FACTORS CONTROLLING THE DISTRIBUTION OF THE HIGH INTERTIDAL GREEN
ALGA, PRASIOLA MERIDIONALIS
A Thesis
Presented. to
The Faculty of the Department of Biology
San Jose State University
In Partial Fulfillment
of the Requirements for the Degree
Master of Arts
By
Brian Scott Anderson
December, 1987
ACKNOWLEDGEMENTS
· Several people were instrumental in the success of this project .
. I would first like to thank Drs. Gregor Cailliet and James Nybakken
of the Moss Landing Marine Laboratories for their review of the
manuscript and generous advice. Special thanks goes to Dr. Michael
Foster whose friendship, enthusiasm, and unfailing good humor
supported me during my stay at Moss Landing.
I would also like
to thank my wife Barbara for holding the flashlight in the middle of
the night and for remaining patient under adverse conditions.
This
work was supported by a grant from the David and Lucille Packard
Foundation.
ABSTRACT
Field and laboratory experiments were used to
determine the effects of insolation, gastropod
herbivory, and sea bird guano on the distribution of
Prasiola meridionalis, a perennial green alga that
occurs in the high intertidal "splash zone" of guano
covered r.ocks. Field experiments were done at two
sites.
One site was a sun exposed horizontal crown of
rock on which Prasiola occurs seasonally, appearing in
the early spring, and dying back in early summer. A
field shading experiment in which experimental plots
were shaded with plastic mesh and compared to unshaded
controls showed that increased sunlight in spring and
early summer killed the unshaded Prasiola, thereby
limiting it's horizontal distribution at this site.
A
second field experiment was conducted to assess the
effects of grazing on limiting the vertical
distribution of Prasiola.
This experiment was done at
another study site where Prasiola grows year round on a
shaded vertical rock face. Gastropod herbivores were
excluded with fences and anti-fouling paint from long .
vertical swaths that extended from the Prasiola band
(at + Sm) down to the next algal zone, 2 meters below.
The results of this experiment showed that herbivores
set the proximate lower limit of distribution of
Prasiola at this site, while some other undetermined
factor set Erasiola's ultimate lower limit of
distribution. The relative importance of sunlight and
herbivory in limiting the distribution of Erasiola
depended on differences in the physical conditions at
these 2 sites. Sunlight was more important on the
horizontal rock surfaces of the sun exposed site, while
herbivory was more important on the vertical rock at
the shaded site.
A laboratory culture experiment
designed to determine the effect of sea bird guano on
the growth and survival of Erasiola showed that lower
concentrations of guano stimulated its growth, but
higher concentrations inhibited its growth. The results
of these experiments demonstrate that a suite of
factors can interact to control the distribution of
high~r
intertidal algae, and that the relative
importance of these factors depends on site-specific
variability in the physical environment.
INTRODUCTION
Our understanding of the causes of algal zonation
on rocky intertidal shores has changed considerably
with continued research in more areas.
Much of the
early work suggested that upper limits of distribution
were determined by physical factors related to emersion
(Doty 1946, Zanveld 1969, Chapman 1973, Schonbeck and
Norton 1978), while lower limits were a result of
biological factors such as predation and competition
(Burrows and Lodge 1950, Lubchenco 1980, Foster 1982).
However, some early work and more recent studies have
shown this generalization to be
simplis~ic.
Castenholtz (1961), Underwood (1980), and Underwood and
Jernakoff (1981, 1984), for example, have indicated
that.biological factors (e.g. herbivory) can play an
important role in setting the upper limits of
intertidal marine algae. Working at lower and midintertidal levels, Underwood (1980) and Underwood and
Jernakoff (1984), showed that, in New South Wales,
herbivores are more important in setting the upper
limit of distribution of foliose algae.
Physical
factors due to emersion (i.e., desiccation, light, and
temperature) are seasonally important in limiting algal
1
survival, recruitment, and growth, and the variation in
algal cover on these shores can be explained by the
complex interaction between herbivory and seasonal and
site-specific variablity in the physical environment.
With more experimental studies from different
areas, it is becoming increasingly apparent that the
distribution of intertidal plants is influenced by a
suite of interacting factors, and that the degree to
which these factors affect algal distribution is
determined, in part, by where a species occurs.
Because the :highest intertidal, or "splash" zone is
subject to elevated temperatures, insolation, and
desiccation, it was assumed that physical factors
played the major role in limiting algal distribution on
this part of rocky shores (Connell 1975, Menge and
Sutherland 1976, see Luchenco and Gaines 1981, and
Chapman 1986 for reviews).
However, herbivores can
also influence algal distribution in this zone
(Lubchenco and Cubit 1980, Robles
and Cubit 1981,
Robles 1982, Cubit 1984).
Most high intertidal foliose algae recruit and
survive only during the wetter months of winter and
spring.
Studies in this zone have therefore focused on
ephemeral species (Lubchenco and Cubit 1980, Cubit
2
1984).
The green alga Prasiola meridionalis is unique
because it is one of the few perennial algae that occur
in this zone. Populations of Prasiola commonly form
dense monospecific bands in the high spray zone of
guano-covered rocks (Abbott and Hollenberg 1976).
Several workers have described the life history of .E...a.
meridionalis and its European cogener, .E...a. stipitata
(Bravo 1962 and 1963, Edwards 1975), and Friedman
(1963) showed that within bands of Prasiola stipitata
there exists a vertical zonation of meiotic and nonmeiotic plants.
Lewin (1955) found that laboratory
growth rates for .E...a. stipitata were stimulated by "fowl
excretia" (guano), suggesting that by growing on rocks
covered with guano, Prasiola is provided with a ready
source of organic nitrogen.
Other than these observations, little is known of
the
~cology
of this alga.
The experiments discussed in
this paper were designed to investigate the effects of
herbivory, sunlight, and organic nitrogen on the
vertical and horizontal distribution of Prasiola
meridionalis.
The hypothesis to be tested is that
herbivores set the lower vertical limit of distribution
for Prasiola at some sites, while sunlight plays
a more important role in limiting Prasiola's
distribution at other sites. An experiment with sea
3
bird guano tested the hypothesis that guano can both
stimulate and inhibit Prasiola's growth depending on
concentration.
These experiments are intended to
illustrate the fact that, as in lower rocky intertidal
zones, algae in the high intertidal zone are limited by
a complex interaction of physical and biological
factors.
SITES AND SPECIES STUDIED
The field experiments were conducted at two study
sites on the central coast of California, between
October, 1984 and November, 1985. One site was the
Hopkins Marine Station, Pacific Grove, California (36'
37" N, 121' 54" W). This site was a near vertical
granite rock face near the west side of Bird Rock (for
a detailed description of this stuqy area, see Foster
1982).
Prasiola meridionalis forms a distinct
perennial band with a sharp lower boundary in the
splash zone (approximately + 5 m) on the shaded side of
this rock.
Photographs of this band confirm its
stability over several years (personal communication,
M. Foster). During the late winter and early spring
Prasiola recruits to the sunnier areas at this site,
4
but then it dies back in the hotter weather as summer
approaches.
The second study site was located on Pescadero
Rocks in Carmel Bay, Monterey County, California
(36'34" N, 121' 56" W).
This site was on the uppermost
horizontal crown of a sun-exposed island consisting of
hard conglomerate rock with smooth pebbles and cobbles
imbedded in fine-grained sandstone. Prasiola forms a
seasonal band on this crown, appearing in early winter
(November) and persisting until early summer (June).
Prasiola is perennial only in the shaded areas at this
site.
Pescadero Rocks differed from Bird Rock in that
it was mostly sun exposed. Although I chose these sites
because of the different physical environments the two
offered, both sites had sunny and shaded areas.
All study sites were located well above MLLW
in the uppermost splash zone (approximately + 5 meters
down to + 3 meters; Zone 1 in the scheme of Ricketts
~~
1968).
both sites.
~
At this height there were few species at
Besides Prasiola meridionalis, the
ephemeral species Porphyra perforata and Porphyra
lanceolata were seasonally present, as well as a cover
of blue-green algae. Invertebrate grazers at Bird Rock
were the limpets Collisella digitalis and
c.
scabra,
and the littorine snails Littorina planaxis and L.
scutulata. Two species of marine mites were also
5
present at Bird Rock, Affieronthrus lineatus, and
Hyadesia sp.
The flora was similar at Pescadero Rocks,
but invertebrate grazers at this site were few (an
occasional Littorina planaxis in cracks and crevices,
and mites).
METHODS
Shading
To test .the hypothesis that sunlight or some
related physical factor caused a seasonal fluctuation
in Prasiola cover at the Pescadero Rocks site, I
constructed shaded plots and compared these to unshaded
controls.
The shades were constructed by attaching 30
by 30 em frames of fiberglass strips to the rock, and
then placing 2 layers of Vexar mesh over these.
After
two months a third layer of mesh was added to each plot
to
m~ke
up for the increased spring sunlight.
Two
opposite sides of the fiberglass strip frames were
arched 10 em high so that water, grazers, and air could
move freely under the shades.
I oriented the shades
randomly in relation to incoming waves and sunlight to
allow for any affect of the structure on these factor's.
Ten shades were constructed and monitored for three
months. Then the shades were removed and the
experimental plots were monitored for one more month.
6
The shaded plots were compared to ten control plots
containing
dense monospecific stands of Prasiola.
The
control plots were situated far enough away (>O.Sm)
from the shade plots so that the shades did not affect
the control plots, and so that any grazers using the
shades for cover would not have easy access to the
controls. Grazers were rarely observed in any of the
treatment plots throughout the study.
Photosyntheti-
cally active radiation (PAR) was measured in and out of
cages using a Licor model LI-1B5A irradiance meter
equipped with a cosine collector.
Air temperature was
recorded occasionally in the shaded and control plots
using a hand held thermometer.
Organic Nitrogen (Guano)
Prasiola commonly grows just below a layer of
guano on the upper surfaces of off-shore rocks (Abbott
~nd
Hollenberg, 1976). The alga is rarely covered with
water, and presumably depends on the sea spray for its
moisture. A culture experiment was designed to
investigate the relationship between sea bird guano and
the growth of Prasiola.
Guano was collected at Bird
Rock and extracted by pouring 50 ml of boiling filtered
(1u) sea water through 10 gm of quano in a standard
7
coffee filter, then a solution was obtained by · mixing
and
centrifuging.the decanted liquid.
decant was considered 100% strength.
The remaining
I
then diluted
this solution with filtered sea water to give 50%, 10%,
5%, 3%, and 1% solutions of guano extract. The salinity
of each solution was adjusted with distilled water to
34 ppt, the salinity of the control.
Algae grown on
these solutions were compared to control cultures in a
solution of pure filtered sea water.
Test plants collected at Bird Rock were used to
assess growth. Each plant had an attached rhizoid and
was approximately 2 mm long. Plants were introduced to
the test dishes haphazardly.
I
monitored six
replicates of each concentration; each replicate was
started with 0.025 gm (blotted wet weight) of algal
tissue.
woul~
I
felt that weighing the blotted Prasiola
give an accurate measurement of growth because
the individual blades.were so thin that they
were
easily blotted to a uniform dryness. The blotted alga
was ·spread onto #1 Whatman filter paper and placed in
Petri dishes.
Four milliliters of solution were
introduced into each Petri dish; equal increments of
solution were introduced into each Petri dish as needed
to make up for losses due to evaporation.
The tissue
was weighed every ten days, and filters and solutions
8
changed.
All cultures were grown in a culture chamber
under cool white light supplemented with two 15 watt
incandescent light bulbs. The temperature was held
constant at 12.8 'C, and the light levels were
maintained at 150 uE/m"/sec with a 12:12 photoperiod.
The
pH was monitored with a Corning model 10 pH meter,
and salinity with an Atago salinity refractometer.
Grazers
To examine the effect of herbivory on limiting the
lower distribution of Prasiola at Bird Rock, I excluded
grazers from experimental plots using a combination of .
fences and toxic marine antifouling paint.
I
constructed the exclusion fences with 1/8" diameter
Vexar plastic mesh (TWP Plastics, San Francisco) cut
into strips approximately 2 m long and 12 em wide.
Holes were drilled into the granite, plastic wall
anchors placed in these, and the mesh strips then
fastened to the rock with thin fiberglass strips
anchored by stainless steel screws.
I pulled up the
sides of the mesh strips and attached them together at
the top to make a double barrier 5.5 em high.
Marine
caulking compound (Boat Life, Old Bethpage, NY) was
used to seal the base of the fences to the rock. I
installed three of these exclosures so that each
9
extended from the lower limit of the Prasiola band down
to an algal band composed of Endocladia muricata,
approximately 2 m below. The three-sided exclosures
were left open on the side adjacent to the Prasiola
bands' lower boundary to allow a flow of algal spores
into the exclosures.
The dimensions of each exclosure
was 1.5 m wide by 2 m long.
I monitored three control areas of equal
dimensions on either side of the grazer exclosures.
Menge (1976) found that herbivores tended to congregate
under cages and foraged into adjacent control plots.
To eliminate this artifact, I painted a 4 em strip of
toxic marine paint (Z Spar Brand, Kopper Co. Inc.,
Pittsburg, PA with copper powder added) around the
sides and lower borders of the two end exclosures (see
Cubit, 1984, for a discussion of this technique).
This
combination helped to exclude all grazers inside the
fences and prevented them from hiding in the shade of
the fences.
The caulking and a general lack of flowing
seawater prevented the toxic paint from penetrating the
exclosures. In addition, I left a 50 em boundary
between the fenced and control plots. I attempted to
control for the shading artifacts inside the fences by
leaving a 20 em wide by 200 em long strip between first
10
and second and second and third grazer exclusion
fences.
Grazer densities in these two strips were not
altered (although they were lower than in the nonfenced control plots; see Table 2).
If shade cast by
the fences was creating an experimental artifact that
was enhancing growth, I expected an increase in the
Prasiola cover between the fences compared to the nonfenced controls.
Percent cover of algae in all field experiments
was determined using a 90
em~
plastic sheet with 100
random holes in it. Each of the three fenced and
control plots were divided vertically into six adjacent
90 cmA
quadrats. These quadrats were then monitored as
discreet quadrats during the course of the study.
To
reduce the effect of .fence artifacts on my results, I
left a 10 em boundary around the inside perimeter of
each.exclosure unsampled.
Densities of invertebrate
grazers were monitored occasionally throughout the
experiment by counting grazer numbers in randomly
placed 0.25
m~
quadrats.
Incoming light was measured
in the fenced and control plots using the Licor
irradiance meter described above.
The experiment was
run for 13 months.
All of the results were compared using nonparametric statistical tests.
11
Paired comparisons were
analyzed with the Mann Whitney U-Test.
Multiple
comparisons were analyzed with a Kruskal Wallis Test
followed by a non-parametric analog of the StudentNewman-Keuls Test (Zar 1974).
RESULTS
Shading
The mean percent cover of Prasiola in the unshaded
and shaded plots remained similar through March, April,
and May (Figure 1).
In June, the percent cover of
Prasiola in the unshaded plots declined dramatically
from a mean of 82.2% to 16.0%.
During this same period
. the mean percent cover in the shaded plots declined
only slightly from 95.2% to 90.2%. The Prasiola in the
unshaded control plots continued to decline, first
becoming blackened.and brittle and eventually
disappearing altogether.
The mean percent cover in
thes'e plots declined to 3. 3% by July.
In the shaded
plots the mean percent cover also began to decline,
though less dramatically than in the unshaded plots.
A
second layer of Vexar was added to each shade plot in
June because some of the Prasiola thalli were beginning
to turn black due to a lack of adequate shade.
The
mean percent cover in the shaded plots declined from
90.2% in June to 61.4% in July, but was still
12
significantly greater than the Prasiola cover in the
control plots (June control mean = 3.3%, Mann Whitney U
test, p < .05).
The 10 shades were removed on July 1 (mean percent
cover = 61.4%) and by August 1 all of the Prasiola in
these plots was gone (mean percent cover= 0). Figure
2a shows a lush cover of Prasiola at the study site in
March.
By June most of the unshaded
Prasiola has been
denuded by the hot weather (Figure 2b), while that in
the shaded plots persisted (Figure 2c).
Light and temperature were greatly reduced in the
experimental plots.
The two layers of Vexar attenuated
approximately 85% of the.sunlight, while three layers
of Vexar attenuated approximately 92% (Table 1). The
amount of sunlight inside of the shades changed
depending on the time of day.
arch~d,
Because the shades were
the inside of the shades were sunnier earlier
in the day when the sun was at a lower angle (Table 1).
·The two-shade plots were 4'C cooler, and the threeshade plots S'C cooler than the control plots (Table
1).
Organic Nutrients (Guano)
The~ncrease
in blotted wet weight of Prasiola
(Figure 3) was significantly higher in the 3% solution
13
of sea bird quano than in all of the other solutions
(Kruskal Wallis and Student-Newman-Keuls analog,
0.05).
P <
The lower concentrations (1 and 3%) had
significantly higher growth rates than the four higher
concentrations (5, 10, 50, and 100%; Kruskal Wallis and
S-N-K analog, P < .05).
The drop in wet weight in the
four higher nutrient concentrations resulted from the
removal of dead plants from the cultures.
These
concentrations inhibited growth and killed some of the
plants.
The control cultures had significantly higher
growth rates (e.g. lower mortality) than the four
higher concentrations, but lower growth rates than the
two lowest concentrations (S-N-K analog, P < .OS).
Although the salinity levels in all of the guano
concentrations were adjusted to the same level as the
control (34ppt) using distilled water, the pH levels
were, not adjusted, and were as follows (%guano- pH):
100% - 8.19; 50% - 8.26; 10% - 8.42; 5% - 8.46; 3% 8.48; 1% - 8.52; 0% - 8.55.
Grazer Exclusion Experiment
After 5 months (October, 1984- March, 1985)'
there was a significantly higher cover of Prasiola in
the fenced plots than in the control plots (Figure 4).
The percent cover of Prasiola was significantly greater
14
in five of the six exclusion fence quadrats than in the
control quadrats (Kruskal Wallis and S-N-K analog,
.05; Figure 5).
P <
In general, the percent cover of
Prasiola was +significantly greater in the upper
quadrats (Kruskal Wallis and S-N-K analog, P < .05).
Thus, quadrat 1 (nearest the Prasio1a band) had a
significantly greater percent cover than quadrat 2,
which had a significantly greater cover than quadrat 3,
and so on.
However, by October 1986 some of the lower
quadrats, in particular quadrat 4, had an increase in
percent cover which coincided with a general decrease
in percent cover in some of the upper quadrats.
Prasiola never occurred in quadrat 6, the lowest
quadrat; so the data from this quadrat were not
included in Fig. 5.
Prasiola only occurred in quadrat
1 in the fence control plots, so only these data are
give~
in Fig. 5.
The percent cover in the Prasiola band and the
fenced quadrats was variable.
The cover of Prasiola in
the Prasiola band decreased gradually over the course
of the experiment from a mean high of 95.67% in
October, 1984, to a mean low of 56.33% Prasiola in
August, 1985.
By November 1985 the Prasiola cover
began to increase slightly in the band to 68%.
percent cover in all of the quadrats (except 6)
15
The
increased until March 1985, at which time the Prasiola
covers tended to level off (Figure 5).
In August, 1985
the cover in all of the quadrats began to decrease,
then in November it began to increase again as the new
growing season began. The percent cover of Prasiola in
the non-fenced control plots were always 0 to 2%.
The percent cover of Prasiola in the two strips
between the three fenced plots in which grazer
densit~es
were left unaltered differed slightly from
the controls. The mean percent cover of Prasiola in
these two strips was 5.35% for the 13 months of the
experiment vs. 1.68% for the three control plots.
The highest density of grazers occurred in the
control areas (Table 2).
littorines,
~
The majority of these were
scutulata. Lower numbers of herbivores
occurred between the fenced plots and very few grazers
were ever found in the fenced plots.
Light intensity was not significantly different
either between the three fenced plots or between the
fenced plots and the control plots (Mann Whitney U
test, P < .05; Table 3).
The light measurements within
the control and fenced plots did differ with light
levels decreasing from the top to the bottom of the
plots (Table 3).
16
DISCUSSION
The results of these experiments demonstrate
the importance of physical and biological factors in
controlling the distribution of Prasiola meridionalis.
Results from the shading experiment indicate that
sunlight can limit the seasonal abudance and horizontal
distribution of Prasiola.
Prasiola grows as a delicate
monostromatic blade, a thallus form that is susceptible
to the deleterious affects of exposure to direct
sunlight (Smith and Berry 1986).
After several days of
warm weather in June, the Prasiola thalli in the
control plots became blackened and brittle, while those
under the shades remained green and healthy.
The mean
percent cover of Prasiola in the control plots declined
from 82% to 16% within one month (Figure 1), while the
Prasiola cover in the shaded plots declined only
slig~tly
during this time (from 95% to 90%).
The results from the shading experiment do not
allow separation of the several factors associated
with increased insolation (e.g. desiccation,
ultraviolet irradiation, increased tissue temperature).
The shades altered the microhabitats around the plants,
both by increasing the humidity and reducing the
ultraviolet radiation. Although the air temperature
differed by as much as 4 'C inside and outside of the
17
shades (Table 1), I do not feel this was enough of a
difference to account for the extreme disparity in
percent cover in the experimental plots.
The
temperature difference is probably. significant only in
that it affects the rate of evaporation in the
experimental plots.
Tissue temperatures as high as 33
'C have been recorded for intertidal algae (Glynn
1965); .the temperatures at the study site were always
much lower than this.
A more likely explanation is
differences in humidity.
An increase in the relative
humidity under the shades would reduce the dehydration
of the Prasiola blades.
The negative effects of
dehydration has been shown to be one of the major
factors limiting intertidal algal distribution
(Schonbeck and Norton 1978, Hodgson 1980, Johnson
~.
1974, Quadir
~
~
gl. 1979), primarily by disrupting
photosynthesis (Hodgson 1981, Dring and Brown 1982,
Smith and Berry 1986).
Although I did not measure the
relative humidity in the experimental plots, I observed
the Prasiola turf to be generally more moist under the
shades. The high arching sides of the shades were
designed to allow water and air to pass freely under
them, but it is likely that the humidity levels under
the shades were significantly higher than those in the
18
control plots regardless of this modification.
Friedman (1969) suggests that the thallus size of
Prasiola stipitata is negatively affected by decreasing
humidity in the higher levels of the Prasiola band and
at the end of the growing season.
Friedman also
suggests that the reproductive status of Prasiola
stipitata is affected by decreasing humidity levels. He
observed that as the warmer months approached in North
Wales, the proportion of meiotic Prasiola plants
decreased.
In June, plants at the highest levels died
out and, at the remaining levels, only spore producing
plants remained.
Other explanations for the results of this
experiment include ultraviolet irradiation, and wind
and water shear.
Ultraviolet light can damage algae in
a variety of ways (Levitt 1980); it is the shorter
wave~ength
UV-B light (280 - 320nm) that is more
biologically injurious.
Algae exhibit varying degrees
of tolerance to UV light (McLeod and McLachlan 1959,
Calkins and Thordardottir 1980), and this variability
is thought to be related to the ability of plants to
absorb the incoming light with a UV-B absorbing
compound (Levitt 1980, Scelfo
1985).
The shades may
have attenuated this component of the incoming sunlight
and this could explain the results of the experiment.
19
The most likely explanation is that the controls simply
received more light, resulting in higher thallus
temperatures and more desiccation. This question cannot
be answered with the data available.
The possibility
that the shades affected the wind and water shear on
the Prasiola blades is unlikely because of the limited
amount of water motion at this tidal height, and the
arched design and random orientation of the shades.
Because this experiment was conducted at the
extreme southern end of the distribution Prasiola
meridionalis
(~
meridionalis ranges from Carmel Bay,
California to Friday Harbor, Washington, Abbott and
Hollenberg 1976) the negative effects of sunlight might
be more pronounced.
Prasiola is described as a
perennial alga (Abbott and Hollenberg 1976), but might
be better described as a facultative perennial. It
appears to be a perennial in more benign (i.e. shaded)
habitats but is ephemeral on sun-exposed surfaces.
This seasonal fluctuation might be
co~on
southern extreme of Prasiolas' range.
only at the
In addition, no
attempt was made to verify that all of the Prasiola was
killed in the sun-exposed plots.
It is possible that
some part of the plants not visible to me were
persisting (e.g. the rhizoidal holdfast) and the plants
might grow back from these when cooler weather
20
returned.
As at Pescadero Rocks, the Prasiola at Bird Rock
that appeared on the sun-exposed horizontal rock
surfaces in early spring, died back in the summer.
Although I did not .quantify this, sunlight was
apparently important in limiting the horizontal
distribution of Prasiola in the sunnier places at this
site.
The results of the guano experiment indicate that
sea bird guano can either stimulate or inhibit the
growth of Prasiola meridionalis depending on the
concentration (Figure 3).
The Prasiola blades more
than doubled in blotted wet weight in the 3% guano
solution while the blades in the 0% and 1% solutions
grew only slightly.
Guano concentrations of 5% and
higher inhibited Prasiola growth and caused mortality.
Lewin (1955) performed a similar experiment on the
growth of Prasiola stipitata using "fowl excretia."
Although his experimental conditions were somewhat
different from mine, the concentrations were the same.
His results were similar in that growth (in length) of
~
stipitata appears to be optimum in a 3% solution of
fowl excretia extract and was reduced above an
approximately 5% solution (Lewin 1955, Fig. 3).
It is
unclear why the higher concentrations of guano extract
inhibited Prasiola growth in my experiment.
21
The
majority of the blades in these concentrations appeared
to be dark green but very mushy and were
disintegrating. By the final weeks of the experiment,
the majority of the higher concentration replicates
were made up of only one or two very large, apparently
healthy plants.
The majority of the lower
concentration replicates were made up of several
smaller, healthy plants. So, while there was greater
mortality in the higher concentrations, there were one
or two plants in each replicate that were able to
tolerate higher concentrations.
The salinity in each of the guano concentrations
was adjusted to 34 ppt, so salinity was not a factor.
The pH in the guano solutions decreased slightly from
the lower to higher guano concentrations, but it not
clear whether this difference was extreme enough to
cause the observed differences in growth and survival.
There have been few studies on the effects of lowered
pH on algal growth and survival.
Ogata and Matsui
(1964) showed that pH changes affect algal
photosynthesis in two ways, by affecting cellular
protoplasm at pH levels greater than 9.8, and.by
changing the carbon dioxide-bicarbonate- carbonate
equilibria in the media.
At pH levels greater than
9.5, the carbon dioxide supply to the plant material is
limited. Lower pH solutions might disrupt intracellular
22
enzyme systems
(Lobban~~
1985).
suggests that urea is inhibitory to
all concentrations.
Lewin (1955)
~
stipitata at
Uric acid, a major component of
sea bird waste (Sturkie 1986) might have a similar
effect.
Although no nutrient analyses were performed on
the guano solutions or Prasiola blades, it is likely
that the stimulatory effect of the lower guano
concentrations was a result of higher nitrogen,
phosphate, and potassium levels.
Although variable
depending on the species of birds and local physical
conditions, sea bird guano contains 11-16% nitrogen, 812% phosphoric acid, and 2-3% potash (Whittow and Rahn
1984).
The notion that marine plant growth is influenced
by animal excretia has been suggested by numerous
researchers. Hanson (1972) found that the nutrients
ass~ciated
with pinniped wastes enriched low intertidal
algal standing crops in central California.
Hockey
(1983) suggested that sea bird guano enhances the
nutrient status of South African intertidal and
nearshore ecosystems resulting in increased primary and
secondary productivity.
These affects are most
pronounced around breeding islands.
Smith (1978) found
increased nitrogen and phosphorous levels associated
with seabirds and seals on a subantarctic island, and
23
suggested that the input of organic nutrients greatly
enhanced plant growth and vitality.
Bosman
·gt ~
(1986) found elevated levels of nitrogen and
phosphorous associated with sea bird guano which
enhanced algal growth rates in tide pools on guano
covered islands, relative to control sites on the
mainland.
This latter study also included a field
experiment where a nutrient mixture made up of a guano
solution that corresponded in concentration to my 1%
guano concentration was dripped onto intertidal algae.
The guano-dripped zones had increased algal growth and
settlement when compared to the controls.
A few
studies have also indicated that higher concentrations
of guano can inhibit algae.
Golovkin (1967) for
example, showed that a solution greater than 5 mg/1
mixed in "springtime" seawater or greater than 10 mg/1
mixed in "autummal" seawater inhibited the growth of
the phytoplankton Nitzschia seriata.
The results of my experiment do not indicate that
guano is a requirement for growth by Prasiola
meridionalis.
(1955)
for~
The same conlusion was reached by Lewin
stipitata.
Nutrient limitation is one of
the major factors limiting intertidal algal growth
rates (DeBoer 1981), and Prasiola occurs in an
intertidal habitat that is the farthest from marine
nutrient inputs; an environment that is undoubtably
24
nutrient depleted. It is possible that guano is
required by Prasiola for some other more subtle
physiological function than growth. It might be that
growth was not a sensitive enough experimental end
point to resolve whether or not guano was a strict
requirement for the survival of Prasiola.
For example,
it is possible that Prasiola requires guano in order to
reproduce. Moreover, it is difficult to determine the
ecological relevance of this experiment without knowing
how closely the guano concentrations in this experiment
duplicate those in the field.
Concentrations in the
field undoubtably vary with seasonal fluctuations in
bird numbers and rainfall. The apparent inhibitory
effect of higher guano concentrations indicated in my
experiment might occur during periods of high rainfall
or at tidal levels closer to the guano deposits.
Clearly, more research on these factors is needed to
fully understand the effects of guano on the growth and
survival of Prasiola.
The results of the grazer exclusion experiment
indicate that gastropod grazers can set the lower limit
of distribution of Prasiola meridionalis. The fleshy,
monostromatic thallus morphology that makes Prasiola
susceptible to solar damage also makes it suceptible to
grazers (Lubchenco and Cubit 1980).
The cover of
Prasiola in the fenced plots was significantly greater
25
that the cover in the non-fenced controls (Fig. 4b and
5).
The grazer densities inside the fenced plots were
significantly lower than in the non-fenced controls
(Table 2). This was the only observed difference
between the control and fenced treatments.
As
expected, the upper quadrats nearest the Prasiola band
had significantly higher growth of Prasiola than the
lower quadrats (Fig. 5), presumably because they were
nearest the source of spores.
This experiment extended through two growing
seasons, yet no Prasiola occured in the lowest quadrat
(#6).
Although it is possible the distance of the 6th
quadrat from the Prasiola band (2m) was prohibitive for
dispersal in the time alloted for the experiment, a
more plausible explanation is that some physical factor
was preventing either recruitment or growth.
For
example, at high tide the lowest quadrat was
periodically scoured by waves washing through a surge
channel.
The fast moving water could have limited the
growth of recruits.
The rock in the lowest quadrats
was for the most part bare, although occasional
Porphyra perfotata and
~
lanceolata occured there.
It is also possible that the light levels at these
lower levels on the rock face were inadequate for
photosynthesis. Light was the only physical factor
26
measured (Table 2), and these measurements showed that
although light levels were similar in all of the
treatments,
·the levels decreased
lower quadrats.
from the upper to
This decrease, an affect of the
rounding of the rock surface,
resulted in light levels
in the lowest (#6) quadrats that were lower than the
levels recorded in the triple shaded treatments in the
Pescadero Rocks shading experiment (Table 1).
The
light levels in quadrat #6 in the Bird Rock grazer
exclusion experiment were 43 and 48 uE/ m~jsec , for
the fenced and control plots respectively, and 94
uE/mA/sec in the 3 shaded plots on Pescadero Rocks.
It
is possible the low light levels observed in quadrat #6
on the Bird .Rock experiment were too low to support
Prasiola. It should be noted, however, that the light
levels given in Table 3 reflect light levels at only
one time of day on one day of the year; the mean light
levels on the lower portions of the rock could easily
have been adequate during other times of the day or
year.
· Also, other species did occasionally occur in the
sixth quadrats, and a thick stand of Endocladia
muricata occurred just below the sixth quadrats.
Although competition has been shown to limit the lower
'distribution of intertidal algae (Hruby 1976, Foster
1982), it was not a limiting factor in this experiment
27
measured (Table 2), and these measurements showed that
although light levels were similar in all of the
treatments,
the levels decreased
lower quadrats.
from the upper to
This decrease, an affect of the
rounding of the rock surface,
resulted in light levels
in the lowest (#6) quadrats that were lower than the
levels recorded in the triple shaded treatments in the
Pescadero Rocks shading experiment (Table 1).
The
light levels in quadrat #6 in the Bird Rock grazer
exclusion experiment were 43 and 48 uE/ rnA/sec , for
the fenced and control plots respectively, and 94
uE/mA/sec in the 3 shaded plots on Pescadero Rocks.
It
is possible the low light levels observed in quadrat #6
on the Bird Rock experiment were too low to support
Prasiola. It should be noted, however, that the light
levels given in Table 3 reflect light levels at only
one time of day on one day of the year; the mean light
levels on the lower portions of the rock could easily
have been adequate during other times of the day or
year.
Also, other species did occasionally occur in the
sixth quadrats, and a thick stand of Endocladia
m~ricata
occurred just below the sixth quadrats.
Although competition has been shown to limit the lower
distribution of intertidal algae (Hruby 1976, Foster
1982), it was not a limiting factor in this experiment
27
because other alga
species were only occasionally
found in any of the lower quadrats and bare space was
common. The long exclusion plots in this experiment
made it possible to show that the proximate lower limit
of distribution of Prasiola is set by grazers; the
ultimate lower limit of distribution is set by some
other factor(s).
There have been several recent studies showing
that herbivory plays an important role in limiting the
distribution of high intertidal algae (Castenholz 1961,
Robles and Cubit 1981, Lubchenco and Cubit 1980, Robles
1982, and Cubit 1984).
Most high intertidal studies
have apparently focused on algae in zones that are
within the effective grazing limits of intertidal
herbivores.
Prasiola is somewhat unique in that,
unlike most other high intertidal alga species, it is a
perennial that forms a dense yearly band higher than
all other marine alga species.
I have observed the
upper limit of the Prasiola band at other sites to
often be in direct contact with the lowest terrestrial
zone, occasionally in contact with the soil.
Because Prasiola occurs on the highest
intertidal rock faces, it is apparently above the
effective grazing limit of intertidal gastropods and
therefore has a refuge from gastropod herbivory.
Thus,
Prasiola can apparently only grow down, and its lower
28
vertical distribution is limited by grazing.
When the
grazers are eliminated, the Prasiola band extends
further down. Gastropod herbivores were rare and
apparently did not limit Prasiola's distribution at the
Pescadero Rocks study site, probably because this site
was too dry for them to graze effectively. However, I
did observe limpets and littorines below the Prasiola
band on the shaded vertical rock surfaces adjacent to
the Pescadero Rocks study site. Grazers were
undoubtably limiting the lower limit of Prasiola in
these areas.
Herbivores have also been shown to set the lower
vertical limit of distribution for some lower interidal
algae.
For example, Burrows and Lodge (1950) showed
that limpets set the lower limit of distribution for
Fucus spiralis.
Similarly, Moreno and Jaramillo (1983)
showed that the lower bound of Iridaea boryana is
determined by gastropod grazing.
Although these
studies were done in lower zones, they demonstrate that
similar mechanisms are controlling intertidal algal
distribution in different zones.
Prasiola growth within and between replicates of
the fenced plots was variable and apparently related to
several factors.
The main reason for the variability
in the data was the relatively low number of replicates
used in the experiment.
29
To minimize the effects of
heterogeneity of the rock surface, this experiment was
done on a limited part of the rock face where the lower
boundary of the Prasiola band was well defined.
Because of the space limitations and the design
requirements that the fenced exclosures be wide and
long, I was only able to use three replicates.
Another source of variability was the shape of the
rock itself.
Although I chose this study site to
minimize differences, the experimental plots were
located on a rounded surface so that the six plots had
subtle differences in sunlight and wind exposure.
The
light measurements showed that light levels did not
vary significantly between the fenced and control plots
but did decrease from the upper to lower quadrats
within replicates (as discussed previously).
This
decrease was an effect of the rounding of the rock and
could have contributed to the variability in the data.
The control plots were located on either side of the
fenced plots and all had similarly low growth of
Prasiola, so it is unlikely that differences in sun
exposure accounted for the dramatic differences in
Prasiola growth between the fenced and control plots.
There was a slight increase in Prasiola cover in the
between-fence control strips compared to the fence
controls (mean percent cover Prasiola in the strips =
5.35% vs. 1.68% in the controls).
30
This difference
could not be tested statistically because there were
only two replicates of the between-fence treatment. The
majority of growth in these strips occurred in the
upper part of the right strip (Figure 4).
Again, note that the grazer densities were much
lower between the fences than in the non-fenced
controls (Table 2). The general decrease in Prasiola
cover in the later months of the experiment (June September, 1984) was apparently related to increases in
desiccation in the spring and summer months.
decrease was found
for~
A similar
stipitata by Friedman (1963).
Because Prasiola occupies the very highest intertidal
splash zone, it receives only the spray from waves at
high tide.
During the spring and summer there is less
splash and this decrease in moisture coupled with
increased temperature slows the growth of Prasiola.
Quadir et al. (1979) found that as desiccation
increased beyond 50% of the wet tissue weight,
photosynthesis declined leading to a decline in tissue
productivity.
Other intertidal studies have found
similar reductions in the summer months (Cubit 1984,
Robles and Cubit 1981, Underwood 1980, Underwood and
Jernakoff 1981 & 1984).
Cubit (1984) found a general
decrease in high intertidal algal cover in the summer
months. He suggested that the wintertime rates of
primary production were higher than the consumption
31
rates of herbivores, so standing crops increased. In
the summer months as the algae became more dry, rates
of production decreased below rates of consumption by
herbivores and algal cover declined.
Other grazers besides gastropods have also been
shown to limit high intertidal algal production.
For
example, Robles and Cubit (1981) and Robles (1982)
found that dipteran fly larvae could sometimes
dramatically reduce the cover and species composition
of high intertidal ephemeral algae. In my study
dipteran larvae were not observed, however, large
numbers of mites were found in all of my study plots,
and at Pescadero Rocks.
High densities of mites
occured in all Prasiola samples taken during the 13
months of the study. The two species, Ameronthrus
lineatus and Hyadesia
~,
are both coastal intertidal
transitional species that feed upon green algae
(Schulte and Weigmann 1977, Schuster 1979).
Although
no experiments were done to quantify the extent to
which mite grazing affected the growth of Prasiola,
large brown areas infested with mites within the
Prasiola band and to a lesser extent within the fenced
plots were undoubtably partially attributable to mite
grazing.
This, coupled with low productivity during
the warm spring and summer probably played a role in
the decrease in Prasiola cover during these months. The
32
Prasiola cover began to increase in all of the quadrats
(except #6) at the onset of cooler weather in October
1985 (Fig. 5).
The results of the shading, guano, and grazer
exclusion experiments on Prasiola meridionalis
illustrate the suite of factors that interact to affect
high intertidal community ecology.
Prasjola is limited
by physical factors (sunlight and desiccation) to shady
surfaces, and by biological factors (grazing) to the
highest intertidal zone.
Perhaps because it is
restricted to a relatively nutrient poor habitat, it
must be able to derive some of its nutrients from sea
bird wastes.
However, its growth can be limited if sea
bird waste concentrations become extreme.
These
experiments also demonstrate the importance of sitespecific physical and biological conditions in
controlling the distribution of Prasiola. At sunexposed sites grazers are restricted to shaded
microhabitats, and thus have less of an impact.
For
example, Prasiola occurs mainly on the shaded side of
Bird Rock where grazing plays a more important role in
controlling it's distribution.
At Pescadero Rocks,
Prasiola occurs on the more sun-exposed surfaces and is
more stongly affected by increasing insolation during
the warmer summer months; grazers are rare at this
site. Sunlight is more important than grazing in
33
restricting Prasiola's distribution overall because it
physically limits where it may occur.
Further
experimentation is needed to determine the degree to
which seabird guano affects Prasiola's
distribution.
SUMMARY
Algal distribution is controlled in all rocky
intertidal habitats by a combination of physical and
biological factors.
Although early research suggested
that biological factors were more important in lower
intertidal zones and physical factors dominated higher
zones, more recent work has shown that many factors
interact to control distribution in all intertidal
zones.
High intertidal habitats are probably more
strongly impacted by physical extremes than lower
intertidal habitats, but during periods of benign
weather, or in protected areas, biological factors can
play an important role in controlling high intertidal
algal distribution;
the relative importance of the
various controlling factors depends on seasonal and
site-specific variability.
34
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39
Table 1. Mean temperatures and light levels in Vexar shaded and
control plots on Pescadero Rocks (light in uE/ m~/
temperature in °C); N = 10 (1 S.D.). Measurements taken on one
day, March 30, 1985 in 10 control and shade plots.
s;
CONTROL
VEXAR
2 layers
e
0
•
M
e
II
II
a
a
o
3 layers
o
o
o
o
e
o
0
o
I
0
0
o
o o
e
0
e
o
o
o
o
01
o
II
0
o
6
e
0
0
a
o
0
o
0
•
0
Time
Light 10:00
172 ( 43)
72
(23)
1090 (0)
Light 12:00
151 ( 45)
94
(42)
1000 (0)
17
(O)
Temp. 12:00
18
(0)
40
22
(0)
0
0
0
Table 2. Grazer densities in control, fenced, and between fence
plots at Bird Rock ( #/m' (1 S.D.). Data obtained once. No
standard deviation values for between-fence controls because N =
2.
h planaxis
digitalis +
C. scabra
~
L. scutulata
.........................................................
Fenced(n=3)
Control(n=3)
Between(n=2)
1.0
(2.2)
0
107.1 (148.9)
18 7 .1 ( 238. 7)
16.7 (-)
14.9 (-)
41
1.6
(3.3)
205.5 (193.1)
29.3 (-)
TABLE 3. Bird Rock fenced and control plot light measurements on
November 26, 1985 in uE/m 2 /sec (1 S.D.), N=3.
Quadrat #
0
0
•
0
* •
1
0
0
0
0
0
control
·fenced plot
8
•
0
0
0
0
o
0
o
0
0
0
0
II
0
..
0
103
0
0
0
(5)
0
0
* •
0
0
0
0
e
0
0
0
0
•
0
0
100
0
0
0
0
0
(6)
2
89 (17)
74 ( 24)
3
69 (
70 (11)
4
53
5
54 (10)
56 (10)
6
43
(2)
48 (11)
42
)
(8)
65
(9)
FIGURE CAPTIONS
Figure
1
Mean percent cover of Prasiola in shaded and control
at Pescadero Rocks; N = 10 for each sampling period.
2
Pescadero Rocks shading experiment. Study site in March,
1985 (a), and June, 1985 (b). Prasiola under removed shade
in June, 1985 (c).
3
Changes
in Prasiola wet weight (+ 1 s.d.)
in
six
concentrations of guano and control solution; N = 6 for each
concentration.
4
Bird Rock grazer exclusion experiment. Fenced plots at the
start
the experiment in October, 1984 (a), and after 5
months in March, 1985 (h).
5
Percent cover of Prasiola in Bird Rock grazer exclusion
experiment.
Prasiola control is the percent cover of
Prasiola in the hand above the fenced exclosures; quadrat 1
is the upper quadrat nearest the Prasiola hand; quadrat 2 is
the next lowest, and so on down to qu~drat 5. Quadrat 6 is
not included because no Prasiola occurred in it. Only
quadrat 1 is shown for the fence control because no Prasiola
occurred in the lower control quadrats. N = 3 for each
treatment. Expressed as mean percent cover± 1 s.d.
43
plots
~
r:J)
'l"""f
+I
. .K Shades Removed
""'
>
0
Q)
u
~
Cl
Q)
~
""'
Q)
~
=
~
Q)
~
0
March Apri I May June July August
Time (months)
Figure 1
44
(a)
Figure 2
45
(
(
(b)
Figure 2
(
46
r
\
(c)
Figure 2
47
60
Mean
Wet Weight
(mg)
3%
50
40
1%
30
0%
20
5%
100%
10
10%
50%
0
May
Jun
Jul
Time (months)
Figure 3·.
48
Aug
Sep
Figure 4 (a)
49
Figure 4 (b)
so
xclusion
Grazer
,lfA-....-....-e--.,
100
'-
Q)
;r tf/--t t--1,,'i \ -H
>
0
()
60
¥
.......
c
Q)
V1
f-l.
.
u
40
'-
(])
a..
20
c
t'j
(])
0
~,
,$
80
~
~
-·~
,--
'¥
·It-····"· .. ·~··.
~
~
'•,
it!
.
·&·... .. .. ..
f)
•• llr ••
·It>·'""'' •.
.:........,.. -•..,.. .a; L·=.~Jr;r~ • .........,_...~ -~ ~·.:.::.tn •
0 N D J
F M A M J
J
Time (months)
Figure 5
A S 0 N
• - Prasiola control
---~~-~
Quadrat #
1
·--o--.....,.._..
3
•6
4
I
I
#
ll.
.........
2
5
- o - Fence control