes................................................................................................ 32
2.3.2. Glacial Till....................................................................................................36
2.3.3. Sandstone....................................................................................................38
2.4. PEI’s north shore sandy beaches and their geology...................................... 41
2.4.1. Cavendish coastal compartment................................................................ 43
2.4.2. Rustico-Brackley coastal compartment......................................................43
2.4.3. Stanhope-Tracadie-Deroche coastal compartment..................................45
2.5. References........................................................................................................46
CHAPTER 3. A SPATIAL COMPARISON OF SANDY BEACHES IN A
VULNERABLE SYSTEM IN THE GULF OF ST. LAWRENCE: SPECIES
COMPOSITION, RICHNESS AND ABUNDANCE IN RELATION TO
SHORELINE TYPE AND EROSION...................................................................... 54
3.1. Abstract........................................................................................................... 55
3.2. Keywords.......................................................................................................... 56
3.3. Introduction....................................................................................................... 56
3.4. Methods................
58
3.4.1. Study area....................................................................................................58
59
3.4.2. Sampling protocol...........................................................
3.4.3. Data analysis.............................................................................................. 62
3.5. Results.............................................................................................................. 64
3.5.1. Physical properties of the sandy beaches.................................................64
3.5.2. Macrofauna communities
....................................................................68
3.6. Discussion.........................................................................................................76
3.7. Acknowledgements.........................................................
82
3.8. References........................................................................................................82
CHAPTER 4. STRANDED MACROPHYTES AS A PATCHY RESOURCE:
WRACK FEATURES INFLUENCE MACROFAUNAL ABUNDANCE IN AN
87
ATLANTIC CANADA SANDY BEACH SYSTEM.....................
4.1. Abstract............................................................................................................. 88
4.2. Keywords.......................................................................................................... 89
4.3. Introduction.......................................................................................................89
4.4. Materials and Methods..................................................................................... 93
4.4.1. Study Area and Stranded Macrophyte Survey.......................................... 93
4.4.2. Field experiment: Stranded seaweed colonization................................... 95
4.4.3. Plant tissue nutrients and amphipod feeding rates...................................97
4.4.4. Statistical analyses.................................................
99
4.5. Results....................
100
4.5.1. Stranded Macrophyte Survey.................................................................. 100
4.5.2. Wrack colonization experiments.............................................................. 103
4.5.3. Nutritional Quality Analysis and amphipod feeding rates........................108
4.6. Discussion.......................................................................................................112
4.6.1. Macrophyte survey and spatial variation..................................................112
4.6.2. Wrack colonization experiments.............................................................. 116
4.6.3. Nutritional Quality and amphipod feeding rates...................................... 118
4.7. Acknowledgements
.................................................................................. 120
4.8. References......................................................................................................121
CHAPTER 5. SUMMARY OF RESULTS AND FUTURE RESEARCH............ 126
5.1. Spatial variation and coastal erosion.............................................................127
5.2. Spatial variation and allochthonous wrack input.......................................... 129
5.3. Future research.............................................................................................. 131
5.4. References..................................................................................................... 133
X
LIST OF FIGURES
Figure 2.1. Characteristic reflective and dissipative beaches. The mode of
transition between states is indicated by the arrows.................................... 14
Figure 2.2. A typical sand dune, located at Brackley Beach, Prince Edward
Island National Park, Prince Edward Island.................................................33
Figure 2.3. A typical glacial till bluff, located at Dalvay, Prince Edward Island
National Park, Prince Edward Island............................................................. 37
Figure 2.4. A typical sandstone cliff, located at Doyles Cove, Prince Edward
Island National Park, Prince Edward Island.................................................40
Figure 2.5. The three coastal cells a)Cavendish, b)Rustico-Brackley and
c)Stanhope-Tracadie-Deroche housing the sandy beaches sampled in this
study. Figure modified from Forbes and Manson (2002)..............................44
Figure 3.1. Approximate location of the sandy beaches sampled along the north
shore of Prince Edward Island, southern Gulf of St. Lawrence. 1.Cavendish
west II, 2.Cavendish west I, 3.Cavendish east, 4.Mackenzies Brook, 5.Cape
Turner, 6.Doyles Cove west, 7.Doyles cove east, 8.Brackley, 9.Ross Lane,
10.Stanhope east, 11.Stanhope west,12.Dalvay west II, 13.Dalvay west I,
14.Dalvay east............................................................................................... 60
Figure 3.2. Relationships between the mean physical characteristics measured
and shoreline type for 14 sandy beaches sampled on the north shore of
Prince Edward Island, summer 2009 / 2010. Error bars represent one
standard error. Identical letters indicate no significant differences among
coast types............................................................. ....................................... 67
Figure 3.3. Relationships between beach physical characteristics and erosion
rate for 14 sandy beaches sampled on the north shore of Prince Edward
Island, summer 2009 / 2010..........................................................................69
Figure 3.4. Relationships between physical characteristics and mean species
richness for the macrofauna community of 14 sandy beaches on the north
shore of Prince Edward Island, summer 2009 / 2010. Error bars represent
the standard error of four replicates............................................................. 71
Figure 3.5. Relationships between physical characteristics and mean abundance
of the macrofauna community of 14 sandy beaches on the north shore of
Prince Edward Island, summer 2009 / 2010. Error bars represent the
standard error of four replicates.....................................................................72
Figure 3.6. Relationships between physical characteristics and mean abundance
of the dominant polychaete Scolelepis squamata on 14 sandy beaches on
the north shore of Prince Edward Island, summer 2009 / 2010. Error bars
represent the standard error of four replicates............................................. 74
Figure 3.7. Multidimensional scaling plot illustrating macrofauna community
similarity among sandy beaches associated with sandstone cliffs (black
symbols), till bluffs (gray) and sand dunes (white). The oval surrounding
samples from beaches associated with till bluffs and sand dunes is based
on ANOSIM results and indicates that community structure in these
samples were not significantly different.........................................................75
Figure 4.1. Approximate location of the sandy beaches sampled along the north
shore of Prince Edward Island, southern Gulf of St.Lawrence.1.Cavendish,
2.Cape Turner, 3.Doyles Cove, 4.Brackley, 5.Ross Lane,6.Dalvay west II,
7.Dalvay west I and 8.Dalvay east................................................................ 94
Figure 4.2. Mean density, cover and water content of wrack from seven sandy
beaches on the north shore of PEI. Bar filling relates to type of shoreline:
black - sand dunes, light grey - till bluffs and dark grey - sandstone cliffs.
BRA: Brackley; ROL: Ross Lane, CAV: Cavendish; DA-I: Dalvay west I; DAII: Dalvay west II; DOC: Doyles Cove, CAT: Cape Turner. Error bars
represent one standard error. Identical letters indicate no significant
differences among coast types.....................................................................102
Figure 4.3. Mean abundance of macrofauna in wrack versus bare sediments.
BRA: Brackley; ROL: Ross Lane, CAV: Cavendish; DA-I: Dalvay west I; DAII: Dalvay west II; DOC: Doyles Cove, CAT: Cape Turner. Identical letters
indicate no significant differences between cover. Error bars represent one
standard error. No statistical tests were conducted for Doyles Cove and
Cape Turner due to the lack of fauna in the bare sediments......................104
Figure 4.4. Mean abundance of macrofauna in fresh and dried rockweed and
eelgrass patches placed on Dalvay east and Brackley beaches. Error bars
represent one standard error. Identical letters indicate no significant
differences among treatment. The mean abundance for the control samples
(bare sediments) are also presented but were not included in the statistical
analyses........................................................................................................107
Figure 4.5. Results of the regression analyses between physical characteristics
of the wrack and macrofauna abundance across experimentally
manipulated wrackpatches.......................................................................... 109
Figure 4.6. Mean percentage of dry weight for proteins, lipids and carbohydrates
present in the tissues of rockweed and eelgrass from samples collected in
Dalvay east and Brackely beach. Identical letters indicate no significant
differences among treatments. Error bars represent one standard
error................................................
111
Figure 4.7. Mean feeding rates by amphipods in laboratory conditions collected
at Dalvay east and Brackely beach. Identical letters indicate no significant
differences among treatments. Error bars represent one standard
error............................................................................................................... 113
XIII
LIST OF TABLES
Table 3.1. Summary of physical characteristics of the 14 sandy beaches
sampled on the north shore of PEI. Sorting categories are based on Folk
and Ward (1957)............................................................................................. 65
Table 3.2. Results of one-way ANOVAs comparing physical features (1/slope,
Beach Deposit Index, Erosion rate) among the 14 beaches surveyed. DF:
degrees of freedom, MS: Mean Squares. For simplicity, Sum of Squares
and F-values have been omitted.................................................................. 66
Table 3.3. Composition of macrofauna communities of 14 sandy beaches
sampled on the north shore of Prince Edward Island, summer 2009 / 2010.
Brackets denote the following: (P)olychaete, (A)mphipod, (O)ligochaete,
(N)emertea......................................................................................................70
Table 4.1. Results of one-way ANOVAs comparing wrack features (patch
density, cover and water content) among the seven beaches surveyed.
One-way ANOVAs comparing the density of macrofauna in wrack versus
bare sediments at each of these beaches are also presented (Doyles Cove
and Cape Turner were not compared statistically). DF: degrees of freedom,
MS: Mean Squares. For simplicity, Sum of Squares and F-values have
been omitted..................................................................................................101
Table 4.2. Results of two-way ANOVAs comparing field colonization rates
(number of invertebrates) in patches of wrack placed at Dalvay east and
Brackley beaches. Wrack species and state refer to seaweed (rockweed vs
eelgrass) and age (fresh vs dried), respectively. DF: degrees of freedom,
MS: Mean Squares. For simplicity, Sum of Squares and F-values have
been omitted................................................................................................. 106
Table 4.3. Results of two-way ANOVAs comparing indicators of nutritional value
in stranded seaweeds. Site and wrack species refer to location (Dalvay east
vs Brackley) and macrophyte (rockweed vs eelgrass), respectively. The
results of a t-test comparing amphipod feeding rates upon the same wrack
species is also presented. DF: degrees of freedom, MS: Mean Squares. For
simplicity, Sum of Squares and F-values have been
omitted..................
110
CHAPTER 1:
INTRODUCTION AND OBJECTIVES
2
1.1 Introduction
At first glimpse, sandy beaches appear to lack obvious signs of life.
However, there are at least two distinct faunistic groups associated with these
habitats (McLachlan and Brown, 2006): the “water breathers”, primarily
suspension-feeding species generally found in the mid and low intertidal zones,
and the “air breathers” or herbivore and detritivore species located primarily at
the supralittoral and upper intertidal levels. Although these faunistic groups are
known to play a key role as food sources for resident and migratory species
(Dugan et al., 2003), two basic aspects of their ecology must be explored before
outlining their trophic role at the ecosystem level: their composition and
abundance, and the factors that structure them as populations and communities.
This is especially important in areas where general information on composition
and the physical factors that influence sandy beach invertebrates have not been
documented. This thesis addresses both aspects in an area where sandy
beaches are a prominent feature of the coastal habitat: the north shore of Prince
Edward Island (PEI).
The Autecological Hypothesis developed by Noy-Meir (1979) states that in
physically controlled environments like sandy beaches, animal populations have
little influence on each other, and communities are structured by their individual
response to the physical environment. The strong correlations found between
purely physical parameters and macrofauna abundance and diversity on
exposed sandy beaches worldwide, suggest that this hypothesis is broadly
applicable to these systems (McLachlan, 1990). Sandy beaches are classified
according to their “morphodynamic state”, which is the result of a combination of
factors such as sediment grain size, slope and wave characteristics. At one
extreme, dissipative sandy beaches result from an abundant supply of fine sand
and high wave energy from breaking waves washing over gentle beach face
slopes. These beaches represent the erosional end of the spectrum of beach
types (Short and Wright, 1983). At the other extreme, reflective sandy beaches
are characterized by steep beach face slopes where lower energy waves break
on the beach face and water flows quickly off the beach. These beaches are
composed of coarse sediments, and represent the accretional extreme of beach
types (Short and Wright, 1983). Between these two extremes, intermediate
sandy beaches include an array transitional states are by far the most common
(Short and Wright, 1983).
One of the most supported generalizations in sandy beach ecology is the
trend of increasing species richness of invertebrates from reflective (coarse
sediment, steep slope) to dissipative (fine sediment, gentle slope) states
(Brazeiro, 2001). The slope, sediment grain size, and a number of indices can be
used as indicators of beach state, and therefore predict their suitability as habitat
for macrofauna. These characteristics do not affect the macrofauna directly;
rather they correlate with the “swash climate” (swash period, speed, turbulence
and water movement over the beach face) experienced by the macrofauna
(McArdle and McLachlan, 1991, 1992; McLachlan and Dorvlo, 2005). Swash
climates are harshest on steep reflective states and lessen as the slope flattens
towards dissipative, more erodible beaches (McLachlan and Dorvlo, 2005). The
macrofauna, particularly the fraction of aquatic organisms located in the mid and
low intertidal levels depend on the swash action (the runup and backwash of
water on the beach face resulting from waves approaching the shore) for food.
The more dissipative a beach, the wider its surf zone where wave energy is
dissipated, resulting in swashes of longer length and period. For these
organisms, the longer swashes observed at flat dissipative beaches provide
better feeding conditions than the shorter swash periods characteristic of steep
reflective beaches, since they are underwater for longer periods of time
(McLachlan, 1990).
A key characteristic of sandy beach morphodynamic states and their
associated swash climate is that they are extremely dynamic, and change over
time and among sites. Shifts in storm behaviour, for example, will alter the
amount and direction of wave energy approaching the shoreline (Slott et al.,
2006), influencing shore profiles (e.g. Matthew et al., 2010; Morton et al., 1994)
and their rates of erosion (Pethick, 2001). Therefore the suitability of sandy
beaches as habitat for macrofauna changes in response to storms and calm
weather, that is, in response to erosion or accretion of sand on the beach
(McLachlan, 2001). The same applies to physical variation among sandy
beaches located in different areas or along shorelines: differences in swash
climates should result in differences in macrofauna assemblages. The first part of
this thesis uses snapshot surveys to compare an array of sandy beaches
presumably exposed to variable physical characteristics and erosion levels. The
null hypothesis used in this part of the thesis is that:
5
Ho: different physical characteristics have no effect on the community
structure of the macrofauna.
Based on information published elsewhere (e.g. McLachlan and Dorvlo, 2005), it
is expected that sandy beaches closer to the erosional extreme (dissipative
states) will support increased macrofauna abundances and diversities, as
opposed to sandy beaches at the accretional extreme (reflective states).
Spatial variation in physical characteristics also influences other
processes on sandy beaches. One such feature is the arrival, stranding and
decomposition of macrophyte wrack (Orr et al., 2005). Regardless of which •
specific physical factors explain the biomass and distribution of stranded
macroalgae and seagrasses, they constitute a valuable resource or subsidy for
the organisms found primarily on the upper shoreline. Food webs of different
habitats are often linked through the transfer of energy or nutrients from donor to
recipient habitats (Polis et al., 1997). The influence of these allochthonous inputs
is expected to be greatest where a highly productive system interfaces with and
exports materials to a relatively less productive system which functions as a sink
(Barrett et al., 2005). Sandy beaches have low autochthonous production, as all
over the world these habitats are characterized by shifting sands devoid of large
plants. However, beaches act as sinks for displaced algal material (Jaramillo et
al., 2006). Thus, sandy beach ecosystems provide an excellent opportunity to
study the influence of allochthonous inputs from marine systems on the resident
macrofauna communities.
6
Unlike the macrofauna of the lower levels of the intertidal zone, the
supralittoral macrofauna are generally more affected by the availability of wrack
(Koop and Field, 1980) than by swash climate. This is due to the fact that they
usually live buried beyond the intertidal areas directly affected by wave action
(Dugan et al., 2003; Jaramillo et al., 2006). Algal wrack deposited on beaches
serves two important purposes for upper shore detritus feeders: it represents
their main food source (Colombini et al., 2000; Dugan et al., 2003; Rodil et al.,
2008), and acts as a refuge against harsh physical c.onditions by providing
shelter from the surrounding environment (Rodil et al., 2008). In temperate
regions, supralittoral macrofauna in areas with moderate macrophyte input are
often dominated by talitrid amphipods. These amphipods are considered primary
colonizers of newly stranded wrack, which in turn attract secondary (predatory)
species from terrestrial systems (Colombini et al., 2000).
Because stranded macrophytes on exposed sandy beaches harbour
distinctive macrofaunal organisms (primarily talitrid amphipods) generally not
found in lower intertidal levels, they are also an important contributor to overall
sandy beach community biodiversity. The deposition of wrack on sandy shores is
affected by the same factors that promote spatial variation: storms (Ochieng and
Erftemeijer, 1999), wave exposure (Orr et al., 2005), and coastal erosion (Lastra
et al., 2008). All these factors play a role on the density and cover characteristics
of stranded seaweed patches, creating spatial differences within and between
individual sandy beaches. Such variation is relevant considering that
macrofaunal organisms are known to respond to the amount and composition of
the stranded patches (Ince et al., 2007; Rodil et al., 2008). In order to
characterize and quantify these relationships, this section of the thesis uses the
following null hypothesis:
Ho: the amount and characteristics of the wrack are uniform among sandy
beaches and have no discernable influence on the upper shore
macrofauna.
Expected results for this particular study include some degree of variation among
beach types located on PEI’s north shore, and an influence on the abundance of
macrofauna in wrack patches compared to bare sediments. Moreover,
characterization of the use of the main seaweed species forming these stranded
patches by macrofauna, as well as their nutritional quality should shed light on
their relative roles and the preferences exhibited by the macrofauna.
1.2 Study Objectives
The general objective of this thesis is to explore the influence of spatial
physical variability and macrophyte wrack on the community structure of the
sandy beach macrofauna on the north shore of PEI. Additionally, relationships
with macrofauna in relation to physical variables relevant to the region but not
necessarily well studied elsewhere, such as shoreline type and erosion rates, will
be explored. My specific objectives are as follows:
(1) to explore spatial variability in the physical characteristics of sandy
beaches on the north shore of PEI (beach face slope, mean sediment
8
grain size, Beach Deposit Index, rates of coastal erosion and shoreline
type) and how these variables relate to macrofauna community
descriptors, specifically abundance and species richness.
(2) to assess the influence of stranded seaweed on the composition and
abundance of upper shore macrofauna, and how aspects such as age and
nutritional value of the wrack play a role on the use of these stranded
seaweeds by the macrofauna.
1.3 References
Barrett, K., Anderson, W.B., Wait, D.A., Grismer, L.L., Polis, G.A., Rose, M.D.,
2005. Marine subsidies alter the diet and abundance of insular and coastal
lizard populations. Oikos 109, 145-153.
Brazeiro, A., 2001. Relationship between species richness and morphodynamics
in sandy beaches: what are the underlying factors? Mar. Ecol. Prog. Ser.
224, 35-44.
Colombini, I., Aloia, A., Fallaci, M., Pezzoli, G., Chelazzi, L., 2000. Temporal and
spatial use of stranded wrack by the macrofauna of a tropical sandy beach.
Mar. Biol. 136, 351-541.
Dugan, J.E., Hubbard, D.M., McCrary, M.D., Pierson, M.O., 2003. The response
of macrofauna communities and shorebirds to macrophyte wrack subsidies
on exposed sandy beaches of southern California. Estuar. Coast. Shelf Sci.
58s, 25-40.
Ince, R., Hyndes, G.A., Lavery, P.S., Vanderklift, M.A., 2007. Marine
macrophytes directly enhance abundances of sandy beach fauna through
provision of food and habitat. Estuar. Coast. Shelf Sci. 74, 77-86.
Jaramillo, E., de la Huz, R., Duarte, C., Contreras, H., 2006. Algal wrack deposits
and macroinfaunal arthropods on sandy beaches of the Chilean coast. Rev.
Chil. Hist. Nat. 79, 337-351.
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Koop, K., Field, F.J., 1980. The influence of food availability on population
dynamics of a supralittoral isopod, Ligia dilatata (Brandt). J. Exp. Mar. Biol.
Ecol. 48, 61-72.
Lastra, M., Page, H.M., Dugan, J.E., Hubbard, D.M., Rodil, I.F., 2008. Processing
of allochthonous macrophyte subsidies by sandy beach consumers:
estimates of feeding rates and impacts on food resources. Mar. Biol. 154,
163-174.
Matthew, S., Davidson-Arnott, R.G.D., Ollerhead, J., 2010. Evolution of a beachdune system following a catastrophic storm overwash event: Greenwich
Dunes, Prince Edward Island, 1936-2005. Can. J. Earth. Sci. 47, 273-290.
McArdle, S.B., McLachlan, A., 1991. Dynamics of the swash zone and effluent
line on sandy beaches. Mar. Ecol. Prog. Ser. 76, 91-99.
McArdle, S.B., McLachlan, A., 1992. Sandy beach ecology: swash features
relevant to the macrofauna. J. Coast. Res. 8, 398-407.
McLachlan, A., 1990. Dissipative beaches and macrofaunal communities on
exposed intertidal sands. J. Coast. Res. 6, 57-71.
McLachlan, A., 2001. Coastal beach ecosystems, in: Lewin, R. (Ed.),
Encyclopedia of Biodiversity. Academic Press, New York, pp. 741-751.
McLachlan, A., Brown, A.C., 2006. The Ecology of Sandy Shores, second ed.
Academic Press, New York.
McLachlan, A., Dorvlo, A., 2005. Global patterns in sandy beach macrobenthic
communities. J. Coast. Res. 21, 674-687.
Morton, R.A., Paine, J.G., Gibeaut, J.C., 1994. Stages and durations of post­
storm beach recovery, southeastern Texas coast, U.S.A. J. Coast. Res. 10,
884-908.
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Ochieng, C.A., Erftemeijer, P.L.A., 1999. Accumulation of seagrass beach cast
along the Kenyan coast: a quantitative assessment. Aquat. Bot. 65, 221238.
Orr, M., Zimmer, M., Jelinski, D.E., Mews, M., 2005. Wrack deposition on
different beach types: spatial and temporal variation in the pattern of
subsidy. Ecology 86, 1496-1507.
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Pethick, J., 2001. Coastal management and sea-level rise. Catena 42, 307-322.
Polis, G.A., Anderson, W.B., Holt, R.D., 1997. Toward an integration of
landscape and food web ecology: the dynamics of spatially subsidized food
webs. Annu. Rev. Ecol. Syst. 28, 289-316.
Rodil, I.F., Olabarria, C., Lastra, M., Lopez, J., 2008. Differential effects of native
and invasive algal wrack on macrofaunal assemblages inhabiting exposed
sandy beaches. J. Exp. Mar. Biol. Ecol. 358, 1-13.
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McLachlan, A., Erasmus, T. (Eds.), Sandy Beaches as Ecosystems. Dr. W.
Junk Publishers, The Hague, pp. 133-144.
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to changing storm patterns. Geophys. Res. Lett. 33, L18404.
t
CHAPTER 2:
LITERATURE REVIEW
Exposed sandy beaches are amongst the harshest aquatic ecosystems on
earth, so much so that they have been compared to marine deserts (McLachlan,
1983). As in desert ecosystems, interactions among populations are less
important in structuring communities than species responding independently to
the physical environment (McLachlan, 1990). If this perception is correct, then
community attributes should correlate closely with one or more of the physical
parameters that characterize sandy beaches (McLachlan and Dorvlo, 2005).
Sandy beach systems are also the depositary of natural (stranded
seaweeds) and anthropogenic (waste) stranded material (e.g. Marsden, 1991a).
Both are expected to play a role on the structure of sandy beach communities,
particularly at the upper levels of the intertidal. It is in these levels where
invertebrate communities are less dependent on the dynamics of waves and
currents, and more sensitive to the habitat and conditions in the supralittoral zone
(e.g. de la Huz and Lastra, 2008; Dugan et al., 2003; Jaramillo et al., 2006).
This literature review focuses on three main themes. First, I review the
literature available on the physical factors that control sandy beach communities
and how are they classified worldwide. Second, I review what is known about the
influence of stranded seaweeds on the communities associated with the upper
tidal levels. Lastly, I provide a brief overview of the main geomorphological
features of Prince Edward Island (PEI) sandy beaches, and describe the main
characteristics of the sandy beach systems located on the north shore of PEI.
13
2.1 Control of sandy beach invertebrate communities by the physical
environment
Sandy beaches are dynamic in both space and time, are apparently
featureless, consisting primarily of §and and water. They can be defined in terms
of wave climate, sand particle size, and tide range (McLachlan et al., 1993).
Indeed, these three variables have been traditionally combined to produce a
range of beach morphodynamic types (Defeo and McLachlan, 2005; see also
below).
On a global scale, exposed sandy beaches may be divided into three
broad categories based on their morphodynamics: reflective, intermediate and
dissipative sandy beaches (Short and Wright, 1983). Reflective beaches are
characterized by coarse sand, low wave energy and often small tide ranges
(Short and Wright, 1983). They have steep faces and no surf zones, therefore
waves move unbroken to the shore where they collapse or surge up the beach
face (Fig. 2.1). Because waves break at the base of the beach face they must
expend all their remaining energy in the swash zone. Much of the energy goes
into the wave uprush and strong backwash, which is reflected back out to sea as
a reflected wave (Short, 1999). As a result, reflective beaches have short,
frequent swashes (McArdle and McLachlan, 1991). At the other extreme of this
range, dissipative beaches are associated with fine sands, heavy wave action
and often larger tide ranges (Short and Wright, 1983). Waves break over a broad
outer surf zone, dissipating most of their energy. This dissipated wave energy
coupled with the fine sand produces a flat beach face slope (Short, 1999; Fig.
14
Reflective
Erosion
Accretion
Swash zone
Dissipative
Swash zone
_
-----
Figure 2.1. Characteristic reflective and dissipative beaches. The mode of
transition between states is indicated by the arrows.
2.1). The gentle slopes of dissipative beaches result in longer swashes (in terms
of period and distance; Mcardle and McLachlan, 1991, 1992). Between these two
extremes, intermediate beaches have fine to medium sands, moderate to heavy
wave action and a range of tide types. They also have intermediate slopes and
surf zones characterized by bars, channels and rip currents (Short and Wright,
1983; McLachlan and Brown, 2006). Dissipative beaches represent the high
energy end of the beach spectrum, the state to which all beaches head during
periods of high waves and resulting beach erosion (Short, 1999). Beach erosion
widens the surf zone by moving sand from the beach and inner surf zone, and
depositing it in the outer surf zone. This process causes waves to break further
seaward thereby lowering wave height and energy at the shore, and providing a
wider surf zone for energy dissipation (Short, 1999). In regions where very large
tides occur, this classification becomes more complex, as macrotidal beaches
have reflective upper shores and dissipative lower shores (Wright et al., 1982).
The changes in drainage, water retention, and other beach face processes that
occur in this spectrum of beach types exert a strong influence on beach faunal
zonation, abundance, and diversity (McLachlan, 1990).
The earliest quantitative demonstration of a biological-physical relationship
in sandy beach literature was a series of correlations of species richness and
abundance with sand particle size and beach face slope for a range of microtidal
South African sandy beaches (McLachlan et al., 1981). These authors
demonstrated that species richness, density, and total abundance of macrofauna
(invertebrates >500 pm) all increased from steep beaches of coarse sandy
sediments towards flatter beaches of finer sands. From these correlations they
were able to predict that steep reflective beaches would support an impoverished
fauna (0-5 species, <100 individuals • m'1), whereas flat dissipative beaches
would support the richest faunas (14-20 species, >1000 individualsnrf1). The
standardized measurement individuals • nrf1 is a measure employed to avoid
biased results as a consequence of changing beach profile or width during rough
and calm conditions (Brazeiro and Defeo, 1996), resulting in dramatic
contraction/expansions of the across-shore distribution of macrofauna (Defeo
and Rueda, 2002).
A subsequent study by McLachlan (1990) included macrofaunal
responses to morphodynamics states from sites in North America, South Africa
and Western Australia. This broad array of sandy beaches revealed three major
trends across the reflective-dissipative gradient. First, there was an increase in
intertidal macrofaunal species richness with decreasing slope, decreasing
sediment particle size, or increased dimensionless fall velocity (an index of beach
type incorporating measures of wave energy and sand mobility; McLachlan and
Brown, 2006). Second, there was a logarithmic increase in total abundance with
an increase in beach width. And third, there was a logarithmic increase in total
biomass toward the dissipative beach state (McLachlan et al., 1993). Subsequent
work over an even wider range of beach types and geographical regions
confirmed this trend and related it to beach morphodynamic types (Brazeiro,
1999; Defeo et al., 1992; Jaramillo and McLachlan, 1993; McLachlan et al., 1996,
1998). In general, macrotidal dissipative beaches supported communities of
greater richness, abundance, and biomass than microtidal reflective beaches.
McLachlan et al. (1993) concluded that species richness, abundance, and
biomass respond to changes in beach type in a remarkably consistent manner,
although the variation in abundance and biomass was less predictable than the
variation in species richness. The results of that study implied that primary
physical control was of overriding importance for beach communities and that
zoogeographic considerations were secondary. The robust correlations found
between purely physical parameters and faunal abundances suggest that the
hypothesis originally proposed by McLachlan (1990) suggesting that sandy
beach communities were primarily physically controlled (the “Autecological
Hypothesis”) was generally applicable to these systems.
McLachlan and Dorvlo (2005) then analyzed data from an even greater
number of sites: 161 quantitative sandy beach transect surveys from ten
countries (South Africa, Australia, United States, Chile, Oman, Brazil, Uruguay,
New Zealand, Madagascar and Belgium). Once again, significant correlations
were found between species richness and mean sand particle size, beach face
slope (log(1/slope), tidal range (log(maximum tidal range)) and a series of beach
state indices. Significant correlations were also found between abundance
(log(abundance)) and mean sand particle size, beach face slope (log(1 /slope)),
tide range (log(maximum tidal range)), wave height and a series of beach state
indices. Based on this new set of results, McLachlan and Dorvlo (2005)
concluded that species richness was a conservative trait of sandy beach
macrobenthic communities, increasing predictably from microtidal reflective
toward macrotidal dissipative beaches. At the same spatial scale, the response
of abundance and biomass to changes in physical factors was similar to, but
more variable than, the response of species richness. Both biological variables
correlated the best with log (1/beach face slope).
Sandy beach macrofaunal communities experience and respond to three
suites of physical factors: the sediment texture and movement, the swash
climate, and the exposure/moisture gradient on the beach face (McLachlan and
Brown, 2006). The first suite of factors (sediment texture and movement) is
associated with features such as particle size, sorting, fluidity and accretion and
erosion dynamics. Because beach sands at the level of the swash zone (lower
intertidal) are generally well sorted, the most important feature of the sand is its
mean particle size. As sand particle size decreases, porosity (water holding
capacity) increases and permeability (the rate of water flow through the
sediment) decreases, resulting in increased nutrient concentrations (McLachlan
and Turner, 1994). Sediment grain size also strongly influences the burrowing
efficiency of macrofauna (e.g. Alexander et al., 1993), with coarse sand making
burrowing difficult or impossible for invertebrates (Mclachlan, 2001). The second
suite of factors or “swash climate”, is associated with features such as the period,
speed and turbulence of wave and tide driven water movement over the beach
face (swash) that is experienced by macrofaunal organisms. These features are
closely related to the beach morphodynamic state, with frequent harsh swashes
on reflective beaches and less frequent benign swashes on dissipative beaches
(McArdle and McLachlan, 1991, 1992). Since macrofaunal organisms living in the
19
intertidal zone of exposed sandy beaches do not inhabit permanent burrows, all
macrofauna interact with the swash at some point (McArdle and McLachlan,
1992). It can be argued that most species are to some extent dependent on the
swash in order to move, feed, burrow and reproduce. Thus these species are
directly affected by, and adapted to the swash climate (McArdle and McLachlan,
1992). The third suite of factors relates to the gradient of moisture within the
intertidal zone. Water retention decreases dramatically from dissipative to
reflective conditions, and creates opportunities for species with different
desiccation tolerances to inhabit different levels on the shore (McLachlan and
Brown, 2006).
2.1.1 Main hypotheses explaining sandy beach species richness
At large (global) scales, the species richness of sandy beaches seems to
be controlled by two processes: An ecological process whereby harsh
environmental conditions (in terms of sediment, swash and exposure
characteristics) allow fewer species to establish populations on reflective than in
dissipative beaches; and an evolutionary process that has led to greater species
pools in the tropics than in temperate zones (Soares, 2003). Several hypotheses
have been proposed to explain these large-scale patterns, all to some extent
related to the Autecological Hypothesis proposed by McLachlan (1990).
The Swash Exclusion Hypothesis (McLachlan et al., 1993) proposed that
sand particle size, wave energy, and beach slope were the ultimate factors
affecting macrofauna through their combined effects on beach face climate.
However, it was not the beach state or type itself that was important for the
fauna, but the swash climate associated with it. These authors refined an earlier
“Swash Control Hypothesis” (McLachlan, 1990) to the Swash Exclusion
Hypothesis, by suggesting that swash climate associated with dissipative
beaches is sufficiently accommodating and varied to enable virtually all
macrofauna species encountered on exposed beaches to maintain viable
populations. As beach type changes through intermediate states to reflective
conditions, increasingly inhospitable swash climates exclude more and more
species until only supralittoral forms such as talitrid amphipods (Crustacea)
which live above the swash zone can remain.
The Multi-causal Environmental Severity Hypothesis (Brazeiro, 2001)
states that changes in swash climate co-vary with changes in grain size and
erosion-accretion dynamics. Given that there are cause-effect pathways between
biological processes and swash frequency and velocity, grain size and erosionaccretion dynamics, these three environmental factors are capable of affecting
the distribution of species along a morphodynamic gradient. This author
suggested that the reduction of species towards the reflective extreme was
caused by increasing environmental severity generated by the sum of the
independent effects of these three variables.
The Habitat Harshness Hypothesis (Defeo et al., 2001, 2003) also
considers the harshness of the swash environment, and proposes that at the
reflective end of the spectrum, the harsh physical conditions force the
macrofauna to divert more energy toward maintenance and less to reproduction.
21
Ultimately, the fecundity of these invertebrates is potentially lower and their
population become more vulnerable to events of mortality from where they
cannot recover. Because of its nature, this hypothesis is more applicable at the
population than at the community level.
The Swash and Sand Control Hypothesis (McLachlan, 2001) refined
earlier hypotheses by also including a mechanism for the reduction in species.
He observed that reflective beaches have harsh swash climates in that they have
high turbulence, short swash periods, and rapid swash drainage resulting in
lower, more seaward effluent lines. The effluent line separates sands saturated
with water from those that are unsaturated in the intertidal zone, above which
macrofauna experience difficulty burrowing in (McArdle and McLachlan, 1991).
For example, coarse sand appears to exclude small or delicate faunistic forms by
crushing and abrasion, and consequently, most species experience decreasing
burrowing efficiency in coarse rather than fine sands. Thus harsh swash climates
and coarse sand associated with reflective beaches appear to exclude many
species.
Finally, the Hypothesis of Macroscale Physical Control (McLachlan and
Dorvlo, 2005) attempts to combine the two levels of factors responsible for global
patterns in sandy beach community species richness outlined previously. Primary
control is by a) tide range, which defines the dimensions of the intertidal habitat
and the number of species/niches that can be accommodated; and b) latitude,
which influences the size of the species pool available to colonize a beach.
Secondary control is by a) harsh swash climates, and b) coarse sand and
22
sediment instability observed towards reflective beaches that result in the
exclusion of species. According to this hypothesis, the primary factors determine
the maximum number of species that could occur under ideal conditions on a
dissipative beach in a particular region. Meanwhile, the secondary factors limit
how many of these species are actually able to establish populations across the
range of beach types by excluding the less well-adapted species to the harsher
conditions developed towards reflective beach states.
All of the above hypotheses imply that post-settlement processes prevent
species that are less robust/well adapted to the harshness of the physical
environment (i.e. swash climate, coarse sand) from developing large populations
on reflective beaches. It does seem clear that on the large scale and toward the
reflective extreme that physical control is overriding. In contrast, towards
dissipative conditions and on finer scales the more benign environment and
greater densities of organisms may allow biological interactions to become
relatively more important (McLachlan and Brown, 2006).
2.2 Influence of Wrack on Macrofauna Assemblages
In general, sandy beach primary consumers include two main groups. The
first is located at within the intertidal zone and is directly affected by the swash
climate described above, and includes suspension feeders such as polychaetes,
hippid crabs (Crustacea) and bivalves (Mollusca) that feed on phytoplankton and
associated particulate organic material. The second group is located at or near
the high tide level, where it is not as exposed to the effects of waves as the first
23
group and includes herbivores/detritivores such as crustaceans (talitrid
amphipods and isopods) and insects which consume macrophytes and other
stranded materials (Dugan et al., 2003). This section focuses on the influence of
macrophyte wrack on the second group.
2.2.1 Wrack as Habitat for Macro fauna
One of the most notable features of exposed sandy beaches is the relative
lack of in situ primary production (Duarte et al., 2010; Dugan et al., 2003;
Jaramillo et al., 2006; McLachlan and Brown, 2006). Except where sand dunes
transition into the supralittoral zone of the beach face, there are no attached
plants found on sandy beaches. This is because beach sediments are too
abrasive and mobile for macrophytes or dense benthic diatom communities to
establish (Griffiths et al., 1983). With so little in situ primary production,
macrofaunal invertebrates must rely on allochthonous inputs (those entering from
outside the system) from other adjacent marine ecosystems as one of their main
feeding resources. These inputs are generally represented by phytoplankton and
drifting marine macrophytes such as macroalgae and seagrasses (Adin and
Riera, 2003; Dugan et al., 2003). Thus, the structure of sandy beach macrofauna
communities are not only related to oceanographic processes such as upwelling
and currents that deliver nutrients and transport phytoplankton onshore, they are
also related to the production and input of nearshore macroalgae and seagrass
beds (Dugan et al., 2003).
24
Beach wrack, the plant and animal litter cast ashore by waves and tides, is
a highly variable habitat providing food and shelter for both aquatic and terrestrial
animals (Behbehani and Croker, 1982). Wrack is known to be highly transient,
with material moving daily between the shallow subtidal (surf zone) and the
beach in some locations, or persisting for relatively long periods in others
(Kirkman and Kendrick, 1997). Even in areas which receive little wave action
such as estuaries, storm surges and spring flood tides are able to transport
wrack onto the upper beach levels (Behbehani and Croker, 1982). Turnover rates
may relate to spring tides, as reported by Stenton-Dozey and Griffiths (1983)
where total replacement of wrack occurred over a 14 day cycle, or be more rapid,
occurring in as little as eight days (Koop and Field, 1980; Koop et al., 1982).
Changes in prevailing weather conditions (e.g. wind direction casting different
materials ashore) not only affects deposition of wrack, but also its composition
(Colombini et al., 2000). These can also relate to season: Stenton-Dozey and
Griffiths (1983) found maximal kelp deposition occurred during the winter when
large offshore swells uproot whole plants and drive them ashore.
The fauna of beach wrack generally changes in response to location of the
wrack on the beach, beach morphology, season, climate and vegetation cover
(Colombini and Chelazzi, 2003). On the west coast of South Africa, StentonDozey and Griffiths (1983) recorded 35 macrofaunal taxa in stranded wrack
comprising crustaceans (amphipods, isopods), molluscs (bivalves, gastropods)
and insects (flies (Diptera) and beetles (Coleoptera)). Talitrid amphipods are an
important component of macrofaunal assemblages inhabiting wrack (Behbehani
25
and Croker, 1982; Griffiths et al., 1983). They are responsible for most of the
primary consumption of surface material (e.g. Griffiths et al., 1983) and function
as key detritivores (Griffiths and Stenton-Dozey, 1981; Inglis, 1989; Marsden,
1991a).
All of these groups that are well represented in wrack communities (e.g.
crustaceans such as isopods, molluscs and insects, especially flies and beetles),
have been reported to show seasonal peaks in abundance (Stenton-Dozey and
Griffiths, 1983). For example, isopod abundances may decline during the winter
months if the swash climate becomes inhospitable due to heavy wave action,
and dipteran flies show increases in abundance during the summer and autumn
coinciding with breeding periods (Stenton-Dozey and Griffiths, 1983). Colombini
et al. (1998, 2000) found that staphylinid beetles adopted different spatial
strategies according to season, and were more abundant in the wet season than
during the dry season in wrack deposits. However, Stenton-Dozey and Griffiths
(1983) found that the overall abundance of Coleoptera throughout the year was
erratic with no discernable seasonal patterns. They attributed this to the fact that
they are not permanent residents of the intertidal, and instead, they migrate
seaward from the sand dunes.
Spatial patterns have also been shown to vary on shorter temporal scales.
For example Colombini et al. (2000) found that macrofauna showed pronounced
spatial differences in response to their use of wrack during semi-lunar cycles.
The talitrid amphipod Talorchestia martensii and the dipteran flies moved toward
seaward wrack during neap tides and toward landward wrack during spring tides.
This was not the case for all macrofauna, however, as these migrations were not
observed in gastropods (snails) and staphylinid beetles. Stenton-Dozey and
Griffiths (1983) reported that isopod species were restricted to wrack near the
water’s edge, where they feed on organic matter and prey on small animals such
as amphipods. furthermore, Colombini et al. (1998) demonstrated that species
with nocturnal or diurnal surface regimes demonstrated extended activity into the
following day or night. Therefore, wrack deposits may represent not only the
diurnal resting grounds for nocturnal species, but also a microhabitat where
activity continues in the absence of large predators and unfavourable climatic
conditions.
Anthropogenic disturbances which affect wrack deposition can also affect
beach fauna. For example, wrack, trash and debris are often removed from
popular public sandy beaches (grooming; McLachlan and Brown, 2006;
Colombini and Chelazzi, 2003). Dugan et al. (2003) investigated the effects of
beach grooming on the macrofauna and found that the abundance of wrackassociated fauna was significantly lower on groomed than un-groomed beaches.
Species richness also varied significantly among groomed and un-groomed
beaches with low and high standing crops of wrack. Fewer than three wrackassociated species occurred on groomed beaches, while un-groomed beaches
had from 6-13 species depending on the level of wrack cover. The depressed
species richness in groomed beaches was particularly evident in the
coleopterans (beetles) and the amphipod crustaceans. Their main finding was
that the mean species richness and abundance of wrack-associated macrofauna,
specifically talitrid amphipods and flies (primarily larvae and pupae), were both
positively correlated with wrack cover. Other authors have also reported
increased abundance of macrofauna with macrophyte cover or volume (Ince et
al., 2007; Jaramillo et al., 2006; Rodil et al., 2008; Stenton-Dozey and Griffiths et
al., 1983). In addition to increased abundances, Behbehani and Croker (1982)
found increased seasonal growth and reproductive development in Platorchestia
platensis with higher availability of wrack.
The spatial distribution of the wrack debris along the beach profile is a
relevant feature because the higher the seaweed is located on the beach, the
longer it is presumably present on the intertidal zone (Rodil et al., 2008), where it
is prone to desiccation and can be reworked either physically by wind or waves,
or biologically through detritivores and decomposers (Ince et al., 2007). Some
wrack-associated macrofauna on sandy beaches have been shown to have
preferences for wrack in various states of decomposition. For example, Jaramillo
et al. (2006) analyzed the population abundances of three wrack associated
macrofaunal species on the Chilean coast associated with fresh macroalgae at
low tide levels, and older dried algae deposited further up the shore. The mean
population abundances of the talitrid amphipod Orchestoidea tuberculata were
significantly higher in the lower band of fresher wrack as compared to the upper
aged band at all beaches except one. In contrast, the tenebrionid beetle
Phalerisidea maculata was significantly more abundant in the wrack located on
the upper beach levels, while the isopod Tylos spinulosus did not show a
preference for either upper and lower levels of wrack (Jaramillo et al., 2006).
28
2.2.2 Wrack as a Food Source for Macrofauna
Algal wrack promotes increased population abundances of sandy beach
macrofauna by providing a source of food and/or a refuge from harsh
environmental conditions and/or predation (Colombini et al., 2000; Ince et al.,
2007; Inglis 1989; Rodil et al., 2008). Population abundances of a variety of
consumers can be potentially determined by food availability (e.g. Polis and
Hurd, 1996), especially in habitats where food sources are patchy or limited,
such as the allochthonous sources on sandy beaches. Marine algae which
makes up the majority of beach wrack, vary greatly in nutritional quality (Duarte
et al., 2010), which also influences feeding preferences. For example, Adin and
Riera (2003) investigated food sources of the talitrid amphipod Talitrus saltator
by stable isotope analysis and showed that they preferentially used certain
species of macroalgae as a food source. Another talitrid amphipod, O.
tuberculata, preferentially consumed Durvillaea antarctica over Lessonia
nigrescens and Macrocystis pyrifera when given the choice of the three species
(Duarte et al,, 2010). The preferred species was found to have significantly
greater protein and carbohydrate contents than the other two species (Duarte et
al., 2010). Other studies have shown similar trends, with amphipods
preferentially consuming algae containing higher quantities of protein (CruzRivera and Hay, 2000; Jimenez et al., 1996) or carbohydrates and chlorophyll a
(Rodil etal., 2008).
These feeding preferences may drive the species patterns found in wrack.
Several authors have found differences in macrofaunal communities in response
to differences in the species that comprise wrack beds (e.g. Colombini et al.,
2000; Rodil et al., 2008), as well as the size of these wrack beds (Olabarria et al.,
2007; Rodil et al., 2008). In addition to food considerations, the composition and
level of compactness of the algal wrack beds provide different microclimatic
conditions within the patches which influence the colonization and exploitation of
the wrack by macrofauna (Colombini et al., 2000). Morphological differences
among seaweeds cause variability in habitat quality, including the quality of the
wrack as a shelter from predation (e.g. Colombini et al., 2000; Rodil et al., 2008;
Vandendriessche et al., 2006). Therefore, the presence of different species may
relate to a variety of factors. Some species like O. tuberculata may be more
influenced by nutritional quality than algal structural traits (Duarte et al., 2010).
These amphipods preferred D. antarctica regardless of whether it was fresh, or it
had all of its structural characteristics removed by grinding it into a fine powder.
This was not the case for the remaining two species of macrophytes tested.
Although they were consumed at rates significantly less than that of D. antartica,
L. nigrescens were consumed at a greater rate than M. pyrifera when fresh, while
this trend was reversed after removal of structural traits (Duarte et al., 2010).
Thus structural and nutritional traits play varying roles in different taxa, so
species patterns rely on complex interplay of all of these factors.
2.2.3 Succession of Macrofauna species in Wrack
Another factor that contributes to the complexity of wrack communities is
that ephemeral patches of wrack and carrion are generally characterized by a
successional change in species composition. This is due to the fact that different
groups of organisms associate with wrack at different stages of decomposition or
age (Colombini and Chelazzi, 2003). Analyses of the succession of species in
the colonization of wrack have shown that not all the invertebrate species invade
the wrack at the same time (Colombini et al., 2000). This suggests a different use
of the wrack according to species’ metabolic and trophic needs, and the
appearance or disappearance of species due to microclimatic changes related to
the position of the wrack on the beach (Colombini et al., 2000). A number of
authors have reported amphipod crustaceans as the primary colonizers of wrack
(Colombini et al., 2000; Griffiths and Stenton-Dozey, 1981; Inglis, 1989;
Jedrzejczak, 2002), although some authors have reported that many wrackassociated species, and not necessarily just amphipods, are rapid colonizers of
wrack (Olabarria et al., 2007; Rodil et al., 2008).
Species colonize wrack based on a combination of feeding and habitat
requirements, and these can change depending on the amount of time that the
wrack has been present. Amphipods prefer moist, fresh wrack as a food source
because these invertebrates are more susceptible to desiccation (Marsden,
1991b). This is likely the case for the larvae of flies (Diptera; Griffiths and
Stenton-Dozey, 1981) and crustacean isopods as well (Colombini et al., 2000). In
contrast, beetles (Coleoptera) prefer older wrack deposits (Griffiths and StentonDozey, 1981). Herbivorous beetles, such as those in the families Tenebrionidae,
Hydrophilidae, Curculionidae and Scarabaeidae, invade wrack once it has dried
out while stranded on the shore (Colombini and Chelazzi, 2003). Predaceous
31
beetles in the families Staphylinidae, Histeridae and Carabidae also form a large
component of the wrack fauna and usually follow the establishment of their prey
which include amphipods and insects, notably the larvae of dipterans (flies;
Colombini and Chelazzi, 2003, Colombini et al., 2000; Griffiths and StentonDozey, 1981).
Meiofaunal assemblages consisting of animals usually less than 1 mm in
size, play an important part in the colonization of very old wrack (Jedrzejczak,
2002; Inglis, 1989). This is probably related to the fact that the older material is
more readily available to microbial decomposers on which meiofauna feed, and
does not require macrofauna to break the material into smaller pieces to assist
decay (Jedrzejczak, 2002). Microorganisms are thus likely to be of primary
importance in the breakdown of seaweeds in the supralittoral zone of sandy
beaches (Jedrzejczak, 2002).
2.3 Main geomorphological features of PEI coasts
The sandy beaches of Prince Edward Island (PEI) are associated with
three predominant shoreline types featuring sand dunes, glacial till bluffs, and
sandstone cliffs. Sand dunes are accumulations of sediment resulting from
Aeolian (wind-driven) processes. Till bluffs on PEI are reddish-brown sediments
containing clay, sand, stones and boulders, all of which are derived from glacial
redeposition of material derived from underlying bedrock. Finally, sandstone
bedrock deposits are composed of very fine to very coarse sand-sized minerals
that form prominent cliffs along the coastline. Although these dunes, bluffs or
cliffs are not generally exposed to the action of waves and currents, they are
related to the dynamics of the entire sandy beach ecosystem. Sediment eroded
from dunes, bluffs and cliffs may be incorporated into the intertidal zone, and
sediments from the intertidal zone may become incorporated into dunes. The
resulting shoreline position and shape may either protect or contribute to beach
erosion. The paragraphs below describe in detail these three main geological
configurations.
2.3.1 Sand Dunes
Coastal sand dunes characteristically develop landward of most sandy
beaches as a result of aeolian sediment transport by onshore winds (Fig. 2.2;
Davidson-Arnott and Law, 1990). To move sand from the beach to the dunes,
wind speed must exceed a threshold velocity for the particular size of sand
available. If the sand is damp or if the grains must move up a slope, the wind
velocities required for sediment transport are greatly increased. The foreshore of
the beach may act as a source of sand if it dries between tidal cycles. This is
especially true in areas where there are diurnal tides (as opposed to semidiurnal
or mixed semidurinal tides), allowing a greater amount of time for the foreshore
to dry between inundations (Morang et al., 2002).
The pattern of dune development in the absence of any anthropogenic
influence involves the formation of dome dunes, oval or circular mounds of sand,
followed by gradual development of parabolic dunes (curved sand ridges with the
concave portion facing the beach; Catto et al., 2002). This type of dune often
33
Figure 2.2. A typical sand dune, located at Brackley Beach, Prince Edward
Island National Park, Prince Edward Island.
0
34
forms downwind of pools or damp areas (Morang et al., 2002).Two factors
operating at the regional scale indirectly influence the development and character
of the coastal dunes of the region: rising sea level and climate. In Atlantic
Canada, rising sea level has resulted in relatively rapid landward migration of
beaches and spits as well as their associated sand dunes, facilitated by the late
onset of spring and slow growth of dune stabilizing vegetation until mid-June
(McCann, 1990).
Most PEI coastal dunes are simple foredunes oriented parallel to the
shoreline (Nutt and McCann, 1990) which serve as storm buffers (Morang et al.,
2002), and they are common along the north shore of PEI. Exceptions occur
where the shoreline is subject to severe anthropogenic disturbance (such as
large numbers of people traversing the dunes) or recent coastal erosion (Catto et
al., 2002) from wind-driven waves, wave run-up and overwash (Gribbin, 1990). In
the most severely disturbed sites, such as Cavendish, Stanhope and Cabot Head
Provincial Park, these two factors have combined to erode and degrade the
foredunes extensively. For example, individual coastal foredunes exceeded 20 m
in height before recent anthropogenic disturbance (Catto et al, 2002).
Sand dunes on PEI are derived from local sources. On the north shore,
the sands consist mainly of well rounded, spherical grains of quartz, often with a
residual red coating. They are derived almost entirely from the erosion and
breakdown of the underlying sandstone bedrock (Gribbin, 1990), creating small
particles that are driven by onshore winds, and adhere onto moist and snowcovered surfaces (Catto et al,, 2002). Generally most of the sediment transported
35
landward from the backshore is trapped initially by vegetation colonizing the area
just beyond the limit of wave action, leading to the development of an incipient
foredune parallel to the shoreline. Exceptions to this occur where vegetation is
sparse because of limited moisture, or where sediment supply is so large as to
prevent the establishment of vegetation (Davidson-Arnott and Law, 1990).
Shifting and blowing sand creates a difficult environment for plant
establishment, so dune plants must have specific adaptations to survive in these
environments. American beach grass (Ammophila breviligulata) is the most
common dune plant in the region (Matthew et al., 2010), and the first colonizer
where incipient foredunes are developing (Nutt and McCann, 1990). The plants
extend rhizomes to spread rapidly to help stabilize the surface of the dune, then
grow vertically as they are buried by sand, allowing plants to keep pace with
sand deposition on the dune. The leaves of the plant also help trap wind-blown
sand on the dune, facilitating dune growth (Morang et al., 2002).
Dunes vary over time, based on vegetation patterns and sediment
transport. With continued sediment supply, the foredune grows in height and
width, and on accreting shorelines (seaward growth of beach by accumulation of
sediment), a sequence of transverse dunes may form (Davidson-Arnott and Law,
1990). Transverse dunes are asymmetrical ridges with steep lee and gentle
upwind slopes oriented perpendicular or oblique to the dominant winds (Morang
et al., 2002). Over the short term (weeks or months), the rates of foredune
growth are dependent on the volume of sand transport from the beach, the wind
climate, and the sediment characteristics. Such characteristics include grain size,
/
36
mineralogy, moisture content, salt crusting, and indirectly, beach width which can
influence the threshold of sediment motion and sand transport (Davidson-Arnott
and Law, 1990). Since dunes form from onshore winds transporting beach sand,
sediment supply to the dune involves a loss to the beach deposits. Therefore,
over the long term (years to decades) for foredunes to grow, sand must continue
to be deposited on the beach. Otherwise, a negative feedback cycle will be
initiated through narrowing of the beach and erosion of the dune by wave actions
(Psuty, 1988).
2.3.2 Glacial Till
Prince Edward Island was completely covered with ice only fifteen
thousand years ago (Crowl and Frankel, 1970). Sediments of glacial origin,
known collectively as glacial till, are found in central PEI. Their distribution, along
with abrasion features on the bedrock and deposition of erratic boulders, confirm
the former presence of a glacier (Crowl and Frankel, 1970). The bedrock strata Of
PEI are generally covered by glacial till ranging from several centimetres to
several meters thick. Over much of PEI the drift thickness is not more than 3-4.5
m (Fig. 2.3) but along the north coast, 7.5-9 m of till are readily visible (Crowl and
Frankel, 1970).
The deposits left by the ice include dense tills made up of clay and sand
that were mainly derived from the underlying bedrock as the glacier slowly
ground its way across the Island. These deposits include loose textured sandy
tills resulting from the melting of the debris-laden glacier ice (Prest, 1973). The
37
Figure 2.3. A typical glacial till bluff, located at Dalvay, Prince Edward Island
National Park, Prince Edward Island.
38
sediment composition of PEI tills range from clay to silt and sand with a variable
stone content. Despite variations in composition (i.e. proportions of sand, clay
and silt), they are all related to common Wisconsin glaciations (Prest, 1973).
Typical sand-rich or clay-rich till exhibit the following characteristics. Upon
impact, the dry material of sandy till either collapses or breaks into small chips
which have little or no cohesive strength. When this till is wet, its plasticity is low
such that banks cut in it (bluffs) tend to slump readily. However, with certain grain
size mixtures they may retain their form. In contrast, clay till breaks with blocky
fractures when dry and these have a much greater cohesive strength. Claystone
fragments may be locally abundant and very plastic when wet so that banks cut
in it tend to retain a steep face. However, with certain grain size mixtures they
slump readily (Crowl and Frankel, 1970). Till types at either extreme, i.e. those
dominated by clay or those dominated by sand, are readily distinguishable.
However, the intermediate types are not because their characteristics seem to
depend on the amount of silt grains replacing either sand or clay-size materials,
and these also vary greatly with changes in moisture content (Crowl and Frankel,
1970).
2.3.3 Sandstone
Bedrock layers of PEI are relatively soft and are made up of (in order of
abundance) freshwater sandstone, mudstone and conglomerate (sedimentary
rocks, consisting of rounded fragments cemented together) laid down during the
Late Pennsylvanian and Early Permian (300 to 250 million years ago). Some of
the Permian strata may be as young as 225 million years old, younger than most
of those found in Nova Scotia and New Brunswick (Prest, 1973). Three major
intervals have been identified from the Island and the Maritime provinces. The
first (earliest) two sequences are widely exposed in parts of New Brunswick and
Nova Scotia. The third upper sequence consist of redbeds which are exposed
throughout PEI and extend beyond the coastal regions of the Island beneath the
Gulf of St. Lawrence and parts of the Northumberland Strait (Fig. 2.4; van de
Poll, 1989).The upper red conglomerate-sandstone interval consists of layers of
mainly red, fine- to very fine-grained sandstone and mudstone.
The bedrock sediments come from material carried by streams originating
in ancient highlands of what are now New Brunswick and Nova Scotia (Prest,
1973), and deposited as a delta at the river mouth located at the present day PEI
and Gulf of St. Lawrence. As the streams slowed, the mineral grains were
deposited in their channels and on the adjoining flood plains. The ever increasing
load of sediment depressed the low regions while the highlands to the west and
south continued to rise, and several thousand feet of sediments accumulated.
Oxidizing conditions and aerobic bacteria destroyed the carbonaceous material
and formed the oxide of iron that gives the sediments their characteristic redbrown color (Prest, 1973).
Sandstone dominates the PEI bedrock, making up about 60% of the
surface area (van de Poll, 1983). It ranges in grain size from very fine to very
coarse, and its color depends on the grain size. It varies from pale orange (fine to
very fine sandstone) to dark purplish-red (medium to very coarse sandstone) and
Figure 2.4. A typical sandstone cliff, located at Doyles Cove, Prince Edward
Island National Park, Prince Edward Island.
41
local insertions of grey, grayish-red, or greenish-grey sandstone occur where the
iron matrix is in the reduced rather than oxidized form (van de Poll, 1983).
2.4 PEI’s north shore sandy beaches and their geology
Prince Edward Island is approximately 230 km long and 6.5-50 km wide,
and its surface rises to a maximum of 127 m above sea level (van de Poll, 1983).
There are approximately 1,260 km of coastline along PEI (Owens and Bowen,
1977), and according to Shaw et al. (1998a,b) PEI has among the most sensitive
coastlines to sea-level rise in Canada. Factors contributing to this vulnerability
include the predominance of soft (friable) sandstone bedrock, a dynamic sandy
shore which is locally sediment-starved (loses more sediment than it
accumulates), and an indented shoreline with extensive salt marsh areas. The
terrain behind the dunes is also low in relief so there is significant flooding
potential, and high rates of shoreline retreat and coastal submergence have been
documented (Shaw et al., 1998a). Parts of the north shore of PEI were rated
especially sensitive to sea level rise because this coast is exposed to the wave
action of the open Gulf of St. Lawrence, where potential wave-generating fetches
of several hundred kilometers occur, depending on wind direction and ice cover
(Shaw et al., 1998b).
The development of the coast of northern PEI is controlled by a
combination of longshore currents and sediment movement, aeolian activity, sea
level rise, and human action (Catto et al., 2002). This entire shoreline system
depends upon having a large supply of sand to replace what is being eroded and
42
transported laterally along the shoreline. The north shore has a relatively uniform
geology, with sandstone and minor mudstone and conglomerate (van de Poll,
1983) and a variable cover of sandy glacial till or glacial outwash deposits
(sediments deposited by glacial meltwater; Prest, 1973). These easily eroded
deposits provide a moderately abundant source of sand to the coastal system.
The beach and nearshore sediments are predominately fine to medium sands
(Owens and Bowen, 1977), but lag gravel (gravel resulting from the removal of
fine sediment by wind or wave action) is also present in the vicinity of rock cliffs.
Gravel is also present in minor quantities elsewhere on the shoreface near
headlands and in most of the nearshore bar troughs (Forbes, 1987). Another
prominent feature of this coastal system is a series of extensive sandy barriers
with well developed coastal dunes that appear on the seaward margin of
numerous bays and estuaries (Forbes and Manson, 2002).
Investigations within the last decade suggest the existence of distinctive
coastal cells, regions separated by headlands experiencing variable longshore or
shore-normal sediment exchange, along the western, northern, and eastern
coasts of the Island (Shaw et al. 2000, modified by Forbes and Manson, 2002).
These authors subdivided the central portion of PEI’s north shore into six distinct
and largely independent compartments. Three of these compartments harbour
the sandy beaches sampled in this thesis and are therefore described here in
detail. In general, high sandstone cliffs occupy part of the coast, most
prominently in the vicinity of Orby Head between Cavendish and Rustico Bay
(Forbes and Manson, 2002). The central-eastern sections of the coast consist of
43
sand dunes as well as lower cliffs cut into glacial till deposits, sometimes resting
on sandstone, and typically bounded by a narrow gravelly beach (Forbes and
Manson, 2002).
2.4.1 Cavendish coastal compartment
This segment of the coast forms the slight embayment between Cape
Tryon on the west and Orby head on the east (Fig. 2.5a), a distance of about 15
km. The spit at Cavendish beach extends as an almost linear barrier
approximately 5 km westward across the bay to Cavendish Inlet at the western
shore. The barrier is typically 200 to 300 m wide with a very discontinuous dune
line broken by numerous channels cut into the dunes by storms. This long beach
is associated with a system of multiple nearshore bars (Forbes and Manson,
2002). Cavendish Beach extends eastward in front of a low wetland near the east
side of New London Bay. High dunes in this area have a very irregular crest,
reflecting in part the many years of heavy pedestrian traffic across the dunes to
the beach. At the east end of Cavendish where the backshore terrain rises out of
the valley, the beach gives way to sandstone cliffs with a variable cover of glacial
sediment. Under northeasterly storm conditions, sand and gravel derived from
wave erosion of those cliffs is transported westward (Forbes and Manson, 2002).
'*•
2.4.2 Rustico-Brackley coastal compartment
This part of the coast occupies a pronounced embayment between Orby
Head on the west and Cape Stanhope on the east (Fig. 2.5b), encompassing
44
Gulf of St.
Lawrence
NW Atlantic
61 °W
i5?W
Cape Tryon
Orby Head
Covehead |
Gape
< Robinson's
\
I Stanhope
Rustico A
Island
Brackley
Beach A i / i ./ . Ross Lane
~NorthA
New London
/
BaV
Rustico
Bay
7 .5
15.
Dalvay
Stanhope
lane -
Point
Deroche
Blooming
Point
— j Tracadie
Bay
kilometers
Figure 2.5. The three coastal cells a)Cavendish, b)Rustico-Brackley and
c)Stanhope-Tracadie-Deroche housing the sandy beaches sampled in this
study. Figure modified from Forbes and Manson (2002).
45
nearly 15 km of shoreline. The shore in this area is characterized by extensive
sandy beaches and barriers, and foredunes up to 15 m high (Forbes and
Manson, 2002). Sections with sandstone cliffs are also present in this
component. Glacial till deposits are exposed along Robinson’s Island and Cape
Stanhope, and locally at the base of dunes as well (Forbes and Manson, 2002).
The underlying glacial deposits extend seaward as a lag gravel shoal (sandbar
comprised of gravel, resulting from the removal of fine sediments by wind and
wave action; Forbes and Manson, 2002).
The Brackley region has almost continuous thin deposits of sand (<1 m
thick) on the shoreface and inner shelf in areas underlain by former river valleys
with sand and mud infill (Forbes and Manson, 2002). The largest volumes of
sand are stored in the coastal dunes and in flood-delta deposits at North Rustico
and Covehead (Forbes and Manson, 2002). Brackley beach forms a long reach
of unbroken beach and dunes extending approximately 6.6 km from the low
headland at the Robinson’s Island causeway to Covehead. The beach is fronted
by multiple nearshore bars and backed by dunes that vary from long, linear,
shore-parallel foredune ridges with crest elevations of up to 7- 8 m, to highly
dissected, residual dunes of up to 12 m high (Forbes and Manson, 2002).
2.4.3 Stanhope-Tracadie-Deroche coastal compartment
This more subtle embayment extends from Cape Stanhope in the west to
Point Deroche in the east (Fig. 2.5c). It is erosiohal in places, notably at Cape
Stanhope and where low cliffs are cut into glacial tills at and west of Stanhope
46
Lane, at Dalvay, and at Point Deroche (Forbes, 1987). The beach, while nearly
continuous, narrows and typically has more gravel in areas fronting erosional
cliffs. Except in areas of cliff backshore, the beach throughout this compartment
is sandy, typically 50-100 m in width, and fronted by multiple nearshore bars
(Boczar-Karakiewicz et al., 1995; Forbes and Manson, 2002). The number of
bars is generally higher in the middle of the compartment (i.e. Stanhope lane to
east Tracadie Bay), where sand is more abundant, and less at the western and
eastern ends near Cape Stanhope and Point Deroche, where shoreface sand is
sparse. Typically three to four bars are present, however abrupt changes in bar
number and position occur in places, reflecting alongshore variation in the
nearshore morphodynamic status (Wijnberg and Kroon, 2002; Wijnberg and
Terwindt, 1995).
The beach throughout this compartment is backed by a single, narrow,
sharp-crested foredune ridge in places (e.g. west of Ross Lane, east of Stanhope
Lane, parts of Point Deroche), with typical crest elevations of approximately 7-9
m, and more complex multiple dune ridges up to 11 m at Blooming Point. While
the extent of white spruce growth on these dunes suggests considerable age and
stability, some parts sustained significant wave trimming, blowouts, and landward
migration of sand during the middle part of the 20th century (Simmons, 1982).
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Zealand. I. Drift composition and distribution. J. Exp. Mar. Biol. Ecol. 152,
61-74.
Marsden, I.D., 1991b. Kelp-sandhopper interactions on a sand beach in New
Zealand. II. Population dynamics of Talorchestia quoyana (Milne-Edwards).
J. Exp. Mar. Biol. Ecol. 152, 75-90.
Matthew, S., Davidson-Arnott, R.G.D., Ollerhead, J., 2010. Evolution of a beachdune system following a catastrophic storm overwash event: Greenwich
Dunes, Prince Edward Island, 1936-2005. Can. J. Earth Sci. 4, 273-290.
McArdle, S.B., McLachlan, A., 1991. Dynamics of the swash zone and effluent
line on sandy beaches. Mar. Ecol. Prog. Ser. 76, 91-99.
McArdle, S.B., McLachlan, A., 1992. Sandy beach ecology: swash features
relevant to the macrofauna. J. Coast. Res. 8, 398-407.
McCann, S.B., 1990. An introduction to the coastal dunes of Atlantic Canada, in:
Davidson-Arnott, R. (Ed.), Proceedings of the Symposium on Coastal Sand
Dunes 1990. National Research Council, Ottawa, pp. 89-107.
McLachlan, A., 1983. Sandy beach e c o lo g y -a review, in: McLachlan, A.,
Erasmus, T. (Eds.), Sandy Beaches as Ecosystems. Dr. W. Junk
Publishers, The Hague, pp. 321-380.
McLachlan, A., 1990. Dissipative beaches and macrofauna communities on
exposed intertidal sands. J. Coast. Res. 6, 57-71.
51
McLachlan, A., 2001. Coastal beach ecosystems, in: Lewin, R. (Ed.),
Encyclopedia of Biodiversity. Academic Press, New York, pp. 741-751.
McLachlan, A., Brown, A.C., 2006. The Ecology of Sandy Shores, second ed.
Elsevier, New York.
McLachlan, A., de Ruyck, A., Hacking, N., 1996. Community structure on sandy
beaches: patterns of richness and zonation in relation to tide range and
latitude. Rev. Chil. Hist. Nat. 69, 451-467.
McLachlan, A., Dorvlo, A., 2005. Global patterns in sandy beach macrobenthic
communities. J. Coast. Res. 21, 674-687.
McLachlan, A., Fisher, M., Al-Habsi, H.N., Al-Shukairi, S.S., Al-Habsi, A.M.,
1998. Ecology of sandy beaches in Oman. J. Coast. Conserv. 4, 181-190.
McLachlan, A., Jaramillo, E., Donn, T.E., Wessels, F., 1993. Sandy beach
macrofauna communities and their control by the physical environment: a
geographical comparison. J. Coast. Res. 15, 27-38.
McLachlan, A., Turner, I., 1994. The interstitial environment of sandy beaches.
Mar. Ecol. 15, 177-211.
McLachlan, A., Wooldridge, T., Dye, A.H., 1981. The ecology of sandy beaches
in southern Africa. S. Afr. J. Zool. 16, 219-231.
Morang, A., Gorman, L.T., King, D.B., Meisburger, E., 2002. Coastal
Classification and Morphology, in: Morang, A. (Ed.), Coastal Engineering
Manual. U.S. Army Corps of Engineers, Washington D.C., pp. 1-77
Nutt, L.A., McCann, S.B., 1990. Foredune evolution near Point Deroche on the
north shore of Prince Edward Island, in: Davidson-Arnott, R. (Ed.),
Proceedings of the Symposium on Coastal Sand Dunes 1990. National
Research Council, Ottawa, pp. 109-115.
Owens, E.H., Bowen, A.J., 1977. Coastal environments of the Maritime
Provinces. Atlantic Geol. 13, 1-31.
Olabarria, C., Lastra, M., Garrido, J., 2007. Succession of macrofauna on
macroalgal wrack of an exposed sandy beach: effects of patch size and
site. Mar. Environ. Res. 63, 19-40.
Polis, G.A., Hurd, S.D., 1996. Linking marine and terrestrial food webs:
allochthonous input from the ocean supports high secondary productivity on
small island and coastal land communities. Am. Nat. 147, 396-423.
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Prest, V.K., 1973. Surficial Deposits of Prince Edward Island. Geological Survey
of Canada, Ottawa.
Psuty, N.P., 1988. Sediment budget and beach/dune interaction. J. Coast. Res.
3, 1-4.
Rodil, I.F., Olabarria, C., Lastra, M., Lopez, J., 2008. Differential effects of native
and invasive algal wrack on macrofaunal assemblages inhabiting exposed
sandy beaches. J. Exp. Mar. Biol. Ecol. 358, 1-13.
Shaw, J., Taylor, R.B., Forbes, D.L., Ruz, M.-H., Solomon, S., 1998a. Sensitivity
of the coasts of Canada to sea-level rise. Geological Survey of Canada,
Ottawa.
Shaw, J., Taylor, R.B., Li, M., Forbel, D., 2000. Role of a submarine bank in the
long-term evolution of the coast of Prince Edward Island: new evidence
from multibeam bathymetry mapping systems. Period. Biol. 102, 589-594.
Shaw, J., Taylor, R.B., Solomon, S., Christian, H.A., Forbes, D.L., 1998b.
Potential impacts of global sea-level rise on Canadian coasts. Can. Geogr.
42, 365-379.
Short, A.D., 1999. Handbook of Beach and Shoreface Morphodynamics. John
Wiley and Sons, Ltd., Toronto.
Short, A.D., Wright, L.D., 1983. Physical variability of sandy beaches, in:
McLachlan, A., Erasmus, T. (Eds.), Sandy Beaches as Ecosystems. Dr. W.
Junk Publishers, The Hague, pp. 133-144.
Simmons, M.D., 1982. Have catastrophic storms shaped P.E.I.’s North Shore?
Some observations which suggest an important role for unusually large
storms, in: Associate Committee for Research on Shoreline Erosion and
Sedimentation (Eds.), Proceedings Workshop on Atlantic Coastal Erosion
and Sedimentation, Halifax. National Research Council Canada, Ottawa,
pp. 23-36.
Soares, A., 2003. Sandy beach morphodynamics and macrobenthic communities
in temperate, subtropical and tropical regions: a macroecological approach.
Ph.D thesis, University of Port Elizabeth, South Africa.
Stenton-Dozey, J.M.E., Griffiths, C.L., 1983. The fauna associated with kelp
stranded on a sandy beach, in: McLachlan, A., Erasmus, T. (Eds.), Sandy
Beaches as Ecosystems. Dr. W. Junk Publishers, The Hague, pp. 557-568.
53
van de Poll, H.W., 1983. Geology of Prince Edward Island. PEI Department of
Energy and Forestry, Charlottetown.
van de Poll, H.W., 1989. Lithostratigraphy of the Prince Edward Island redbeds.
Atlantic Geol. 25, 23-35.
Vandendriessche, S., De Keersmaecker, G., Vinex, M., Degraer, S., 2006. Food
and habitat choice in floating seaweed clumps: the obligate opportunistic
nature of the associated macrofauna. Mar. Biol. 149, 1499-1507.
Wijnberg, K.M., Kroon, A., 2002. Barred beaches. Geomorphology 48, 103-120.
Wijnberg, K.M., Terwindt, J.H.J., 1995. Extracting decadal morphological
behavior from high-resolution, long-term bathymetric surveys along the
Holland coast using eigenfunction analysis. Mar. Geol. 126, 301-330.
Wright, L.D., Nielsen, P., Short, A.D., Green, M.O., 1982. Morphodynamics of a
macrotidal beach. Mar. Geol. 50, 97-128.
54
CHAPTER 3:
A SPATIAL COMPARISON OF SANDY BEACHES IN A VULNERABLE
SYSTEM IN THE GULF OF ST. LAWRENCE: SPECIES COMPOSITION,
RICHNESS AND ABUNDANCE IN RELATION TO SHORELINE TYPE AND
EROSION
55
MacMillan, M.R., Quijon, P.A., Submitted. A spatial comparison of Sandy
beaches in a vulnerable system in the Gulf of St. Lawrence: species
composition, richness and abundance in relation to shoreline type and
erosion. Ecological Indicators.
3.1 Abstract
Sandy beaches associated with sand dunes, till bluffs and sandstone cliffs
constitute the most prominent feature of Prince Edward Island’s north shore. The
entire area is vulnerable to sea-level rise and storm-related erosion, and
constitutes a model for the study of sandy beach macrofauna in relation to
physical variation. This study used snapshot surveys to document the species
composition, diversity and abundance of macrofauna communities at 14 sandy
beaches, and explored their relationship with shoreline type, erosion rates, slope,
sediment grain size and the Beach Deposit Index (BDI). At each beach, 20
samples (0.03 m2) were collected from replicated transects across the intertidal
zone in order to characterize the macrofauna, and additional samples and
measurements were taken to characterize physical descriptors. The lower
intertidal of all sandy beaches was numerically dominated by the spionid
polychaete Scolelepis squamata while the upper intertidal was characterized by
the amphipods Americorchestia megalophthalma and Haustorius canadensis on
beaches associated with sand dunes and till bluffs, and by Platorchestia
platensis on beaches associated with sandstone cliffs. Average species richness
was low and abundances were highly variable among beaches and shoreline
types. Regression analyses identified positive relationships between erosion rate
and species richness and abundance, but failed to detect any significant
56
relationship between faunistic variables and the other physical variables.
Similarity analyses indicated that beaches associated with sandstone cliffs, which
coincidentally exhibited the lowest rates of coastal erosion, sustained
communities significantly different from those collected from beaches associated
with till bluffs and sand dunes. This exploratory study represents a first step
towards the potential use of sandy beach invertebrates as indicators of weather
related phenomena affecting sandy beaches.
3.2 Keywords
Sandy beaches; Shoreline erosion; Macrofauna; Biological indicators; Snapshot
survey; Gulf of St. Lawrence
3.3 Introduction
Sandy beaches are characterized by the interactions of the wave energy
they experience, their tidal regimes and the nature of the sand available for
sorting and transport by tides and waves (McLachlan, 2001). Sandy beaches can
be classified into three basic morphodynamic states: dissipative, intermediate
and reflective. Dissipative beaches are at the erosional end of the spectrum, and
result from a combination of high wave energy dissipating over a wide surf zone,
an abundance of fine sands, and are characterized by gentle beach face slopes
(Short and Wright, 1983). Reflective beaches are at the accretionary end of the
spectrum, and result from a combination of low energy waves reflecting off of a
narrow beach. This type of beach is characterized by coarse sands and steep
57
beach face slopes (Short and Wright, 1983). Between these two extremes,
intermediate beach types represent a transition from dissipative to reflective
states, and exhibit intermediate physical characteristics (Short and Wright, 1983).
Sandy beach invertebrate communities are primarily structured by the
environmental conditions they experience. The autecological hypothesis (NoyMeir, 1979) states that in physically controlled environments, animal populations
have little influence on each other, and communities are structured by species
responding independently to the physical environment rather than to biological
interactions. McLachlan (1990) provided initial evidence supporting the
application of this hypothesis to an array of sandy beaches, suggesting that the
swash climate controlled macrofaunal community structure. More supporting
evidence came from a subsequent study by McLachlan and Dorvlo (2005) who
compiled published data from 161 quantitative sandy beach transect surveys
from tropical, subtropical, warm temperate and cold temperate regions. That
study identified global patterns of community structure in which species richness
and abundance increased from narrow reflective to broad dissipative systems.
Since sandy beach macrofauna communities are structured by their
physical environment, changes to beaches as a result of erosion from weather
related phenomena are expected to result in changes to macrofauna
communities. For instance, sea-level rise is believed to be responsible for long
term beach erosion on United States east coast barrier beaches (Zhang et al.,
2004). Additionally, shifts in storm behaviour (i.e. frequency, intensity) alter the
amount and direction of wave energy approaching the shoreline (Slott et al.,
58
2006) and influence coastal erosion rates (Pethick, 2001). Sandy beaches are
not locked into single morphodynamic states, and respond to changes in wave
energy by moving toward dissipative states during storms, and towards reflective
conditions during calm weather. During this process, sand erodes or accretes on
the beach face as wave height changes (McLachlan, 2001). Similar differences
can be expected at the spatial scale, where physical variation among sites is
expected to be reflected in infaunal communities.
The objective of this study was to document the composition, diversity and
abundance of sandy beach macrofaunal communities on the north shore of
Prince Edward Island (PEI) in relation to a number of physical characteristics.
PEI sandy beaches are associated with three predominant shoreline types: sand
dunes, till bluffs and sandstone cliffs. Given the apparent visual discrepancy
between these three shoreline types it is hypothesized that sandy beaches
associated with them will differ in their physical characteristics, and therefore
exhibit differences in their macrofauna communities.
3.4 Methods
3.4.1 Study area
The north shore of PEI is part of a major estuarine system, the southern
Gulf of St. Lawrence. Surface water salinity shows little spatial fluctuation along
the study area during the summer months (June 29.53 ± 1.07%o, July 28.29 ±
1.42%o, and August 28.11 ± 0.76%o; Petrie et al., 1996). The region has a cool
temperate maritime climate and mean tide range of 0.7 m (Forbes et al., 2004).
59
The coast is dominated by wave-generated processes, exposed to waves from a
direction between northwest and east (Owens and Bowen, 1977). Fourteen
sandy beaches along the north shore of Prince Edward Island, all within Prince
Edward Island National Park (PEINP) were selected for sampling during the
summer seasons of 2009-2010 (see Fig. 3.1 for approximate locations). These
beaches were associated with three distinctive shoreline types: sandstone cliffs
(Doyles Cove east, Doyles Cove west, MacKenzie’s Brook, Cape Turner), till
bluffs (Davlay west I, Dalvay west II, Stanhope east, Stanhope west) and sand
dune deposits (Brackley, Cavendish east, Cavendish west I, Cavendish west II,
Ross lane, Dalvay east). These sites were sampled on the following dates:
Brackley (July 15, 2009), Cavendish east (July 16, 2009), Ross lane and Dalvay
east (July 20, 2009), Doyles Cove east and McKenzie’s Brook (July 21, 2009),
Doyles Cove west and Cape Turner (July 22, 2009), Dalvay west I (July 29,
2009), Stanhope east (July 31, 2009), Stanhope west (August 3, 2009),
Cavendish west I (June 22, 2010), Dalvay west II (June 23, 2010), Cavendish
west II (July 5, 2010).
3.4.2 Sampling protocol
Snapshot surveys, a one-time sampling of each site with values for each
beach regarded as a single point in space and time, are considered the standard
for spatial comparisons of sandy beaches, and were utilized in this study.
Sampling was conducted during a narrow time frame over the summer. Each site
was sampled during spring low tides, with 1-2 beaches sampled per day.
60
Gulf of St.
Lawrence
45° N - -
N W A tla n tic
"65?W
6 1 °W
O Sand dune
•S a n d s to n e cliff
• T ill bluff
*1
12 13
kilometers
Figure 3.1. Approximate location of the sandy beaches sampled along the north
shore of Prince Edvyard Island, southern Gulf of St. Lawrence. 1.Cavendish
west II, 2.Cavendish west I, 3.Cavendish east, 4.Mackenzies Brook, 5.Cape
Turner, 6.Doyles Cove west, 7.Doyles cove east, 8.Brackley, 9.Ross Lane,
10-Stanhope east, 11.Stanhope west, 12.Dalvay west II, 13.Dalvay west I,
14. Dalvay east.
61
For characterizing macrofauna communities, four transects spaced two
meters apart extending from the drift line (high tide level) to the lower limit of the
swash zone (low tide level) were sampled at each beach. Along each transect, a
0.03 m2 PVC core was used to collect sediment up to a depth of approximately
25 cm at five equally spaced levels from high tide to low tide levels. The
sediment samples collected were sieved through a 1 mm mesh screen, and the
macrofauna retained were stained with Rose Bengal solution to facilitate sorting,
and stored in 70% ethanol. Macrofauna were identified to the lowest possible
taxonomic level using the key by Bousfield (1973) and Bromley and Bleakney
(1984), and counted in order to determine abundance and taxa richness.
Physical characteristics of each beach were determined with the following
protocols. Approximately 250 mL of surface sediment was collected to a depth of
5 cm at each level of the first transect for grain size analysis (n=5 per beach).
Sediment samples were wet-sieved through a nested series of mesh sizes (i.e.
1000, 500, 250and 125 pm) then dried and weighed. Themean sediment grain
size as well as sorting was calculated using the logarithmic Folk and Ward (1957)
graphical measures in GRADISTAT 4.0 (Blott and Pye, 2001). Beach face slope
was measured in an area that was undisturbed during sampling, immediately
adjacent to one of the sampled transects. This was done using graduated rods
and the horizon (Emery, 1961) in two meter increments from the drift line to the
swash zone. The Beach Deposit Index (BDI) was then calculated according to
BDI = (—
) ■ (\M—z J)
Vtan b J
(1)
' ’
62
where tan B is the average intertidal beach slope, a is the median grain size of
the sand particle size classification scale (1.03125 mm) and Mz is the average
intertidal sand grain size in mm (Soares, 2003). This index is lowest for beaches
with steep slopes and coarser sands, and increases with flat slopes and finer
sands in a gradient resembling that of the transition from reflective to dissipative
beaches.
The erosion rate for each sandy beach was estimated from the
measurements recorded at the nearest erosion monitoring site within PEINP. The
distance between sampled beaches and erosion monitoring sites ranged from 02 km. These sites were part of a suite of sites established by the PEI Department
of the Environment in 1985 to measure coastal erosion. They were monitored
sporadically through to 1996, and until the monitoring program was acquired by
Parks Canada in 2002. The amount of shoreline eroded at each site was
determined by measuring the distance from a reference point on a cliff, bluff or
dune edge/scarp to survey pins installed at each site. The rate of erosion (m •
yr'1) was determined by dividing the total distance eroded from the coast by the
number of years the site was sampled (Hawkins, 2008).
3.4.3 Data analysis
Physical characteristics (1/slope, sediment grain size, erosion rate) and
the BDI were compared among beaches associated with the three shoreline
types using one-way Analysis of Variance (ANOVA). Pairwise multiple
comparisons were made using the Holm-Sidak method when significant
63
differences were found between shoreline types. Sediment grain size was not
normally distributed nor could it be normalized by transformation, so differences
between shoreline types were assessed using Kruskal-Wallis one-way ANOVA
on ranks. The relationship between physical descriptors of beach state (1/slope,
sediment grain size and the BDI) and erosion rate were also explored using
linear regression analyses.
Relationships between faunistic and physical variables were explored
using linear regression analysis. Preliminary analyses were also conducted with
non-linear regressions, and although in a few cases the r2 values were slightly
higher, they did not change the outcome or the overall interpretation of the
regressions. Therefore, for consistency, only linear regressions are presented.
Data on species composition and abundance per transect (all five samples
combined for each transect; cf. Brazeiro, 2001) were used as single replicates to
characterize each sandy beach (n=4 per beach). Mean species richness and
abundance per sandy beach, as well as abundance of the dominant species
(Scolelepis squamata) were logio(x + 1) transformed when necessary in order to
meet assumptions of normality and homoscedasticity for the regression model.
Similarity among communities associated with the three shoreline types
was also assessed. Transects were used as replicates for non-metric
multidimensional scaling ordinations (MDS) based on the Bray-Curtis similarity
index (de la Huz and Lastra, 2008; PRIMER Version 6.1.13) to interpret
differences among communities. A measurement of goodness-of-fit of the MDS
ordination was given by the stress value, where a low stress factor (< 0.2) was
64
considered an ordination with no serious prospect of a misleading interpretation
(Clarke and Warwick, 1994). Three transects which had no macrofauna, all from
sandy beaches associated with sandstone cliffs, were excluded from the
similarity analysis. The contribution of rare species (against a single highly
abundant species) was accounted for by applying a logio (x + 1) transformation
before data were analyzed. Pairwise multiple comparisons using Analysis of
Similarity (ANOSIM; Clarke, 1993) were used in order to test the null hypothesis
of no difference among infaunal assemblages from beaches associated with the
three different shoreline types. SIMPER analysis was subsequently used to
identify the species that contributed the most to the differences between
communities associated with each shoreline type.
3.5 Results
3.5.1 Physical properties of the sandy beaches
The physical characteristics of the 14 sandy beaches surveyed exhibited
substantial variation. The inverse of beach face slope ranged from 10 to 53 and
the BDI ranged from a low of 28 on the steepest beaches to 246 (Table 3.1).
Values for both variables were slightly greater on beaches associated with sand
dunes, but the differences were borderline or not significant (one-way ANOVA;
p=0.055 and p=0.153 respectively; Table 3.2, Fig. 3.2a,b). Mean sediment grain
size ranged from fine to medium sands and sorting varied from well sorted to
poorly sorted (Table 3.1), but there was no difference between shoreline types
(Kruskal-Wallis; p=0.997; Fig. 3.2c). Erosion rates, however, were significantly
65
Table 3.1. Summary of physical characteristics of the 14 sandy beaches sampled on the north shore of PEI. Sorting
categories are based on Folk and Ward (1957).
Site
Location (N,W)
Shoreline
type
Doyles Cove East
Doyles Cove West
MacKenzies Brook
Cape Turner
Dalvay west I
Dalvay west II
Stanhope East
Stanhope West
Brackley
Cavendish east
Cavendish west I
Cavendish west II
Ross Lane
Dalvay East
46.47479,-63.30448
46.47603, -63.30550
46.49664, -63.34588
46.484737,-63.311872
46.41736, -63.07385
46.418518, -63.085419
46.422928, -63.108484
46.423553, -63.111156
46.430635,-63.202985
46.499377,-63.392177
46.50103,-63.40291
46.50216, -63.41856
46.42580, -63.12159
46.41690, -63.07039
Sandstone
Sandstone
Sandstone
Sandstone
Till
Till
Till
Till
Dune
Dune
Dune
Dune
Dune
Dune
1/Slope
BDI
Mean erosion
rate (m ■yr'1)
24
28
11
26
21
21
16
10
40
53
29
12
31
44
135
147
28
114
113
107
82
28
216
184
100
49
171
246
0.20
0.20
0.17
0.16
0.94
0.81
1.10
1.10
0.41
0.36
0.59
0.61
0.99
0.94
Mean
sediment
grain
size (0)
2.47
2.36
1.35
2.07
2.41
2.33
2.32
1.47
2.37
1.74
1.74
1.98
2.41
2.44
Sediment sorting
Well Sorted
Moderately Well Sorted
Poorly Sorted
Moderately Sorted
Well Sorted
Moderately Well Sorted
Moderately Well Sorted
Poorly Sorted
Moderately Well Sorted
Moderately Well Sorted
Well Sorted
Moderately Well Sorted
Moderately Well Sorted
Well Sorted
66
Table 3.2. Results of one-way ANOVAs comparing physical features (1/slope,
Beach Deposit Index, erosion rate) among the 14 beaches surveyed. DF:
degrees of freedom, MS: Mean Squares. For simplicity, Sum of Squares
and F-values have been omitted.
Comparison
Beach
Characteristics
Dependent variable
1/Slope
Source of Variation
Between Groups
Error
DF
2
11
MS
444.601
116.439
P
0.055
BDI
Between Groups
Error
2
11
8179.579
3654.760
0.153
Erosion rate
Between Groups
Error
2
11
0.650
0.0368
<0.001
67
60
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200
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0
§
a
g
3.0
a>
a
150
100
20
10
b)
250
50
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50
0 A
c)
~
y 2 .5
> •
<o 2.0
1.2
1.0
c: 1.5
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a>
I
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a>
d)
as
0 .6
u.
1.0
a>
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£ o.o
CO
L2
Li
0.2
0.0
Sandstone
Till
Dune
Shoreline type
Sandstone
Till
D une
Shoreline type
Figure 3.2. Relationships between the mean physical characteristics measured
and shoreline type for 14 sandy beaches sampled on the north shore of
Prince Edward Island, summer 2009 / 2010. Error bars represent one
standard error. Identical letters indicate no significant differences among
coast types.
68
lower on sandstone shorelines than sand dune and till shorelines (one-way
ANOVA; p=0.006 and p<0.001 respectively; Table 3.2), which also differed
significantly (p=0.020; Fig. 3.2d). There were no significant relationships between
erosion rate and the three physical descriptors (Regression analysis; Fig. 3.3):
1/slope (p=0.543), sediment grain size (p=0.576) and BDI (p=0.850).
3.5.2 Macro fauna communities
Macrofauna species richness was poor, ranging from 0 to 5 species per
transect. Abundance was highly variable, ranging between 0 and 830 individuals
per transect. The most abundant species was the spionid polychaete Scolelepis
squamata, which dominated sediments near the low tide level and represented
approximately 95% of the total abundance (Table 3.3). The next most abundant
species were all amphipods, two of which are characteristic of beaches
associated with till bluffs and sand dunes, Haustorius canadensis and
Americorchestia megalophthalma which represented 1.49% and 0.50% of total
abundance respectively. A third amphipod species, Platorchestia platensis, was
characteristic of beaches associated with sandstone cliffs and represented
0.75% of the total abundance (Table 3.3).
Regression analysis revealed a significant relationship between
macrofauna and erosion rates, but not other measured physical characteristics or
the BDI. Species richness and abundance both increased as erosion rate
increased (Regression analysis; p<0.001 and p=0.006, r2=0.721 and r2=0.485
respectively; Figs. 3.4a & 3.5a). However, there were no significant relationships
69
60
50
(U 40
|
30
^
20
10
0
2.6
2.4
g 2.2
2.0
</>
g 1.8
n
ro
O 1-6
1.4
1.2
300
250
200
g 1go
100
50
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Erosion rate (m • yr~1)
Figure 3.3. Relationships between beach physical characteristics and erosion
rate for 14 sandy beaches sampled on the north shore of Prince Edward
Island, summer 2009 / 2010.
70
Table 3.3. Composition of macrofauna communities of 14 sandy beaches
sampled on the north shore of Prince Edward Island, summer 2009 / 2010.
Brackets denote the following: (P)olychaete, (A)mphipod, (O)ligochaete,
(N)emertea.
CD
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CD
s
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co
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east
Doyles Cove
west
Mackenzies
Brook
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Figure 3.4. Relationships between physical characteristics and mean species
richness for the macrofauna community of 14 sandy beaches on the north
shore of Prince Edward Island, summer 2009 / 2010. Error bars represent
the standard error of four replicates.
Log(Mean Abundance + 1)
72
3.0
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100
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Figure 3.5. Relationships between physical characteristics and mean abundance
of the macrofauna community of 14 sandy beaches on the north shore of
Prince Edward Island, summer 2009 / 2010. Error bars represent the
standard error of four replicates.
between mean species richness and the beach face slope, sediment grain size
or BDI (Regression analysis; Fig. 3.4b,c,d): 1/slope (p=0.714), sediment grain
size (p=0.373) and BDI (p=0.817), or between those variables and abundance
(Fig. 3.5b,c,d): 1/slope (p=0.432), sediment grain size (p=0.550) and BDI
(p=0.148). The mean abundance of S. squamata also increased with erosion rate
(Regression analysis; p=0.004, r2=0.504; Fig. 3.6a). However, there were no
significant relationships between the abundance of this species and the physical
characteristics described or BDI (Fig. 3.6b,c,d): 1/slope (p=0.429), sediment
grain size (p=0.761) and BDI (p=0.412).
Macrofauna communities differed significantly between some shoreline
types, but not others. Based on species composition and abundance, no
significant differences were observed between communities from beaches
associated with till bluffs and sand dunes (ANOSIM; p=0.127). However,
communities on beaches associated with sandstone cliffs were significantly
different from the other two (p<0.001 in both pairwise comparisons; Fig. 3.7).
This distinction among communities matched an arbitrary categorization of
beaches based on their rates of coastal erosion: shorelines experiencing
relatively low rates of erosion (0.16-0.20 m/year, beaches associated with
sandstone cliffs) and shorelines experiencing relatively high levels of erosion
(0.36-1.1 m/year, beaches associated with till bluffs and sand dunes; Fig. 3.2d).
SIMPER analysis revealed that average similarity values were much lower
between samples associated with sandstone beaches (18.64%) than in those
associated with till and dune beaches (69.43% and 66.06%, respectively). The
Log(Mean S. squamata
Abundance + 1)
74
3.0
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40
50
60
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1/Slope
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3.0
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2.6
0
50
100
150
BDI
Figure 3.6. Relationships between physical characteristics and mean abundance
of the dominant polychaete Scolelepis squamata on 14 sandy beaches on
the north shore of Prince Edward Island, summer 2009 / 2010. Error bars
represent the standard error of four replicates.
75
Figure 3.7. Multidimensional scaling plot illustrating macrofauna community
similarity among sandy beaches associated with sandstone cliffs (black
symbols), till bluffs (gray) and sand dunes (white). The oval surrounding
samples from beaches associated with till bluffs and sand dunes is based
on ANOSIM results and indicates that community structure in these
samples were not significantly different.
76
most important species driving similarity among samples in the three shoreline
types was S. squamata, followed by Platorchestia platensis (sandstone
beaches), Americorchestia megalophthalma (till beaches) and Haustorius
canadensis (sand dune beaches).
3.6 Discussion
Worldwide, the diversity and abundance of macrofauna on sandy beaches
increases predictably from reflective to dissipative states (Defeo and Mclachlan,
2005). Although such trends have been primarily reported in exposed oceanic
beaches, it has recently been demonstrated to be applicable to some beaches
associated with estuarine systems (e.g. Lercari and Defeo, 2006). Sandy
beaches on the north shore of PEI are embedded within a large estuarine system
(Forbes et al., 2004), and despite their poor complement of species, they were
expected to follow a similar reflective-dissipative pattern in abundance and
diversity. Surprisingly, no significant relationships were detected between infauna
and the physical variables most closely related to reflective-dissipative gradients
(slope and grain size) or the Beach Deposit Index. Instead, consistently
significant relationships were found between macrofauna and the rates of coastal
erosion: as erosion rates increased, the abundance and diversity of the
macrofauna did as well. Among other factors, this relationship can be explained
by the supply of nutrients to the suspension-feeding organisms dominating this
system.
77
Food sources for sandy beach macrofauna include planktonic surf and
epipsammic diatoms, particulate and dissolved organic matter, detritus and
carrion (McLachlan and Brown, 2006). The main driving force maintaining sandy
beaches is wave energy which transports energy and matter (including
suspended food) into, within, and out of sandy beach ecosystems (McLachlan et
al., 1981). Menn (2002) reported that highly dynamic beaches, such as those
studied here, are affected by high levels of erosion, filter large volumes of
seawater and result in high fluxes of particulate organic matter through the
beach. Suspension feeders like Scolelepis squamata, by far the numerically
dominant species in this system, feed directly on these suspended particulates.
Although this polychaete is a facultative suspension / deposit feeder, Dauer
(1983) demonstrated that even in the absence of currents, deposit feeding is rare
in this species. Most commonly (near 95% of the time), when exposed to
currents of 5 cm • s'1or higher this species fed on suspended or re-suspended
particles (Dauer 1983). The second most abundant species in this study was the
haustoriid amphipod Haustorius canadensis. Haustoriid amphipods are also
suspension feeders (Bousfield, 1970; Crocker, 1967; Ivesterand Coull, 1975).
Hudon (1983) reported the amphipod Calliopius laeviusculus was an efficient and
selective consumer of re-suspended particulates. Furthermore, although
amphipods of the genus Gammarus are considered omnivores, Hudon (1983)
observed that the rapid beating of the pleopods necessary for respiration by
Gammarus oceanicus created currents bringing suspended and re-suspended
food particles to the mouth. Thus by their very nature, amphipods appear to be
78
well suited to feeding on suspended particulates despite their functional feeding
preferences. Since sandy beaches, particularly those exposed to erosion, are not
nutrient sinks (McLachlan and McGwynne, 1986), conditions are not favourable
for deposit feeder species (Snelgrove and Butman, 1994), which indeed were
virtually absent from the surveys.
Previous studies have reported that active swimmers such as S.
squamata and the amphipod H. canadensis are predominant on sandy beaches
severely affected by wave action (Eleftheriou and Nicholson, 1975; Menn, 2002).
This is consistent with the results of this study, particularly in the case of S.
squamata. Although found on all sites, this polychaete was much more abundant
on beaches associated with sand dunes and till bluffs which experienced
significantly greater rates of erosion than sandstone cliffs. Similarly, H.
canadensis was not found on the relatively stable sandy beaches associated with
sandstone cliffs, where this species was replaced by the amphipod Platorchestia
platensis, a primary beach colonizer (Bousfield, 1973). A series of other studies
have investigated the effects of erosion on beach invertebrates (e.g. Beentjes et
al., 2006; Jaramillo et al., 1987; Menn, 2002; Walker et al., 2008). However, this
is the first time sandy beach community assemblages have been investigated
across a spatial gradient that include three distinctive shoreline types. Erosion
levels were considerably lower in sandy beaches associated with sandstone
cliffs. This likely implies lower levels of sediment and nutrient re-suspension that
limit or reduce the amount of food available to suspension-feeding organisms, by
far, the best represented in these communities.
Surprisingly, the observed increase in abundance and diversity with rate of
erosion does not appear to be the result of a change in beach state toward
dissipative conditions. Although coastal geomorphology, erosion rates and beach
dynamics are interrelated (Bray and Hooke, 1997), no significant relationships
were detected between the rate of coastal erosion and any of the physical
characteristics studied here (slope, sediment grain size) or the BDI. Furthermore,
no significant relationships were found between macrofauna abundance or
diversity and those physical descriptors. The inverse of beach face slope
generally ranges from 10 to 100 for a gradient between reflective and dissipative
beaches (McLachlan and Brown, 2006). However, in this study the range was
narrower (10-53) indicating that the beaches sampled fall within a reflectiveintermediate range. Thus, the lack of significant relationships between
macrofauna and physical characteristics may be attributed at least in part to the
rather homogeneous morphodynamics of the beaches on the north shore of PEI.
Since the increase in species richness with erosion rate appears to be unrelated
to beach morphodynamics, the increase may be due to a greater number of
predators feeding on the abundant prey species at the eroding sites. For
example, Nephtys bucera is known to feed on the polychaete Scolelepis
squamata (McDermott, 1987), and nemerteans are known to feed on amphipods
and polychaetes (McDermott, 1998; Thiel and Kruse, 2001).
Given the temporal constraints of this survey (a single spatial comparison
applicable to summer conditions) it is impossible to infer whether these results
are applicable to other seasonal conditions. However, the spatial scope of the
comparisons conducted (~40 km of shoreline) is large enough to be relevant for
coastal management. Therefore, the results of the regressions conducted
provide meaningful information despite the fact that their predictive level is only
modest. Further analyses on broader temporal scales are clearly required in
order to determine the robustness of these relationships in light of seasonality
and inter-annual variation. The results of this study also have implications for the
further study and monitoring of sandy beaches. Indeed, the most common and
abundant species reported here may be suitable as an indicator species for
assessing beach conditions over time (Walker et al., 2008).
One reason that erosion fates may not have related well to physical
characteristics was that the erosion monitoring sites used in this study were
established to monitor park infrastructure and not necessarily beach conditions.
Forbes and Manson (2002) noted that sand dune shorelines on the north shore
of PEI were highly mobile and may rapidly retreat and subsequently heal, while
cliff shorelines retreat relatively slowly but persistently (Forbes and Manson,
2002). Given that these shorelines appear to have “characteristic” dynamics of
erosion, the differences in erosion rates are assumed to be meaningful and
representative of each sandy beach. Regardless, differences in erosion levels
recorded over a period of only 10 years should be taken cautiously, as they may
not necessarily represent long-term or future trends at a given site (Dolan et al.,
1991).
There are other factors not considered in this study that may also play a
meaningful role in the spatial variation of the macrofauna. For example, Dugan et
al. (2003) found that species richness, abundance and biomass of macrofauna
were not well predicted by any physical characteristic of the sandy beaches of
the southern California coast. These authors found that overall species richness
and abundance were instead correlated with standing crop of macrophyte wrack.
Talitrid amphipods such as Americorchestia megalophthalma and Platorchestia
platensis found in this study are strongly influenced by wrack inputs and may
indeed contribute to the variability of macrofauna communities (see Chapter 4).
The macrofauna plays a role of upmost importance in sandy-beach food
chains. They consume primary food sources and in turn serve as prey for midand top-level predators (McLachlan and Brown, 2006). A known predator of
sandy beach invertebrates on the north shore of PEI is the Piping Plover
(Charadrius melodus), a species that has been listed as “endangered” in Canada
and the United States (Burger, 1994). Its reliance on sandy beach invertebrates
as a main source of food, adds relevance to studies like this that aim to
document spatial variation of the plover’s food sources during the time this
species nests and feeds in the region. Aiming to predict plover’s food availability
in scenarios of sea level rise and increased erosion levels seems the natural
follow up to this study. Additionally, being able to predict macrofauna community
response to these and other physical and biological factors has relevance for the
management and conservation of biodiversity on sandy beaches (McLachlan and
Dorvlo, 2005).
82
3.7 Acknowledgements
Thanks to Veronique Dufour, Christina Pater, Bradley MacMillan, Jessica Willis,
Megan Tesch and Marianne Parent for their assistance with field work. I would
also like to thank Dr. Pedro Quijon, Dr. Donna Giberson and Dr. Darren Bardati
for reviewing this chapter. Thanks also to Parks Canada for access to PEINP and
their collaboration during the selection of study sites. This research was
supported by a grant from Environment Canada through funding to UPEI’s
climate change research program.
3.8 References
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on abundance and distribution of toheroa (Paphies ventricosa) at Bluecliffs
Beach, Southland, New Zealand. N.Z. J. Mar. Freshwat. Res. 40, 439-453.
Blott, S.J., Pye, K., 2001. GRADISTAT: a grain size distribution and statistics
package for the analysis of unconsolidated sediments. Earth Surf. Process.
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Bousfield, E.L., 1970. Adaptive radiation in sand-burrowing amphipod
crustaceans. Chesap. Sci. 11, 143-154.
Bousfield, E.L., 1973. Shallow-Water Gammaridean Amphipoda of New England.
Comstock Publishing Associates, London.
Bray, M.J., Hooke, J.M., 1997. Prediction of soft-cliff retreat with accelerating
sea-level rise. J. Coast. Res. 13, 453-467.
Brazeiro, A., 2001. Relationship between species richness and morphodynamics
in sandy beaches: what are the underlying factors? Mar. Ecol. Prog. Ser.
224, 34-44.
Bromley, J.E., Bleakney, J.S., 1985. Keys to the fauna and flora of Minas Basin.
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83
Burger, J., 1994. The effect of human disturbance on foraging behavior and
habitat use in Piping Plover (Charadrius melodus). Estuaries 17, 695-701.
Clarke, K.R., 1993. Non-metric multivariate analysis of changes in community
structure. Aust. J. Ecol. 73, 117-143.
Clarke, K.R., Warwick, R.M., 1994. Change in Marine Communities: an Approach
to Statistical Analysis and Interpretation. Natural Environmental Research
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Crocker, R.A., 1967. Niche diversity in five sympatric species of intertidal
amphipods (Crustacea: Haustoriidae). Ecol. Monogr. 37, 173-200.
Dauer, D.M., 1983. Functional morphology and feeding behavior of Scolelepis
squamata (Polychaeta: Spionidae) Mar. Biol. 77, 279-285.
Defeo, O., McLachlan, A., 2005. Patterns, processes and regulatory mechanisms
in sandy beach macrofauna: a multi-scale analysis. Mar. Ecol. Prog. Ser.
295, 1-20.
de la Huz, R., Lastra, M., 2008. Effects of morphodynamic state on macrofauna
community of exposed sandy beaches on Galician coast (NW Spain). Mar.
Ecol. 29, 150-159.
Dolan, R., Fenster, M.S., Holme, S.J., 1991. Temporal analysis of shoreline
recession and accretion. J. Coast. Res. 7, 723-744.
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of macrofauna communities and shorebirds to macrophyte wrack subsidies
on exposed sandy beaches of southern California. Estuar. Coast. Shelf Sci.
58s, 25-40.
Eleftheriou, A., Nicholson, M.D., 1975. The effects of exposure on beach fauna.
Cah. Bio. Mar. 16, 695-710.
Emery, K.O., 1961. A simple method of measuring beach profiles. Limnol.
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Folk, R.L., Ward, W.C., 1957. Brazos River bar: a study in the significance of
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shoreline retreat in the southern Gulf of St. Lawrence. Mar. Geol. 210, 169204.
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Forbes, D.L., Manson, G.K., 2002. Coastal geology and shore-zone processes,
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and Sea-level Rise on Prince Edward Island. Geological Survey of Canada,
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Hawkins, R., 2008. Coastal Erosion Monitoring Protocol. Prince Edward Island
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Hudon, C., 1983. Selection of unicellular algae by the littoral amphipods
Gammarus oceanicus and Calliopius laeviusculus (Crustacea). Mar. Biol.
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Jaramillo, E., Croker, R.A., Hatfield, E.B., 1987. Long-term structure,
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Lercari, D., Defeo, O., 2006. Large-scale diversity and abundance trends in
sandy beach macrofauna along full gradients of salinity and
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56, 177-189.
McDermott, J.J., 1987. The distribution and food habits of Nephtys bucera
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McDermott, J.J., 1998. Observations on feeding in a South African suctorial
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McLachlan, A., Dorvlo, A., 2005. Global patterns in sandy beach macrobenthic
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McLachlan, A., Wooldridge, T., Dye, A.H., 1981. The ecology of sandy beaches
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Petrie, B., Drinkwater, K., Sandstrom, A., Pettipas, R., Gregory, D., Gilbert, D.,
Sekhon, P., 1996. Temperature, Salinity and Sigma-t Atlas for the Gulf of
St. Lawrence. Bedford Institute of Oceanography, Dartmouth.
Short, A., D., Wright, L.D., 1983. Physical variability of sandy beaches, in:
McLachlan, A., Erasmus, T. (Eds.), Sandy Beaches as Ecosystems. Dr. W.
Junk Publishers, The Hague, pp. 133-144.
Slott, J.M., Murray, A.B., Ashton, A.D., Crowley, T.J., 2006. Coastline responses
to changing storm patterns. Geophys. Res. Lett. 33, L18404.
Snelgrove, P.V.R., Butman, C.A., 1994. Animal-sediment relationships revisited:
cause versus effect. Oceanogr. Mar. Biol. Annu. Rev. 32, 111-177.
Soares, A.G., 2003. Sandy Beach Morphodynamics and Macrobenthic
Communities in Temperate, Subtropical and Tropical Regions - A
Macroecological Approach. Ph.D thesis, University of Port Elizabeth, South
Africa.
Thiel, M., Kruse, I., 2001. Status of the Nemertea as predators in marine
ecosystems. Hydrobiologia 456, 21-32.
Walker, S.J., Schlacher, T.A., Thompson, L.M.C., 2008. Habitat modification in a
dynamic environment: the influence of a small artificial groyne on
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86
Zhang, K., Douglas, B., Leatherman, S.P., 2004. Global warming and coastal
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87
CHAPTER 4:
STRANDED MACROPHYTES AS A PATCHY RESOURCE: WRACK
FEATURES INFLUENCE MACROFAUNAL ABUNDANCE IN AN ATLANTIC
CANADA SANDY BEACH SYSTEM
88
MacMillan, M.R., Quijon, P.A., Submitted. Stranded macrophytes as a patchy
resource: wrack features influence macrofaunal abundance in an Atlantic
Canada sandy beach system. Journal of Sea Research.
4.1 Abstract
Patches of stranded macrophytes (wrack) are a distinctive feature of
sandy beaches worldwide and a potential food subsidy for their resident
communities. Despite their relevance, the spatial variation of wrack and its
potential influence on upper shore beach organisms remain poorly understood.
Wrack and macrofauna were surveyed on seven sandy beaches associated with
sand dunes, till bluffs and sandstone cliffs along the north shore of Prince
Edward Island, Atlantic Canada. Wrack patch density, cover, and water content
were measured, and the associated macrofauna were compared to the
communities inhabiting nearby bare sediments. The survey found among-site
spatial differences in wrack characteristics and identified rockweeds (Fucus
serratus) and eelgrass (Zostera marina) as the main macrophyte species in the
area. Macrofaunal abundances were higher in wrack than in bare sediments but
this varied among locations. A field manipulation was conducted at two sandy
beaches to measure macrofauna colonization on patches of fresh and aged
rockweed and eelgrass. Regardless of macrophyte age, macrofaunal organisms
preferentially colonized sediments associated with rockweeds. In addition,
calculations across treatments detected positive relationships between
macrofaunal abundance and wet mass, dry mass and water content of the wrack
patches, regardless of macrophyte species or state. Macrophyte preferences
89
were further explored by comparing the nutritional value of the plant tissues and
assessing macrofauna feeding rates under laboratory conditions. Rockweed
tissues had consistently higher protein, lipid and carbohydrate contents than
eelgrass and were affected by higher invertebrate consumption rates. Overall,
these results suggest that spatial variation and wrack features and species
composition play key roles on the structure of the supralittoral macrofauna.
4.2 Keywords
Sandy beaches; Allochthonous material; Macrofauna; Eelgrass (Zostera marina)]
Rockweed (Fucus serratus); Gulf of St. Lawrence
4.3 Introduction
Sandy beaches around the world are characterized by intertidal zones of
unconsolidated shifting sands devoid of large primary producers (Jaramillo et al.,
2006; Ince et al., 2007). For these habitats, most food availability is
allochthonous and limited to phytoplankton cells and other particulates
transported onshore (McLachlan and Brown, 2006) and the input of nearshore
macroalgae and seagrasses (macrophytes; Dugan et al., 2003). The
accumulation of patches of stranded macrophytes or “wrack” represents a key
food subsidy for resident invertebrate communities, particularly those living at the
upper levels of the intertidal zone (Griffiths et al., 1983; Inglis, 1989; McLachlan
and Brown, 2006; Lastra et al. 2008). Wrack is expected to influence these
communities, and several authors have reported increased abundances of
90
macrofauna with wrack cover, volume or standing stock (Behbehani and Croker,
1982; Dugan et al., 2003; Ince et al., 2007; Jaramillo et al., 2006; Rodil et al.,
2008; Stenton-Dozey and Griffiths, 1983). Wrack also affects the zonation of
macrofauna. For example, some macrofaunal species such as the talitrid
amphipod Talorchestia martensii closely follow the movements of wrack as its
position on the beach changes during the semi-lunar cycle (Colombini et al.,
2000 ).
Marine invertebrates living in the supralittoral zone live buried in the sand
beyond the intertidal zone, and therefore are minimally affected by intertidal
swash conditions (Koop and Field, 1980; Jaramillo et al., 2006). In temperate
regions, the supralittoral fauna in sandy beaches with moderate macrophyte
input are often dominated by talitrid amphipods (Colombini et al., 2000; Griffiths
and Stenton-Dozey, 1981; Inglis, 1989; J^drzejczak, 2002). These organisms are
considered primary colonizers of newly stranded wrack (Behbehani and Croker,
1982; Colombini et al., 2000; Griffiths and Stenton-Dozey, 1981; Inglis, 1989;
Marsden, 1991; Stenton-Dozey and Griffiths, 1983), which subsequently attract
and sustain secondary (predatory) species (Colombini et al., 2000; Dugan et al.,
2003; Griffiths and Stenton-Dozey, 1981; Ince et al., 2007; J^drzejczak, 2002).
Since macrophytes have different physical and nutritive properties (Lastra et al.,
2008), the colonization of wrack by amphipods is expected to relate to its
composition and the amount of time the patches have been stranded over the
beach. Wrack deposits may undergo severe dehydration as they age or become
covered by windblown sand (Ince et al., 2007; Rodil et al., 2008). Both processes
/
91
contribute to their decomposition (Ince et al., 2007) and remineralization by
bacteria (Kirkman and Kendrick, 1997).
Wrack species composition and biomass change spatially and temporally
(Dugan et al., 2003; Lastra et al., 2008; Marsden, 1991; Orr et al., 2005; StentonDozey and Griffiths, 1983) in response to local hydrological processes (Ince et
al., 2007) or larger-scale processes, such as sea-level change and the dynamics
of shoreline erosion (Lastra et al., 2008). Additionally, species of stranded
seaweeds are not uniformly used by colonizing invertebrates (Rodil et al., 2008).
Therefore, changes in composition of the standing crop of wrack may influence
invertebrate communities. For example, Lastra et al. (2008) found that
amphipods of the genera Megalorchestia and Talitrus rapidly consumed brown
algae of the genera Macrocystis and Saccorhiza, respectively, but only
consumed negligible amounts of the seagrass Phyllospadix. The causes for
these differences are diverse. Seaweeds may vary in physical structure (levels of
branching, toughness), nutritional quality and/or quantity, palatability, and
decomposition rates while stranded on the beach face (Duarte et al., 2010;
Dugan et al., 2003; Rodil et al., 2008; Rossi and Underwood, 2002; StentonDozey and Griffiths, 1983). Few studies so far have attempted to describe the
spatial variation of stranded seaweeds and their individual influence on sandy
beach invertebrates, despite the importance of wrack as food and habitat
Several species of seagrasses and rockweeds dominate the wrack on
eastern North American shorelines and each possess different characteristics
that affect their use by invertebrates. Seagrasses possess structural
92
polysaccharides and a low proportion of available nitrogen, limiting its value as a
food source (Inglis, 1989). Meanwhile, rockweeds are high in nitrogen content
and other soluble substances (Inglis, 1989) but contain secondary metabolites
that may deter herbivory and decrease herbivore assimilation efficiency and
growth (Boettcher and Target, 1993; Denton and Chapman, 1991; Pennings et
al., 2000). Both types of macrophytes are common in the supralittoral wrack on
the sandy beaches on the north shore of Prince Edward Island (PEI), but their
composition and abundance is expected to vary with shoreline type. For
example, sandy beaches in this region are associated with sand dunes, till bluffs
and sandstone cliffs, one of which (sandstone cliffs) erode at a much slower rate
(cf. Forbes and Manson, 2002; Flawkins, 2008, see also Chapter 3). Given that
little is known about the influence of wrack on the upper shore macrofauna of
beaches here and elsewhere, this shoreline system offers a unique opportunity to
address this knowledge gap. The goal of this study was to assess the role of
macrophyte wrack on the abundance of the supralittoral macrofauna using a
combination of exploratory and experimental approaches. Specifically, this study
reports (1) a snapshot survey of the standing crop of wrack in seven
representative sandy beaches and compares macrofaunal numbers in wrack and
bare sediments; (2) a field experiment assessing the colonization on the two
most common species of stranded seaweeds; and (3) a comparison of the
nutritional quality of those two seaweed species and their corresponding rates of
consumption by invertebrates in a laboratory setting.
93
4.4 Materials and Methods
4.4.1 Stranded Macrophyte Survey
%
Seven sandy beaches located on the north shore of Prince Edward Island
were sampled during summer 2010: Brackley (July 8), Ross Lane (July 8),
Dalvay west I (July 12), Dalvay west II (July 12), Cavendish (July 26), Doyles
Cove (July 13) and Cape Turner (July 13; Fig. 4.1). The sandy beaches at
Brackley, Cavendish and Ross Lane are backed by sand dunes, Dalvay west I
and II are backed by till bluffs, and Doyles Cove and Cape Turner are backed by
sandstone cliffs. A table of random numbers was used to select the location of
samples along the driftline associated with the last high tide in order to prevent
bias from unintentional selection of areas with high or low macrophyte
accumulation. At each site, the percent cover of macrophytes was visually
estimated to the nearest five persent using 1 x 4 m quadrats placed along the
driftline. The quadrat was then subdivided in four 1 m2 subunits and the number
of distinct macrophyte patches (ranging from individual fronds to large clumps
consisting of multiple plants) within each subunit was counted. The mean of the
four values were used as an individual replicate, and considered an estimator of
seaweed patch density (n=6 per beach).
A 20 cm diameter PVC core was then used to collect invertebrates from a
randomly chosen seaweed patch within each quadrat (n=6 per beach). The core
was inserted into the patch to a depth of approximately 20 cm in order to collect
both the seaweed at the surface and the sediment underneath. The wrack
sample was carefully examined within the core to collect invertebrates in
94
Gulf of St.
Lawrence
47°N ' “
45°N - -
NW Atlantic
l65?W
6 1 °W
O Sand dune
•S a n d s to n e cliff
• T i l l bluff
kilometers
Figure 4.1. Approximate location of the sandy beaches sampled along the north
shore of Prince Edward Island, southern Gulf of St. Lawrence. 1.Cavendish,
2.Cape Turner, 3.Doyles Cove, 4.Brackley, 5.Ross Lane, 6.Dalvay west II,
7.Dalvay west I and 8.Dalvay east.
between the fronds, and sealed in plastic bags for further analyses in the
laboratory. The sediments were then sieved on site through a 1 mm mesh
screen. The macrofauna retained on the screen were stored in ethanol until
subsequent sorting and identification. An additional core sample was taken
approximately 1 m away from each quadrat (n=6 per beach) in order to
characterize invertebrates associated with bare sediments. In the laboratory,
macrophyte samples were carefully re-inspected for the presence of macrofauna
that may have been missed in the field, and then weighed, dried in an oven at
60°C to a constant mass (48 h) and re-weighed to determine water content.
Macrofauna were identified and counted; the majority of the invertebrates
collected were amphipods belonging to Americorchestia megalophthalma and
Americorchestia longicornis (>95%). Due to the difficulty in differentiating
between the juveniles of these two species, these amphipods were quantified as
Americorchestia sp.
4.4.2 Field experiment: Stranded seaweed colonization
Zostera marina (hereafter eelgrass) and Fucus serratus (hereafter
rockweed) were the most common species of stranded macrophytes (see
results) and were therefore chosen to conduct a short-term wrack colonization
experiment. Freshly detached eelgrass and rockweed were harvested from a
nearby shallow subtidal shoreline, spread outdoors on a uniform surface and left
to dry in direct sunlight for one week to simulate the aging process that takes
place in the upper intertidal. The day prior to the implementation of the
96
experiment, a second batch of fresh eelgrass and rockweed were harvested and
stored overnight. On August 10, 2010 at Dalvay east beach and August 18, 2010
at Brackley beach (Fig. 4.1), bundles of approximately 0.031 m2 of fresh and
dried eelgrass and rockweed were prepared and deployed to simulate natural
patches of stranded seaweeds consisting of fresh and dried eelgrass and fresh
and dried rockweed. Four treatments were randomly assigned among the
patches. The bundles (patches) were spaced 2-3 m apart along the drift line of
both beaches. Metal hooks approximately 25 cm in length were pushed into the
sediment to anchor these patches to the shore, and other naturally stranded
macrophytes were carefully removed to avoid interference with the experimental
patches. A total of five and seven replicates per treatment were deployed at
Dalvay east and Brackley, respectively.
After six days of deployment, 20 cm diameter cores were used to collect
patches and sediments underneath using the procedure described above. The
seaweed patches were placed in sealed plastic bags, taken to the laboratory,
weighed, dried at 60°C in an oven and re-weighed to determine water content.
Due to the loss of some samples due to wave action, the number of replicates
per treatment at the end of the experiments ranged from 4-5 at Dalvay east and
3-7 at Brackley. At each site, 10 additional core samples were taken from bare
sediments located approximately 1 m away from the artificial patches, and
processed using the protocol described above.
97
4.4.3 Plant tissue nutrients and amphipod feeding rates
Macrophytes were collected from the driftline of two sandy beaches,
Dalvay east and Brackley, for processing to determine if there were differences in
the nutritional quality of eelgrass and rockweed. Approximately 15 g samples of
each species (n=3 per species per beach) were randomly collected to estimate
the concentration of proteins, lipids and carbohydrates in their tissues. These
samples were stored at -80°C, dried at 60°C until a constant mass (~16 h) and
ground to a fine powder with a mortar and pestle in liquid nitrogen. Protein
content was estimated using a method adapted from Wong and Cheung (2001).
The powdered material for each sample was mixed with a solution of 100 mM
Tris HCI pH 8.0 and 100 mM NaOH and agitated with a wrist-action shaker for
two hours at 4°C before being centrifuged at 10,000xg and 4°C for 20 minutes to
obtain the supernatant. Then 100 mM NaOH and 0.5% 2-mercaptoethanol was
added to the pellet and the samples were agitated with a wrist action shaker for
one hour at 4°C and again centrifuged at 10,000xg and 4°C for 20 minutes. The
supernatants were combined and the protein was precipitated from the solution
by adding an equal volume of acetone containing 0.07% DTT. The samples were
then mixed by vortexing and incubated at -20°C overnight. Afterwards, the
samples were centrifuged at 10,000xg and 4°C for 5 minutes, and the
supernatant discarded. The pellet was dried under a nitrogen stream and the
protein was then dissolved in a solution of 100 mM Tris pH 8.8, 100 mM NaCI
and 0.2% SDS. The protein concentration was determined using the
bicinchoninic acid method (BCA) from Pierce (BCA Protein Assay Kit) using
bovine albumin serum as a standard and measuring absorbance at 562 nm. Lipid
content was estimated from a separate sample of powdered material using an
ASE® 150 Accelerated Solvent Extractor. The lipids were extracted from the
macrophyte powder with hexane 190 and the extracted solution was then
concentrated using a rotary evaporator. The lipid content was then determined
gravimetrically. Finally, carbohydrates were extracted from the macrophyte
powder following Sadasivam and Manickam (1996) and the carbohydrate content
was determined using D-(+)-glucose as a standard and measuring absorbance at
490 nm (Dubois et al., 1956).
Amphipod feeding rates were estimated in the laboratory following the
methodology described by Duarte et al. (2010). Approximately 2-3 g of fresh
macrophytes (eelgrass or rockweed, separately) were weighed and placed in
two-compartment plastic containers (12 x 20 x 5 cm) with a modified top (the top
had a large window covered with a 1 mm mesh to allow gas exchange while
preventing amphipod escape). Each of these containers was then placed within
larger 50 x 70 x 15 cm plastic containers with 2-3 cm of wet sand at the bottom to
keep relatively constant humidity levels. At the beginning of the experiment, five
adult-size amphipods, starved for 48 h to standardize hunger levels, were placed
within one of the compartments of each plastic container and kept there for 24 h.
At the end of that period, the amphipods from each container were carefully
removed and the macrophytes re-weighed to estimate amount of tissue
consumed per amphipod and per day (n=10 per species). The macrophytes in
99
the second compartment were also weighed in order to calibrate and control for
weight loss due to desiccation (Duarte et al., 2010).
4.4.4 Statistical analyses
Data from the survey characterizing wrack and macrofauna, seaweed
percent cover, patch density and water content were compared among sites
(beaches) using one-way Analysis of Variance (ANOVA). Percent cover and
patch density were square-root transformed in order to meet assumptions of
normality and homoscedasticity. When significant differences were found,
pairwise multiple comparisons were made using the Holm-Sidak method.
Comparisons of macrofauna abundance between wrack samples and bare
sediments were done on fourth-root transformed data using a two-way ANOVA
that assessed the influence of site (seven sandy beaches) and cover (wrack
versus bare sediments). A significant site x cover interaction was found (p=0.007;
i.e. these results cannot be properly interpreted because invertebrate abundance
did not vary consistently between the two treatments across different sites), so ttests were used to compare macrofauna abundance between macrophyte wrack
and bare sediment on a site by site basis using log-io (x + 1) transformed data
when necessary to meet assumptions of normality and homoscedasticity.
Macrofauna abundances from the field experiment were logio (x + 1)
transformed and compared using a two-way ANOVA with species (eelgrass
versus rockweed) and age (fresh versus aged) as factors. Additionally,
abundances from wrack samples were compared with bare sediments using a
100
one-way ANOVA. Linear regression analyses between macrofaunal abundance
and wrack patches’ wet mass, dry mass and water content were performed in
Sigmaplot 11.0 (build 11.2.0.5). Macrofauna abundances as well as physical
variables were square-root transformed when necessary in order to meet
assumptions of normality and homoscedasticity.
The concentrations of proteins, lipids and carbohydrates measured in
eelgrass and rockweed tissues were compared using a two-way ANOVA with site
(Brackley versus Dalvay east) and species (eelgrass versus rockweedj as
factors. Protein and lipid contents were logio transformed to meet normality and
equal variance requirements. Differences in amphipod feeding rates upon
eelgrass and rockweed were assessed using t-tests.
4.5 Results
4.5.1 Stranded Macrophyte Survey
The macrophyte wrack was composed primarily of eelgrass (Zostera
marina) and a species of rockweed (Fucus serratus). Other species that were
less frequently collected (<5%) included Irish moss (Chondrus crispus), sea
lettuce (Ulva lactuca) and one or more species of the genus Laminaria. Mean
wrack cover ranged between 0 and 23.8% and was slightly higher (one-way
ANOVA; p=0.062; Table 4.1) at the two beaches associated with sandstone cliffs
(Doyles Cove and Cape Turner; Fig. 4.2). Mean patch density ranged from 0.25
to 15.5 patches per m2 and mean water content ranged between 6.9 and 90.6%
(Fig. 4.2) and in both cases, the differences among sandy beaches were
101
Table 4.1. Results of one-way ANOVAs comparing wrack features (patch
density, cover and water content) among the seven beaches surveyed.
One-way ANOVAs comparing the density of macrofauna in wrack versus
bare sediments at each of these beaches are also presented (Doyles Cove
and Cape Turner were not compared statistically). DF: degrees of freedom,
MS: Mean Squares. For simplicity, Sum of Squares and F-values have
been omitted.
Comparison
Wrack features
Wrack vs bare
sands
Dependent
variable
Patch density
Source of
Variation
Between Groups
Error
Patch cover
Water content
DF
MS
P
6
35
5.189
1.105
0.001
Between Groups
Error
6
35
2.223
0.993
0.062
Between Groups
Error
6
35
3171.6
200.2
<0.001
1.808
0.123
0.003
1.622
0.236
0.026
0.01
Density Brackley
1
Between Groups
Error
Density Ross
Lane
Density
Cavendish
10
1
Between Groups
Error
10
1
Between Groups
Error
10
0.807
0.081
Density Dalvay I
Between Groups
Error
1
10
0.172
0.088
0.191
Density Dalvay II
Between Groups
Error
1
10
0.083
0.617
0.721
102
Sand Dunes
Till Bluffs
Sandstone
Cliffs
■
■I
b
U)
c
a>
ab
ab
TD
■■I
sz
o
-I—•
(0
Q.
a
a
r 3” ! P n
i
i
a
a
. . . ■I
(D
>
o
o
sz
o
(0
a
T
CL
T
I
I
i
r
60
BRA ROL CAV
DA-I DA-II
DOC CAT
Figure 4.2. Mean density, cover and water content of wrack from seven sandy
beaches on the north shore of PEI. Bar filling relates to type of shoreline:
black - sand dunes, light grey - till bluffs and dark grey - sandstone cliffs.
BRA: Brackley; ROL: Ross Lane, CAV: Cavendish; DA-I: Dalvay west I; DAII: Dalvay west II; DOC: Doyles Cove, CAT: Cape Turner. Error bars
represent one standard error. Identical letters indicate no significant
differences among coast types.
103
significant (one-way ANOVA; p=0.001 and p<0.001 respectively; Table 4.1). In
both cases, the highest values were measured at beaches associated with
sandstone cliffs; Cape Turner and Doyles Cove.
The macrofauna was numerically dominated by the amphipod
Americorchestia sp. (97.6%). Only three other species, two amphipods Calliopius
laeviusculus (0.6%), Haustorius canadensis (1.5%) and one polychaete
Scolelepis squamata (0.3%), made up the remainder of invertebrate abundance.
In general, macrofauna abundances were higher in samples associated with
stranded macrophyte wrack than bare sediments. At the three sandy beaches
associated with dunes, Brackley, Ross Lane and Cavendish, the differences
between wrack-covered and bare sediments were significant (t-test; p=0.003,
p=0.026 and p=0.010, respectively; Table 4.1, Fig. 4.3). At the two sandy
beaches associated with till bluffs, Dalvay west I and II, the differences between
wrack and bare sediments were not significant (t-test; p=0.191 and p=0.721
respectively; Table 4.1, Fig. 4.3). In sandy beaches associated with sandstone
cliffs, macrofauna were collected exclusively from samples associated with
wrack. Due to the absence of fauna in bare sediments, no statistical tests were
applied for Doyles Cove and Cape Turner.
4.5.2 Wrack colonization experiments
At Dalvay east, the mean macrofauna abundance in fresh and dried
rockweed patches was 10.2 and 10.0 individuals per core, respectively. In
comparison, mean abundance in fresh and dried eelgrass patches only reached
104
Sand dunes
Till Bliffs
Sandstone cliffs
Bare sediments
DA-I
CAV
CAT
Figure 4.3. Mean abundance of macrofauna in wrack versus bare sediments.
BRA: Brackley; ROL: Ross Lane, CAV: Cavendish; DA-I: Dalvay west I; DAII: Dalvay west II; DOC: Doyles Cove, CAT: Cape Turner. Identical letters
indicate no significant differences between cover. Error bars represent one
standard error. No statistical tests were conducted for Doyles Cove and
Cape Turner due to the lack of fauna in the bare sediments.
105
6.2 and 2.75 individuals per core. Differences among treatments were significant
between seaweed species (two-way ANOVA; p=0.012), but not between ages
(two-way ANOVA; p=0.784). There was no significant interaction between
species and age (two-way ANOVA; p=0.237; Table 4.2, Fig. 4.4) indicating that
the abundance patterns varied consistently between species of different ages.
The majority of the macrofauna were talitrid amphipods, with A.
megalophthalma and A. longicornis making up 86% and 4% of the abundance,
respectively. Insects of the beetle family Histeridae accounted for 2.7%, while
beetles in the Hydrophilidae and Curlionidae, and Formicidae (ants) and Diptera
(flies) each had a lone representative (0.67% each). Combined, unidentified
insect larvae accounted for 4.7% of the total abundance. Control samples
collected from bare sediments had a mean abundance of 0.6 individuals per core
(Fig. 4.4).
Similar results were gathered from the experiment conducted at Brackley
beach. The mean macrofauna abundance in fresh and dried rockweed patches
was 7.3 and 5.1 individuals per core respectively, compared to 2.5 and 1.8
individuals per core, respectively in fresh and dried eelgrass. Americorchestia
megalophthalma comprised 53.1% of the total abundance, followed by the beetle
families Histeridae (35.8%), Hydrophilidae (9.9%) and Curculionidae (1.2%).
Control samples, collected from bare sediments had a mean abundance of 0.4
individuals per core (Fig. 4.4). There were significant differences in abundance
between macrophyte species (two-way ANOVA; p=0.013), but not between ages
(two-way ANOVA; p=0.245). There was no significant interaction between these
106
Table 4.2. Results of two-way ANOVAs comparing field colonization rates
(number of invertebrates) in patches of wrack placed at Dalvay east and
Brackley beaches. Wrack species and state refer to seaweed (rockweed vs
eelgrass) and age (fresh vs dried), respectively. DF: degrees of freedom,
MS: Mean Squares. For simplicity, Sum of Squares and F-values have
been omitted.
Comparison
W rack colonization
Dependent
variable
Density Dalvay
Source of
Variation
W rack species
W rack state
Species x state
Error
Density Brackley
W rack species
W rack state
Species x state
Error
DF
MS
P
1
1
1
14
0.651
0.00616
0.121
0.0791
0.012
0.784
0.237
1
1
1
16
0.591
0.112
0.0278
0.0766
0.013
0.245
0.556
107
Rockweed
Eelgrass
Dalvay
a>
o
o
<n
(0
3
~o
>
TJ
JZ
0
O
c
CD
TJ
C
3
Brackley
jQ
<
C
CD
0
Fresh Dried
Fresh Dried
Bare sedim ents
Figure 4.4. Mean abundance of macrofauna in fresh and dried rockweed and
eelgrass patches placed on Dalvay east and Brackley beaches. Error bars
represent one standard error. Identical letters indicate no significant
differences among treatment. The mean abundance for the control samples
(bare sediments) are also presented but were not included in the statistical
analyses.
108
two factors (two-way ANOVA; p=0.556; Table 4.2, Fig. 4.4) indicating that the
abundance patterns varied consistently between species of different ages.
Figure 4.5 shows the relationship between macrofauna abundance and
wet seaweed mass, dry seaweed mass, and water content for all the
experimental patches combined (across treatments). At Dalvay east, there were
significant positive relationships between macrofauna abundance and each of
the seaweed parameters measured: wet mass (p<0.001, 1^=0.556), dry mass
(p<0.001, 1^=0.550), and water content (p=0.029, 1^=0.265; Fig. 4.5). Similar
relationships between macrofauna abundance and seaweed features were
obtained at Brackley: wet mass (p<0.001,1^=0.506), dry mass (p=0.015
1^=0.231), and water content (p<0.001, ^=0.62; Fig. 4.5).
4.5.3 Nutritional Quality Analysis and amphipod feeding rates
Similar trends were observed in the indicators of nutritional quality
investigated and amphipod feeding rates. The mean amount of proteins
measured for the rockweed was 20.6% and 10.43% dry weight at Brackley and
Dalvay east, respectively. For eelgrass, the mean amount of proteins measured
was 0.58% and 0.31% dry weight at Brackley and Dalvay east, respectively.
There were significant differences between species (two-way ANOVA; p<0.001)
but not between sites (two-way ANOVA; p=0.089). There was no significant
interaction between site and species (two-way ANOVA; p=0.574; Table 4.3, Fig.
4.6) indicating that the concentration of protein varied consistently between
species at both sites. The mean amount of lipids measured for the rockweed was
109
D a lva v E ast
B ra ckle y
y = 0 .6 7 9 + 0 .0 2 9 6 X
0 .5 5 6
y = 0 .7 0 8 + 0 .0 5 3 6 X
r2 = 0 .5 0 6
p<0.001
p<0.001
ro w
20
40
60
80
0
100 120 140
20 40
60 80 100 120 140 160 180 200
W e t m a s s (g)
W e t m ass (g)
b
Sqrt(Abundance)
(Individuals / Core)
6
y = 0 .6 6 2 + 0 .0 3 4 1 X
5 r2 = 0 .5 5 0
4
If
p < 0 .0 0 1
4
as ^
3
T3
C W
3 « 2
3
2
•
■C >
O" TJ
0
20
40
60
80
100
p = 0 .0 1 5
—■
— •
——----,—
•
•
•
•
•
• _-—--- **" ••
1 —
CO C 0
'—
1
y = 0 .2 3 1 + 0 .2 5 6 x
r2 = 0 .2 8 6
••
4
Dry m ass (g)
5
6
7
8
9
10
S q rt( D ry m a s s (g))
6
5
4
y = 1 .7 8 7 + 0 .1 0 1 x
r2 = 0 .2 6 5
• ‘
p = 0 .0 2 9
•
3
c
•
ro w
—
"2
\y = 1 .8 4 9 + 0 .0 9 9 2 X
r2 = 0 .6 2 0
p<0.001
03
T 3
2
•
1
^
2!
tm
•
•
c
0
5
10
15
20
W a te r c o n te n t (g)
25
0
20
40
60
80
100 120
W a te r c o n te n t (g)
Figure 4.5. Results of the regression analyses between physical characteristics
of the wrack and macrofauna abundance across experimentally
manipulated wrack patches.
r
110
Table 4.3. Results of two-way ANOVAs comparing indicators of nutritional value
in stranded seaweeds. Site and wrack species refer to location (Dalvay east
vs Brackley) and macrophyte species (rockweed vs eelgrass), respectively.
The results of a t-test comparing amphipod feeding rates upon the same
wrack species is also presented. DF: degrees of freedom, MS: Mean
Squares. For simplicity, Sum of Squares and F-values have been omitted.
Comparison
Rockweed vs
eelgrass
Rockweed vs
eelgrass
Dependent
variable
Protein
Source of
Variation
DF
MS
P
Site
Wrack species
Site x species
Error
1
1
1
8
0.283
6.778
0.0259
0.0754
0.089
<0.001
0.574
Lipids
Site
Wrack species
Site x species
Error
1
1
1
8
0.0698
2.029
0.0116
0.0236
0.124
<0.001
0.504
Carbohydrates
Site
Wrack species
Site x species
Error
1
1
1
8
1.69
317.7
2.566
4.088
0.538
<0.001
0.451
0.00327
0.00063
0.035
Feeding rates
1
Wrack species
Error
18
Ill
30
£
25
Rockweed
20
Eelgrass
ai
P 15
10
i
5
0
4 </) o
TJ 3
1
-
25
co 2 0
©
4-«
CO
* 15 1
_c
o
n> 10
O
_Q
5-
IP
Brackley
Dalvay
Figure 4.6. Mean percentage of dry weight for proteins, lipids and carbohydrates
present in the tissues of rockweeds and eelgrass from samples collected in
Dalvay east and Brackely beach. Identical letters indicate no significant
differences among treatments. Error bars represent one standard error.
112
3.03% and 3.77% dry weight at Brackley and Dalvay east respectively, compared
to only 0.44% and 0.66% dry weight for eelgrass at Brackley and Dalvay east,
respectively. There were significant differences between species (two-way
ANOVA; p<0.001) but not between sites (two-way ANOVA; p=0.124). There was
no significant interaction between site and species (two-way ANOVA; p=0.504;
Table 4.3, Fig. 4.6) indicating that the concentration of lipids varied consistently
between species at both sites. The mean amount of carbohydrates measured for
the rockweed was 15.7% and 17.4% dry weight at Brackley and Dalvay east
respectively compared to 6.31% and 6.14% dry weight at Brackley and Dalvay
east, respectively. There were significant differences between species (two-way
ANOVA; p<0.001) but not between sites (two-way ANOVA; p=0.538). There was
no significant interaction between site and species (two-way ANOVA; p=0.451;
Table 4.3, Fig. 4.6) indicating that the concentration of carbohydrates varied
consistently between species at both sites. Amphipod feeding rates mirrored the
nutritional patterns observed for the macrophyte species: Amphipods consumed
on average 13.39 mg rockweed/amphipod/day, and 8.27 mg eelgrass/amphipod/
day. There was a significant difference in feeding rates between macrophytes (ttest; p=0.035; Table 4.3, Fig. 4.7).
4.6. Discussion
4.6.1 Macrophyte survey and spatial variation
The re-suspension and re-deposition of wrack is a typical feature of
dynamic systems like sandy beaches (Kirkman and Kendrick, 1997; Ochieng and
113
20
Feeding Rate
CD
Rockweed
Eelgrass
Figure 4.7. Mean feeding rates by amphipods in laboratory conditions collected
at Dalvay east and Brackely beach. Identical letters indicate no significant
differences among treatments. Error bars represent one standard error.
Erftemeijer, 1999). The input of wrack is highly variable and depends on factors
such as tides and wave exposure (Orr et al., 2005; Ince et al., 2007), wind
(Colombini and Chelazzi, 2003), storms uprooting or breaking algal holdfasts
(Griffiths and Stenton-Dozey, 1981; McLachlan and Brown, 2006; Milligan and
DeWreede, 2000; Tolley and Christian, 1999), erosion levels (Lastra et al., 2008),
and even beach substratum characteristics (Orr et al., 2005). Likely, the same
processes operate on the north shore of Prince Edward Island. In this system,
spatial differences in wave exposure and shoreline erosion are likely to create
differences in the amount of stranded seaweed that is being retained above the
intertidal area. A parallel study found that Doyles Cove and Cape Turner, both
sites associated with sandstone cliffs, were considerably less exposed to erosion
than beaches associated with sand dunes and till bluffs (Chapter 3, this thesis),
and these sites had the highest amounts of wrack. Lower erosion and sand
abrasion slow down decomposition and increase wrack residency time
(Colombini and Chelazzi, 2003), and may explain the relatively high density,
cover and water content measured in the wrack of Doyles Cove and Cape
Turner.
Surprisingly, however, the invertebrates did not show the same pattern
among beach types as the quantity and cover of wrack. There was considerable
variation in the level of utilization of the wrack by the macrofauna. Similar to
studies elsewhere (e.g. Behbehani and Croker, 1982; Dugan et al., 2003; Ince et
al., 2007; Jaramillo et al., 2006; Rodil et al., 2008; Stenton-Dozey and Griffiths,
1983), the number of invertebrates colonizing wrack on dune-associated sandy
beaches was consistently and significantly greater than samples from bare
sediments. Interestingly, this was not the case in sandy beaches associated with
till bluffs, which are fairly similar in terms of physical features and macrofaunal
communities (Chapter 3, this thesis). Therefore the pattern of enhanced
communities underneath wrack patches is not as general as the published
literature suggests. At Doyles Cove and Cape Turner, two sites associated with
sandstone cliffs experiencing low rates of erosion (Chapter 3, this thesis),
invertebrates were scarce in bare sand and restricted to sediments associated
with wrack. Although statistics could not be applied, this further demonstrates the
importance of wrack to sandy beach communities. Since communities
associated with these sites are less diverse and abundant than sites
experiencing higher rates of erosion (Chapter 3, this thesis), local availability of
invertebrates for colonization of wrack may be as important as physical variables
to explain spatial differences in wrack utilization.
As a consequence of harboring increased macrofaunal abundances,
wrack is also important for vertebrate predators. Dugan et al. (2003) found the
mean abundance of two species of shorebirds, the Black-bellied Plover (Pluvialis
squatarola) and the Western Snowy Plover (Charadrius alexandrinus nivosus)
were positively correlated with stranding crop of macrophyte wrack and the
abundance of wrack-associated macrofauna. In the Atlantic Maritimes, it has
been demonstrated that an endangered species, Piping Plover (Charadrius
melodus), exploit wrack stranded on sandy beaches (Majka and Shaffer, 2008).
These authors found representatives of five families of herbivorous and
116
predaceous Coleoptera were consumed by Piping Plover, and other authors
have reported amphipods and flies (Shaffer and Laporte, 1994) as well as
polychaetes (Staine and Burger, 1994) in plover diets.
4.6.2 Wrack colonization experiments
Upper shore invertebrates rely on wrack as a food source or refuge
against harsh physical conditions (Jaramillo et al., 2006; Olabarria et al., 2007),
however, predicting the feeding preferences or traits that influence preferences of
marine herbivores is difficult (Pennings et al., 2000). The results of the
colonization experiments between the two most common species of stranded
macrophytes show that rockweeds were colonized by the highest densities of
invertebrates, regardless of age and location. Research elsewhere has reported
that the age of wrack does affect colonization, showing strong associations
between macrofauna and fresh wrack (Jaramillo et al., 2006; Marsden, 1991),
while also reporting only weak associations with aged wrack. The lack of a
relationship between macrofauna abundance with the age of wrack in this study
could be due to the breakdown rates of the two dominant macrophyte species.
For example, Buchsbaum et al. (1991) found that detritus of a related rockweed,
Fucus vesiculosis, typically loses its mass at a much lower rate than other
macrophytes. If F. serratus also exhibits slow breakdown rates, it could help
explain why this species was consistently preferred by the macrofauna of this
beach system since a high quality food resource would be available for a longer
period. In contrast, the colonization of eelgrass was considerably more variable
117
between locations and wrack states. Eelgrass tissues are in general less nutritive
than brown algae tissues (Inglis, 1989; this study) and decompose faster. Both
aspects likely made eelgrass wrack considerably less valuable as a source of
food for macrofauna.
Another factor affecting the colonization of wrack by invertebrates is the
physical structure of the habitat. Stranded macrophytes can differ in shape and
level of compactness (Colombini et al., 2000), and these differences can
translate into distinct microclimates for the macrofauna (Marsden, 1991;
Olabarria et al., 2007). Due to its physical structure and humidity retention,
patches of rockweed likely provide a more hospitable environment than eelgrass
patches. This is consistent with the higher macrofaunal abundances detected in
rockweed in comparison to eelgrass patches of about the same size, though not
consistent with the pattern for lowest abundance on the sandstone cliff shorelines
that had the highest water content. Desiccation of macrophyte tissues not only
affects their water content, it also accelerates the release of cell contents by
leaching. This release contributes to the decomposition of the plant and its loss
of organic matter (Newell et al., 1986; Ochieng and Erftemeijer, 1999; Rodil et
al., 2008), ultimately altering the nutritive value of the wrack. Wrack patch size
and water retention are closely related, and given their importance (Olabarria et
al., 2007), a series of regressions explored the relationships between these
variables and macrofaunal abundance. In spite the rather narrow range of
variation in the wrack properties, this study found significant positive relationships
between macrofaunal abundances and patch’s water content, dry and wet
118
weight. These results suggest that physical features do play a role in the
utilization of wrack by the macrofauna (Inglis, 1989), though factors such as
availability of colonizing invertebrates can also be a factor. Virtually no insects
were found in the survey of naturally stranded wrack at the most recent high tide
levels. However, in the colonization of artificially stranded wrack patches at
Dalvay east and Brackley, insects composed 10 and 47% of the total macrofauna
respectively. This suggests that naturally stranded macrophytes on the north
shore of PEI do not have long enough residence times for the development of
Diptera larvae which are consumed by predaceous Coleoptera (Colombini and
Chelazzi, 2003), and to dry to a level suitable for herbivorous Coleoptera
(Colombini et al., 2000). However, this is likely not the case for wrack stranded
further up the shore during the highest high tides.
4.6.3 Nutritional Quality and amphipod feeding rates
This study also explored whether the differences in colonization rates
between rockweed and eelgrass patches may have been related to their nutritive
values. Rockweed tissues exhibited significantly greater quantities of proteins,
lipids and carbohydrates than eelgrass tissues. Such strong differences provide
evidence to suggest that colonization rates may be driven primarily (but not
exclusively) by nutritional differences between these two macrophytes.
Preference for rockweed has been widely reported, and to our knowledge, only
one study so far (Pennings et al., 2000) has found a complete lack of preference
for species of Fucus sp. based on a study of two fairly unrelated upper level
119
invertebrates, the amphipod Traskorchestia traskiana and the isopod Ligia
pallasii. All the other existing evidence is consistent with the results of this study
and indicates preferential use of brown algae, including Fucus sp. by talitrid
amphipods. For example, Adin and Riera (2003) found that rockweeds of the
genus Fucus (primarily F. serratus) were preferentially used as a food source by
Talitrus saltator, while the use of eelgrass was negligible or null. Similarly, Lastra
et al. (2008) found that brown macroalgae of the genera Macrocystis and
Saccorhiza were heavily consumed by amphipods of the genera Megalorchestia
and Talitrus, respectively. Other macrophytes offered to these invertebrates,
including a seagrass of the genus Phyllospadix, were barely consumed (Lastra et
al., 2008).
Optimal foraging theory suggests that, in general, higher quality foods
enhance fitness and should be selectively eaten when available (Cruz-Rivera
and Hay, 2000; Stephens and Krebs, 1986; Wakefield and Murray, 1998). This
appears to be the case for the supralittoral macrofauna on the north shore of PEI.
The results of an experiment comparing feeding rates on standard amounts of
rockweed and eelgrass were also consistent with the nutritional value of their
tissues: amphipods consumed significantly more (>50%) rockweed than eelgrass
tissues. Although all consumers must choose among prey of different nutritional
value, these choices are particularly critical for herbivores and detritivores: they
rely on food sources that are critically low in protein content compared to what
they require to produce animal biomass (Cruz-Rivera and Hay, 2000; Mattson,
1980). Understanding the relationship between nutrition and the feeding
120
preferences of marine herbivores can be challenging (Pennings et al., 2000)
since the nutritive value of macrophytes is linked to a wide range of factors,
including but not limited to, seaweed’s level of decomposition, blade toughness,
and palatability (Pennings and Paul, 1992; Pennings et al., 1998). Although this
study did not address preference directly; focusing on measurements of
consumption levels (Duarte et al., 2010), and did not address potential
differences in nutritional quality among the structures within a plant (Duarte et al.,
2011), the differences in nutrition and feeding rates between rockweed and
eelgrass provide a convincing explanation for the differences in colonization rates
observed in the field. However, understanding the contribution of food in the
context of among-site spatial variation, and its synergy with physical variability
requires further exploratory and experimental studies.
4.7 Acknowledgements
Thanks to Christina Pater, Megan Tesch, Veronique Dufour, Tyler Wheeler and
Cassandra Mellish for their assistance in the field, Dr. Bourlaye Fofana, Dr.
Kaushik Ghose, David Main and Guru Selvaraj for their assistance in determining
nutritional values, and Kyle Knysh for his assistance during invertebrate
identification. I would also like to thank Dr. Pedro Quijon, Dr. Donna Giberson
and Dr. Darren Bardati for reviewing this chapter. Personnel of Parks Canada
granted access and collaborated during site selection and preliminary sampling
in the PEI National Park. This research was supported by a grant from
121
Environment Canada through UPEI’s Climate Change Research Program.
Additional support came from a NSERC Discovery grant to P.A. Quijon.
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CHAPTER 5:
SUMMARY OF RESULTS AND FUTURE RESEARCH
127
Sandy beach macrofauna play an important role in coastal food webs.
Primary consumers including suspension feeders utilize phytoplankton and
associated particulate organic matter, and herbivores consume drift macrophytes
and other stranded material. Secondary consumers such as crabs and some
beetles prey upon consumers of both types of primary consumers, and
eventually fall prey to vertebrate predators such as shorebirds and fish (Dugan et
al., 2003). Changes in the availability and input of phytoplankton or macrophyte
wrack could shift energy flow to consumers and therefore prey availability to
higher trophic levels (Dugan et al., 2003).
5.1. Spatial variation and coastal erosion
On oceanic sandy beaches, the diversity and abundance of macrofauna
changes predictably with the variation in beach morphodynamics, typically
increasing in numbers and diversity from reflective to dissipative states (de la
Huzand Lastra, 2008; Lercari and Defeo, 2006; McLachlan, 1990; McLachlan
and Dorvlo, 2005; McLachlan et al., 1981). The first part of this thesis shows that
this is not the case for the study area. Surveys of the intertidal zone of 14 sandy
beaches yielded 7,961 individuals belonging to 13 different species, but their
diversity and abundance did not reflect a reflective-dissipative pattern of
variation: No significant regressions were found between biological descriptors
and the physical characteristics frequently used to describe the physical
conditions on sandy beaches such as slope and grain size (McLachlan and
128
Dorvlo, 2005), though the beaches in this study did not cover the full range of
beach types.
However, the macrofauna communities did change spatially among
shoreline types; beaches associated with sandstone cliffs exhibited communities
consisting of the talitrid amphipod Platorchestia platensis and relatively low
densities of Scolelepis squamata. In contrast, beaches associated with sand
dunes and till bluffs exhibited communities consisting of the talitrid
Americorchestia megalophthalma or the haustoriid amphipod Haustorius
canadensis, and much higher densities of Scolelepis squamata. These
differences seemed related, among other factors, to the distinctive levels of
coastal erosion experienced by each of these shoreline types. Significant positive
correlations were found between the rate of coastal erosion and macrofauna
species richness and abundance. The first part of this thesis concludes that the
rate of erosion is indicative of exposure to wave action and therefore food
availability for a fauna composed primarily of suspension.-feeding organisms.
Prince Edward Island (PEI) is embedded within the Gulf of St. Lawrence
estuary system, and although the system is exposed to wave generating fetches
of hundreds of kilometers (Shaw et al., 1998), it does not necessarily behave like
oceanic sandy beaches do. Unfortunately, relatively little research has been
conducted on these habitats in the Canadian Maritimes so it is impossible to
predict if other sandy beaches associated with large estuarine systems will follow
the same patterns (e.g. Lercari and Defeo, 2006). To the author’s knowledge, the
129
present study represents the first known survey of the sandy beach macrofauna
communities on the north shore of PEI and in the Maritimes region, providing a
baseline reference for future studies.
With the exception of erosion levels, it is still unclear what factors structure
sandy beach communities on PEI and elsewhere in the region. The precise role
of shoreline type, for example, is both unknown and intriguing. Shoreline type is
generally not considered an important component in sandy beach studies, so it is
rarely reported. The interpretation offered in this thesis links shoreline types and
erosion levels with food availability for suspension feeders. In the absence of
clear relationships between macrofauna and the other physical variables studied
(grain size, slope and the BDI), more studies at other sandy beaches with more
diverse communities (dominated or not by suspension feeders) will be very
useful. These studies will clarify whether erosion and food availability are the
main structural factors, as suggested here, or whether there are other causal
factors that were not accounted for.
5.2. Spatial variation and allochthonous wrack input
If erosion levels and food availability for suspension feeders play a role in
the structure of intertidal communities, they are unlikely to be relevant for
supralittoral macrofaunal organisms. The second part of this thesis shows that
the differences between number of organisms associated with wrack and bare
sediments were prominent in sandy beaches associated with sand dunes, but
130
irrelevant for similar sandy beaches associate with till bluffs. The scarce literature
available suggests that supralittoral zones associated with dune formations
represent a more gradual transition from terrestrial to marine systems, allowing
for migration and intermixing of fauna associated with more terrestrial habitats
(particularly insects; McLachlan and Brown, 2006). In contrast, supralittoral
zones associated with till bluffs and sandstone cliffs represent an abrupt change
between terrestrial and marine systems, with little potential for immigration of
terrestrial organisms.
Sandy beaches around the world are devoid of attached plants (Jaramillo
et al., 2006), so the input and accumulation of wrack at or behind the high tide
level is expected to play a more substantive role than erosion or other physical
variables for supralittoral organisms living there. Seven sandy beaches
associated with the three predominant shoreline types previously described were
surveyed and spatial differences recorded. Wrack stranded on beaches
associated with sandstone cliffs seemed to accumulate in larger volumes.
However, at beaches associated with dunes, the macrofauna showed a more
clear reliance on sediments associated with stranded macrophytes.
Rockweeds and eelgrasses are common macrophytes of the waters of the
North Atlantic (e.g. Adin and Riera, 2003; Behbehani and Croker, 1982;
Buchsbaum et al., 1991; Robertson and Mann, 1980). Not surprisingly, the two
most abundant species of macrophytes making up the wrack on the north shore
of PEI during this study were rockweed (Fucus serratus) and eelgrass (Zostera
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marina). Unfortunately, the lack of information from similar surveys in other areas
of the region makes it impossible to determine if these species are the dominant
component of the wrack at larger spatial scales. The results of this study also
indicate that these macrophytes were not utilized equally by the macrofauna.
Experimental manipulation of wrack patches indicated that macrofaunal
organisms were associated preferentially with rockweed, regardless of whether
the patches were fresh or dried/aged. Significant correlations were found
between macrofauna abundance and the main physical features of these
patches. Further evidence on the nutritional value of the two wrack species,
revealed that the nutritive value of their tissues may explain the differences in
rates of colonization. This was subsequently supported by the significantly higher
consumption rates of this species’ tissues by talitrid amphipods in laboratory
feeding trials.
5.3. Future Research
Sandy beaches harbour distinct macrofauna communities that are not
found in any other habitat. It is clear that shoreline type plays a role in the
structuring of macrofauna communities in sandy beaches on the north shore of
PEI, but not necessarily in a clear and predictable way. Future research efforts
should take advantage of the baseline information provided in this thesis but
should also be well aware of its limitations in scope and depth.
For understanding large-scale patterns, intensive long-term sampling in a
few areas would be meaningless unless it is complemented with snapshot
surveys covering a wide range of conditions (McLachlan and Dorvlo, 2005; Figs.
3.1, 4.1). Snapshot surveys were the methodology adopted for this study: a
number of sites with varying degrees of erosion and shoreline types were
sampled. It is well known that sandy beach macrofauna exhibit seasonality;
although the sampling conducted here was meant to be representative of
summer conditions, it is recommended that future studies on PEI sandy beaches
focus on temporal scales. Since the beaches on the north shore of PEI undergo
seasonal erosion-accretion cycles, it would be interesting to investigate the short
and long-term responses of macrofauna communities to winter erosion.
The wrack characteristics reported in this study are representative of the
summer standing stock at the time of sampling, and are not necessarily
representative of long-term trends. It is recommended that seasonal inputs of
macrophyte wrack be estimated and investigated, preferably on beaches
associated with sand dunes. These sandy beaches harboured the highest
numbers of wrack-associated macrofauna, and so are the sites where the
macrofauna makes a more intensive use of these ephemeral resources.
Additionally, although rockweed and eelgrass are the most common components
of the wrack, other species present in lower frequency (e.g. Irish moss, Chondrus
crispus; sea lettuce, Ulva lactuca; Laminaria sp.), should be further studied as
potential food and refuge sources. Furthermore, it is recommended that future
studies investigate the feeding rates of adult and juvenile species of amphipods
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on these macrophytes in an effort to determine if the age of these dominant
supralittoral fauna affects their rates of macrophyte consumption.
5.4. References
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macroalgae by Talitrus saltator (Amphipod, Talitridae): a stable isotopes
study in the northern coast of Brittany (France). Estuar. Coast. Shelf Sci.
56, 91-98.
Behbehani, M.I., Croker, R.A., 1982. Ecology of beach wrack in northern New
England with special reference to Orchestia platensis. Estuar. Coast. Shelf
Sci. 15, 611-620.
Buchsbaum, R., Valiela, I., Swain, T., Dzierzeski, M., Allen, S., 1991. Available
and refractory nitrogen in detritus of coastal vascular plants and
macroalgae. Mar. Ecol. Prog. Ser. 72, 131-143.
de la Huz, R., Lastra, M., 2008. Effects of morphodynamic state on macrofauna
community of exposed sandy beaches on Galician coast (NW Spain). Mar.
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Dugan, J.E., Hubbard, D.M., McCrary, M.D., Pierson, M.O., 2003. The response
of macrofauna communities and shorebirds to macrophyte wrack subsidies
on exposed sandy beaches of southern California. Estuar. Coast. Shelf Sci,
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Jaramillo, E., De La Huz, R., Duarte, C., Contreras, H., 2006. Algal wrack
deposits and macroinfaunal arthropods on sandy beaches of the Chilean
coast. Rev. Chil. Hist. Nat. 79, 337-351.
Lercari, D., Defeo, O., 2006. Large-scale diversity and abundance trends in
sandy beach macrofauna along full gradients of salinity and
morphodynamics. Estuar. Coast. Shelf Sci. 68, 27-35.
McLachlan, A., Brown, A.C., 2006. The Ecology of Sandy Shores, second ed.
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McLachlan, A., 1990. Dissipative beaches and macrofauna communities on
exposed intertidal sands. J. Coast. Res. 6, 57-71.
McLachlan, A., Dorvlo, A., 2005. Global patterns in sandy beach macrobenthic
communities. J. Coast. Res. 21, 674-687.
McLachlan, A., Wooldridge, T., Dye, A.H., 1981. The ecology of sandy beaches
in southern Africa. S. Afr. J. Zool. 16, 219-231.
Robertson, A.I., Mann, K.H., 1980. The role of isopods and amphipods in the
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Shaw, J., Taylor, R.B., Solomon, S., Christian, H.A., Forbes, D.L., 1998b.
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