Global Change Biology (2010) 16, 1860–1869, doi: 10.1111/j.1365-2486.2010.02168.x
Decreased precipitation exacerbates the effects of sea
level on coastal dune ecosystems in open ocean islands
T A R A L . G R E AV E R * and L E O N E L S . L . S T E R N B E R G w
*Office of Research and Development, National Center for Environmental Assessment, Tara Greaver, U.S. Environmental Protection
Agency, Mail Drop B-243-01, Research Triangle Park, NC 27711, USA, wDepartment of Biology, Leonel S.L. Sternberg, University
of Miami, Coral Gables, FL, USA
Abstract
The alteration of fresh and marine water cycling is likely to occur in coastal ecosystems as climate change causes the
global redistribution of precipitation while simultaneously driving sea-level rise at a rate of 2–3 mm yr!1. Here, we
examined how precipitation alters the ecological effects of ocean water intrusion to coastal dunes on two oceanic
carbonate islands in the Bahamas. The approach was to compare sites that receive high and low annual rainfall and
are also characterized by seasonal distribution (wet and dry season) of precipitation. The spatial and temporal
variations in precipitation serve as a proxy for conditions of altered precipitation which may occur via climate change.
We used the natural abundances of stable isotopes to identify water sources (e.g., precipitation, groundwater and
ocean water) in the soil–plant continuum and modeled the depth of plant water uptake. Results indicated that
decreased rainfall caused the shallow freshwater table on the dune ecosystem to sink and contract towards the inland,
the lower freshwater head allowed ocean water to penetrate into the deeper soils, while shallow soils became
exceedingly dry. Plants at the drier site that lived nearest to the ocean responded by taking up water from the deeper
and consistently moist soil layers where ocean water intruded. Towards the inland, decreased rainfall caused the
water table to sink to a depth that precluded both recharge to the upper soil layers and access by plants. Consequently,
plants captured water in more shallow soils recharged by infrequent rainfall events. The results demonstrate dune
ecosystems on oceanic islands are more susceptible to ocean water intrusion when annual precipitation decreases.
Periods of diminished precipitation caused drought conditions, increased exposure to saline marine water and altered
water-harvesting strategies. Quantifying species tolerances to ocean water intrusion and drought are necessary to
determine a threshold of community sustainability.
Keywords: Caribbean islands, climate change, coastal plants, eco physiology, ocean water intrusion, oxygen isotopes, water cycle,
water relations
Received 27 May 2009; revised version received 20 October 2009 and accepted 28 October 2009
Introduction
The water cycle of coastal ecosystems will likely be
affected by the combination of altered precipitation and
sea level rise as climate change proceeds. There are 2100
barrier islands, 49 sovereign island nations and 29
island territories that will likely experience an alteration
of fresh and marine water cycling. Global warming is
predicted to redistribute rainfall, causing more frequent
and prolonged drought in some areas, for example the
median value for precipitation in the Caribbean is
projected to decrease by 12% from current values
(Christensen et al., 2007). In addition to altered precipitation, the current average rate of sea level rise,
3 mm yr!1 along many coastlines, is predicted to accelCorrespondence: Tara Greaver, tel. 1 1 919 542 2435, e-mails:
[email protected] and [email protected]
1860
erate with continued climate change (Bindoff et al.,
2007). The rising sea will likely increase maritime
influence on soil hydrology which, in turn, affects the
function of terrestrial coastal ecosystems (White, 1983;
Haines & Dunn, 1985; Sternberg & Swart, 1987; Ross
et al., 1994; Williams et al., 2001; Greaver & Sternberg,
2006, 2007). The ecological effect of simultaneous
changes in precipitation and sea level on coastal ecosystems is poorly understood.
It is vital to determine how alteration of the balance
between fresh and saline water may affect ecotonal
coastal systems. Salinity is a physiologic stress for many
plant species that may lead to decreased reproduction
and survivorship (Williams et al., 1999). Coastal ecosystems protect coastlines from erosion by stabilizing and
accreting sandy soils that mitigate sea surge from daily
tides, episodic storms and tsunami events (Liu et al.,
2005). As these ecosystems protect the interior, they are
Published 2010
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D E C R E A S E D P R E C I P I TAT I O N E X A C E R B AT E S S E A L E V E L
poised to bear the first impacts of the rising sea. It is
estimated that 41% of the world’s population lives
within 100 km of the coasts (Martinez et al., 2007) and
benefit from the services coastal ecosystems provide,
such as stabilizing soils and mitigating erosion.
Typically, ocean water input to coastal ecosystems
may occur from above ground via sea spray or overwash, or from belowground via ocean intrusion to the
water table. Salt spray is aerosolized ocean water that
was recently shown to penetrate the surface soil, percolate downward and mix with the existing soil water of
sand dunes (Greaver & Sternberg, 2007). The robustness
of the freshwater lens in combination with the level of
the sea would govern whether or not ocean water
intrusion occurs. The water table at the base of barrier
and open-ocean islands is typically an unconfined
aquifer (i.e., there is no confinement from below by a
layer of impermeable rock), which is a lens of low
density freshwater that floats above a layer of highdensity saltwater (the so-called Gyben–Hertzberg
Lens). The lens is typically recharged only by precipitation (minus evaporation) in open-ocean islands, however barrier islands may be recharged by an additional
source, the continental aquifer. If precipitation decreases it is possible that the lens would shrink along
its margins causing ocean water to intrude and salinize
areas normally occupied by freshwater. Ocean water
intrusion was not found in a barrier island dune system
in S. Florida (Greaver & Sternberg, 2007). However,
open-ocean islands may be more vulnerable to intrusion because the only source of recharge is precipitation.
Furthermore, the interaction between precipitation and
the frequency and extent of above- and belowground
ocean water input to a coastal ecosystem may alter the
vegetation community structure and function.
We examine how the amount of precipitation alters
the ecological effects of ocean water intrusion to coastal
dunes in sites with high and low annual rainfall that are
also characterized by seasonal (wet and dry season)
distribution of precipitation. The spatial and temporal
variations in the amount of precipitation create in situ
conditions to compare the effects of high and low rainfall. This comparison serves as a proxy for conditions of
altered precipitation which may occur via climate
warming. We test the hypothesis that varying levels of
precipitation influence the severity, frequency and
mechanisms of ocean water input to ecosystems by
using natural abundances of stable isotopes to identify
water sources in the soil in conjunction with soil moisture and salinity measurements. The consequential
effects of the water sources on dune ecosystem function
are assessed using natural abundances of stable isotopes in vegetation, water source apportionment and
uptake modeling.
1861
Methods
Site description
The two coastal dune systems studied were: Dead Man’s Reef
(26155 0 N, 78170 0 W) in Grand Bahama (GB) island, Bahamas;
and East Beach (2413 0 N, 74130 0 W) in San Salvador (SS) island,
Bahamas (Fig. 1). GB and SS islands are located on the
Bahamian bank/platform system separated by the Straits of
Florida from the outer margin of the Florida inner continental
platform (Enos, 1977). These dune sites are characterized by
subtropical vegetation (Greaver & Sternberg, 2006), share
some similar species and are characterized by low elevations;
the maximum elevation of the first and second dune crests
ranging between 1.48 and 2.06 m a.s.l. during the wet season
2001 (Greaver & Sternberg, 2006). These sites all have quartz
and carbonate soils, and humid subtropical climates. Both
islands experience seasonal precipitation with 80% of the
annual rainfall occurring in the wet season from June to
November. The two islands also fall along a rainfall gradient.
The approximate annual rainfall of GB is 1450 mm and SS is
1000 mm.
The first 5–12 m inland of the high tide mark was designated
the fore dune (FD), this area did not extend inland beyond the
middle of the first dune crest. The back dune (BD) was
designated between 35 and 50 m inland from the high tide
mark. The FD and BD positions represent near and far distances from the ocean and are the extremes of a potential
gradient in hydrologic conditions across the dune field.
Stem, soil and water source collection
Soil samples were collected July 20–August 14, 2001 (mid-wet
season); October 27–November 10, 2001 (late wet season);
February 16–March 2, 2002 (mid-dry season); and April 25–
May 21, 2002 (late dry season). Stable isotope analyses were
done for all of the samples except for those collected in the late
Fig. 1
Map of the Bahamas with field sites indicated by circles.
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1862 T . L . G R E AV E R & L . S . L . S T E R N B E R G
dry season. Plant stems and soil samples for the same site were
collected within a period of 2 days of the late wet seasons
(November) and the mid-dry season (February). Ocean water
was collected at the same time as soil and stem samples. Rain
water was collected at each study site over the interval of July
to November for the wet season and over the interval of
February to April for the dry season.
For each dune position (FD and BD), three soil cores were
taken 30 m apart along a transect running parallel to the ocean
(N 5 6 cores/site/season). Soil samples were collected with a
sand auger throughout the vadose layer every 25 cm, from
0.1 m depth to the level of saturation by the water table; depth
varied by site (0.75–2.25 m from the dune surface). Soil samples at each depth were divided at the time of collection into
separate vessels to measure moisture content, conductivity
and stable isotope value of pore water. Soil samples were
weighed and then dried at 60 1C for 5 days to calculate the
gravimetric water content (ym), according to the definition of
Or & Wraith (2001). After samples were dried, 10 mL of
deionized water was added and agitated for 1 min before
measurement of the electrical conductivity (msiemens) of the
soil water (Portable conductivity meter; Oakton Instruments,
Vernon Hills, IL, USA). Salinity (ppt) was calculated from
conductivity and soil water content (Alpha et al., 1996).
Three-way ANOVAs (JMP Version 4.0.4; SAS Institute, Cary,
NC, USA) tested the effects of depth, dune position, season
and their interactions on the moisture content and salinity of
soils at each site.
Stem samples were collected from five species in the FD and
five different species in the BD (Table 1). Species were selected
to represent the vegetation association in each dune position,
Table 1
Species investigated in this study
FD
Ambrosia hispida
Caesalpinia bondoc
Casasia clusifolia
Coccoloba uvifera
Conocarpus erectus
Ipomoea pes-caprae
Iva imbricata
Lantana involcrata
Mallotonia gnaphalodes
Panicum sp.
Scaevola plumieri
Sesuvium portulacastrum
Spartina sp.
Sporobolus virginicus
Suaeda linearis
Strumphia sp.
BD
Grand
Bahama
FD
BD
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
dð%Þ ¼
!
"
Rsample ! Rstandard
& 1000;
Rstandard
ð1Þ
where Rsample represents the molar ratio of heavy to light
isotope in the sample and Rstandard is the molar ratio of the
Vienna-Standard Mean Ocean Water (V-SMOW).
Depth of water uptake
San
Salvador
Species
species were chosen based on their distribution (Greaver, 2005)
and nomenclature followed Wunderlin (1998). For example, a
species collected in the FD either only occurred within the FD
or occurred in the FD with the greatest abundance compared
with other dune positions. The same species were collected
from each site when possible. Stem samples were collected
from four individuals of each species, placed in pyrex tubes
and sealed by threaded caps that were secured with parafilm
for transport to the lab where the samples were frozen until
analysis. To prevent water loss from the stems by transpiration, care was taken to collect suberized stems or in the case of
grasses, to collect stolons or stem sections with multiple sheath
layers.
Water was distilled from the soil and stem samples by
vacuum distillation (Sternberg & Swart, 1987) for isotopic
analysis. Oxygen isotope ratios of water were determined by
a technique originally developed by Epstein & Mayeda (1953),
modified so that a 1 mL aliquot of the sample water was
equilibrated with 5 mL of CO2 gas (at 1 atm and 25 1C) for 2
days. The CO2 was then extracted by cryogenic distillation and
its isotopic value measured using an isotope-ratio gas mass
spectrometer (VG Prism; Micromass, Middlebury, England)
with a precision of " 0.1%. Isotopic values were expressed in
d units, described by the following equation
x
x
x
x
x
Fore dune (FD) and Back dune (BD) association is noted for
each site. Authorities for scientific names given by Wunderlin
(1998).
A model introduced by Romero-Saltos et al. (2005) that couples
the d18O values from the soil profile with the d18O values of
sap water was used to determine the depth from which plants
take up water. The average d18O value of soil water at each
depth constitutes the soil profile, which is unique to each dune
position (fore and back), sampling date (July, November,
February), and site (GB and SS). The isotopic value of the stem
water is evaluated with respect to the soil profile from the
same dune position, date and site to calculate the mean depth
of water uptake. The model assumes: (1) a plant can uptake
water from a single vertical segment of soil, here assumed to
be 20 cm long, (2) water taken up from that segment follows a
normal distribution (Sokal & Rohlf, 1995) and (3) the d18O
signature of plant stem water is equal to the sum of d18O
signature of soil water absorbed by the roots. The results of the
model calculated with water uptake from a 20 cm segment of
the soil profile were linearly correlated with results calculated
with a segment length of 50 cm to determine the sensitivity of
this model to the segment length (r 5 0.94, slope 5 1.02,
Po0.001). The slope of the correlation shows that for a 60%
change in the parameter (i.e., length of soil segment), there is
only a ' 2% change in model output (i.e., in the calculated
mean depths of water uptake), indicating little sensitivity of
the model to segment length.
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When the model yielded two or more possible uptake
depths for a single stem, multiple solutions were averaged if
there was no more than 30 cm difference between the depths
calculated by the model. Stem samples within the same genus,
sampling date, site and dune position (N 5 4 per species) were
analyzed under the following conditions. We assumed individuals of the same species took up water from the same depth;
therefore the uptake depth of individuals with a single solution was compared with the possible uptake depths for
individuals with multiple solutions. If one of the multiple
solutions was within 10 cm of the value of the single solution
it was consider correct and the others were discarded. Oneway nested ANOVA tested the effect of dune position and
season on depth of water uptake as calculated by the model.
Mixing model
Traditionally, it was difficult to determine stem water composition of vegetation in a system with more than two water
sources because of the 2n-member limitation of mass-balance
mixing models. However, a recently published three-source
mixing-model (Zencich et al., 2002) allows us to calculate the
relative contribution of groundwater (saturated region of the
soil), rain water and ocean water to stem water from the d18O
value of the stem water. This model does not give a discrete
solution, but calculates a range of possible solutions. If the
mixing model yielded a range of possible solutions in which
the lower limit is zero, we conclude the species was not an
obligate user of ocean water. Data presented graphically
represent the mean value for the range of possible ocean water
use for each species (N 5 3 or 4 for each species).
Results
The depth of the soil sample had a significant effect on
soil water content at both sites (Table 2). Water content
was least in the shallow soil samples (' 3%) and greatest in the deepest soil layers (' 25%). Sampling date
also significantly affected soil water content at both sites
(Table 2), the driest conditions occurring in the driest
months (Fig. 2). However, only the SS site had statistically greater water content in FD than BD soil (Table 2).
The factors of sampling date, depth, dune position
and the interaction between depth and dune position
significantly affected soil salinity at both sites (Table 2).
The mean salinity of soil pore water across all depths
and seasons was statistically greater in the FD at both
sites (GB 5 6.2 ppt, SS 5 12.9 ppt) than the BD
(GB 5 2.8 ppt, SS 5 3.8 ppt, Tukey–Kramer HSD,
P 5 0.5). Average soil salinity across all depths and
dune positions was highest in February, the mid-dry
season (GB 5 8 ppt, SS 5 5 ppt, Tukey–Kramer HSD
P 5 0.05), significantly greater from all other months at
GB, but February was equal to April at SS. Salinity
increased with depth at GB and SS (Fig. 2). Soil pore
water salinity values obtained at the two deepest pro-
1863
files were between 13 and 16 ppt on GB and the three
deeper soil profiles during the dry season at the SS site
were between 26 and 29 ppt, close to values of 35 ppt
commonly observed for ocean water (Fig. 2).
The d18O value of ocean water remained constant for
both sites (GB 5 1 2.02%, and SS 5 1 1.34%; Fig. 2).
The d18O value of rain water was more depleted during
the rainy season (GB 5!2.6%, SS 5!3.6%), than the
dry season (GB 5!1.1%, and SS 5!1.5%; Fig. 2). Meaningful statistical comparisons of d18O values of soil
water cannot be made among sites or among seasons
because the d18O values of environmental source waters
(ground and rain water) shift. However, a comparison
of dune positions within the same site and during
February revealed that the average d18O of soil across
all depths is significantly more enriched in the FD
(GB 5!1.37%, SS 5!0.32%) than the BD (GB 5!3.4%,
SS 5!2.6%, Tukey–Kramer HSD P 5 0.5, and Fig. 2).
The model to calculate depth of water uptake yielded
solutions for 86% of the 219 stem samples. However,
43% of the solutions yielded two or more possible
uptake depths for a single stem. Multiple solutions
were averaged if there was no more than 30 cm difference between the depths calculated by the model as
discussed in the methods. Of those samples that were
not solved (14%), the majority were from the FD, and
were equally enriched or depleted beyond the values
within the average soil profile.
The mean of all species grouped together indicates
those from the FD at SS took up water deeper in the soil
profile during the dry season (D) compared with the
wet (W) season (Fig. 3; D 5 82.9 " 0.5 cm and
W 5 69.2 " 1.2 cm, Tukey–Kramer HSD, Po0.05); however, at GB the depth of uptake becomes shallower for
plants over the same period (Fig. 3; D 5 57.9 " 0.7 cm
and W 5 50.0 " 0.3 cm, Tukey–Kramer HSD, Po0.05).
Species from the BD association at SS and GB take
up water from significantly more shallow layers of
the profile in the dry than wet season (Fig. 3;
GB: D 5 29.6 " 0.8 cm and W 5 63.4 " 1.3 cm, SS:
D 5 70.7 " 1.1 cm and W 5 102.3 " 1.2 cm, Tukey–
Kramer HSD, Po0.05).
Water-harvesting strategy of the all FD species shifted
from the dry to wet seasons at SS. Three out of the four
FD species (there were insufficient samples to include
Mallotonia sp. in this analysis) shifted from solely freshwater to mixed fresh/marine water uptake (Fig. 4). BD
species used no ocean water, however their uptake of
rainwater increased in the dry season. On average,
27 " 2.2% of stem water from the BD vegetation association was captured rainwater (vs. 73% groundwater)
in the dry season as opposed to 9% " 3.7 rainwater (vs.
91% groundwater) during the wet season (Fig. 4). This
difference in groundwater usage was significant when
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1864 T . L . G R E AV E R & L . S . L . S T E R N B E R G
Table 2 Three-way ANOVA table for the effects of sampling date, depth, dune position and their interactions on gravimetric water
content of soil pore water (A and B), and salinity (C and D)
(A) Grand Bahama (water content)
Whole model
Effects test
Sampling date
Depth
Dune position
Sampling date & depth
Depth & dune position
Sampling date & dune position
Sampling date & depth & dune positions
(B) San Salvador (water content)
Whole model
Effects test
Sampling date
Depth
Dune position
Sampling date & depth
Depth & dune position
Sampling date & dune position
Sampling date & depth & dune positions
(C) Grand Bahama (salinity content)
Whole model
Effects test
Sampling date
Depth
Dune position
Sampling date & depth
Depth & dune position
Sampling date & dune position
Sampling date & depth & dune positions
(D) San Salvador (salinity content)
Whole model
Effects test
Sampling date
Depth
Dune position
Sampling date & depth
Depth & dune position
Sampling date & dune position
Sampling date & depth & dune positions
R2adj
df
Sum of squares
P-value
0.67
31
3529.49
o0.0001*
3
3
1
9
3
3
9
211.06
3016.36
15.93
90.80
54.55
78.85
48.21
0.0012*
0.0001*
0.31
0.76
0.33
0.18
0.95
47
5518.3
o0.0001*
3
5
1
15
5
3
15
622.13
2050.95
1782.89
212.62
759.26
12.29
51.19
o0.0001*
o0.0001*
o0.0001*
o0.0001*
0.98
0.74
0.14
15
1911.77
o0.0001*
3
1
1
3
1
3
3
420.88
148.41
294.8
135.17
309.66
169.39
234.89
0.0012*
0.0152*
0.0008*
0.14
0.0073*
0.0096*
0.0260*
15
7800.72
o0.0001*
3
1
1
3
1
3
3
1157.36
498.32
2694.01
110.27
761.59
1535.89
234.89
o0.0001*
0.0006*
o0.0001*
0.4383
0.0005*
o0.0001*
0.1303
0.78
0.38
0.55
*Statistically significance (a 5 0.05, Po0.01).
all species were considered together (P 5 0.0004). This
mixing model could not be used for stem samples at GB
because the range of stable isotope values in the soil
water was greater than the range for the three water
sources. For example, the FD soil (10 cm) in the wet
season was !10% (Fig. 2) whereas the rain, ocean and
groundwater were 2%, !2% and !3.6%, respectively. It
is unclear why the average soil water was so much
more depleted than the rainwater. However, in this
circumstance the mixing model cannot accurately calculate source water.
Discussion
Our results revealed new insights to how coastal ecosystems may be affected by patterns of precipitation
and sea-level predicted to occur with future global
change. The results indicated that the complex mixture
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1865
Fig. 2 Wet (November) and dry (February) season depth profiles of d18O value and salinity of soil water found in the fore ( & ) and back
(&) dune positions. Each marker indicates the mean value at each depth (N 5 3), error bars are " 1 SE. The d18O value of ocean water is
indicated by the stippled line (- - - -) and the d18O value of rainwater is indicated with the solid line (——) without markers. The d18O
value of ocean water are more enriched than all other water sources on the dune (GB 5 1 2.02%, and SS 5 1 1.34%). The d18O value of
rain water was more depleted during the rainy season (GB 5 !2.6%, SS 5 !3.6%), than the dry season (GB 5 !1.1%, and SS 5 !1.5%).
of precipitation, groundwater and ocean water in coastal soils wax and wane both spatially and temporally.
Ocean water becomes a more dominant component of
the water cycle during the dry season, at the same time
more arid conditions are observed in shallow soils,
especially in higher elevation sites. The water uptake
patterns of vegetation responded in unexpected ways
that suggest salinity and drought may interact to cause
ecosystem vulnerability.
The temporal and spatial distribution of soil moisture
showed that dunes are characterized by a dynamic
interaction between rainfall and elevation. As expected,
the driest soil conditions occurred during the months
receiving the least rainfall (Fig. 2). Surprisingly, the BD
was significantly more arid than the FD at SS, a pattern
not observed at GB (Fig. 1, Table 2). This discrepancy
between sites is explained by their contrasting landward elevations (elevation above mean sea-level
GB 5 0.36 m and SS 5 2.06 m, Greaver & Sternberg,
2006). The sharp rise in elevation of the BD at SS caused
the distance between the soil surface and the water table
to increase to 2 m (Fig. 3). We suggest the deep water
table does not contribute to the water content of shallow
soil layers (o1 m) because the process of capillarity
generally augments the water content of sand only
within 1 m above the water table (Wentworth, 1942).
The results indicate elevation is an extremely important
factor affecting the spatial distribution of water within
the soil profile of dune ecosystems.
Higher soil salinity has been observed in areas nearest to the ocean in previous studies (Greaver & Sternberg, 2007; Forey et al., 2008). In our study, soil nearest to
the ocean became more saline during the dry months,
making the salinity gradient from the ocean towards the
inland more severe at both sites during these times
(Table 2, Fig. 2). We rule out tidal height as the mechanism increasing salinity because more saline conditions
would be expected during periods of higher tides, the
nearest tidal measurements to our study sites indicate
tides were more than 0.5 ft lower on average in the dry
than wet season. (Tidal measurements available for
Waitling Island, SS and Settlement point, GB from
http://www.irbs.com/tides) The depth of elevated salinity and enriched 18O in the soil profile indicated the
mechanism of ocean water input was ocean water
intrusion from below ground more commonly than salt
spray deposition from above ground. There was only
one event (out of eight for both sites) when sea spray
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1866 T . L . G R E AV E R & L . S . L . S T E R N B E R G
Fig. 3 Dune soil profiles indicating soil moisture and depth of water uptake by fore and back dune vegetation in the wet season (dark
gray line) and the dry season (light gray line). The brown horizontal rectangle indicates the depth of the rock in the Grand Bahama back
dune N 5 12–18 plant samples for each season within the same dune position. The X indicates the model solution for the depth of water
uptake by plants.
input was indicated by significantly higher soil salinity
and enriched d18O in the shallow layers (10–25 cm
depth) of the FD soil (SS-Nov Fig. 2). These findings
differ from those reported for barrier island dunes in S.
Florida which indicated ocean water entered dune soils
via salt spray more frequently (Greaver & Sternberg,
2007).
Ocean water intrusion occurred more frequently
and/or to a greater extent in the dry than wet season.
The results indicate the reduced rainfall caused the
freshwater lens to retract inland and exacerbated saltwater intrusion, especially at SS where events were
observed at all sampling dates, but were more pronounced in the dry season (Fig. 2). Ocean water intrusion was only detected once in GB. The second dry
season measurement in April (data not shown) was the
highest observed salinity (' 20 ppt) of all sampling
dates in the lower soil profile (75–100 cm). This pattern
of salinity indicates ocean water intrusion; however, the
stable isotope data for this date is not available for a
secondary confirmation.
Freshwater lens morphology on small carbonate islands, like the sites studied here, is typically governed
by hydraulic conductivity and precipitation (minus
evapotranspiration) (Schneider & Kruse, 2003).
Hydraulic conductivity is often driven by the underpinning geologic material, in this case, carbonate. Porosity
of carbonate increases with age, and therefore hydraulic
conductivity typically increases with age of the parent
material. Since the two study sites were formed in the
same period it is unlikely that age of the material causes
differences in the lens. Our study found that the dunes
which received the lesser amount of annual rainfall
were vulnerable to ocean water intrusion throughout
the year. These islands fall along a rainfall gradient that
inversely correlated with the frequency of ocean water
intrusion to the dunes. GB receives 1450 mm of annual
rainfall and we only recorded one event of ocean water
intrusion at the very end of the dry season. Whereas SS,
which receives 1000 mm of annual rainfall, showed
ocean water intrusion throughout the year and to the
greatest extent in the dry season.
In contrast to our results, ocean water intrusion was
not observed in a study of dunes from Key Biscayne, a
coastal barrier island in Florida (Greaver & Sternberg,
2007). The annual rainfall of Key Biscayne (1325 mm)
is intermediate to the two Bahamian sites in this
study; therefore precipitation probably did not preclude
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D E C R E A S E D P R E C I P I TAT I O N E X A C E R B AT E S S E A L E V E L
Fig. 4 Stem water composition of fore and back dune species at
the San Salvador (SS) site during the November dry (d) and
February wet (w) seasons. Ocean water (OW), groundwater
(GW) and rain water (RW) are shown in solid colors, while areas
with diagonal bars indicate a freshwater mixture of RW and GW.
intrusion. However, Key Biscayne is very close to the
North American continent and its freshwater lens may
be bolstered by the Biscayne continental aquifer, as
occurs on other sandy barrier islands (Schneider &
Kruse, 2003). We suggest ocean water intrusion may
be more common on dunes of coralline rock islands in
this study because the freshwater lens is typically not
recharged by a robust continental aquifer as may occur
on barrier islands.
Spatial and temporal patterns in the depth of water
uptake by vegetation respond to changing interactions
between precipitation, ground and ocean water. First, it
is important to note that our data confirm that biologically meaningful investigations of soil conditions in
dune systems should include measurement to a depth
of at least 1 m. The depth of water uptake was often
from soil layers between depths of 30 cm and 1 m (Fig.
3). Numerous investigations have quantified soil characteristics only in the upper 30 cm (Oosting & Billings,
1867
1942; Boyce, 1954; Barbour et al., 1973; Sykes & Wilson,
1991; Wilson & Sykes, 1999; Stallins, 2001; Forey et al.,
2008). We strongly suggest this is insufficient to understand the full hydrologic conditions from the plant’s
perspective.
There is a significant seasonal effect on the depth of
water uptake (Fig. 3). The dune plants show different
patterns at each site which correspond to the local soil
hydrology. In GB both the fore and BD plants, on
average, took up water from more shallow soil layers
during the dry season (Fig. 3). This may be to avoid the
increased salinity in the deeper soils of the FD (Fig. 2)
and to access the more shallow layers in the BD as the
water table sunk into the limestone rock creating drier
conditions (soil moisture at 75 cm in the soil profile
D 5 14.7 " 4.8 and W 5 21.8 " .0.6 ym). As previously
discussed, the water table at SS is deeper during the dry
than wet season across the dune, likely in response to
low recharge by rainfall. The FD plants at SS avoided
the shallow dry soil and drew from slightly deeper
( '13 cm) layers of soil moisture at SS. The elevation
of the back dune precluded abundant water recharge
from the freshwater lens to the upper soil layers. We
expected that plant water uptake would be deeper in
the dry season when the lens was deeper. Surprisingly,
the BD plants tended to take water from more shallow
levels. This may occur due to the high elevation of this
dune, the distance between the dune surface and water
table increased during the dry season, so much so
(42 m) that plants no longer invested the C resources
needed to grow roots to chase the water. Instead the
roots would catch the episodic rainfall in the more
shallow layers (Fig. 4).
The FD plants had a more complicated hydrologic
environment than the BD because they are exposed to
the osmotic stress of drought in the upper soil layers
and salinity in the deeper layers. In the dry season at SS,
the same time the FD plants avoided the shallow dry
soil and drew from slightly deeper layers, 73% (N 5 15)
of individuals (and three out of four species) took up
ocean water (0.6%–54.0% total stem water, Fig. 4). This
supports that the FD is a mixed marine/freshwater
system. Some FD species demonstrate physiologic plasticity in response to the osmotic challenges caused by
elevated salinity (Greaver & Sternberg, 2007); however,
prolonged exposure may result in decreased biomass
and ultimately death (Goldstein et al., 1996; Greaver &
Sternberg, 2007). Quantifying species-specific tolerances
to ocean water intrusion will be necessary to determine
a threshold of community survivorship. Once the
threshold is surpassed, belowground plant structures
that are important for soil cohesiveness will not regenerate and the land will be susceptible to higher rates of
erosion.
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1868 T . L . G R E AV E R & L . S . L . S T E R N B E R G
The interactions between marine and freshwater cycling that are identified here begin to address how dune
ecosystems will be effected by future scenarios of global
change, namely the redistribution of rainfall (Neelin
et al., 2006; Wang et al., 2006) and sea-level rise (Overpeck et al., 2006). Many tropical and subtropical regions
are anticipated to experience diminished precipitation
as global climate warms (Chen & Taylor, 2002; Kumar
et al., 2006). Our study indicates decreased precipitation
due to climate change may threaten the sustainability of
dune ecosystems by two mechanisms: limiting water
availability and facilitating ocean water intrusion. We
found that rainfall is a greater portion of total water
uptake by plants during dry periods, and we suggest
decreased rainfall could threaten the sustainability of
higher elevation dune ecosystems, which can be especially dependent on rainfall. Additionally decreased
precipitation could lead to retraction of the freshwater
lens and more intense ocean water intrusion into the
ecosystem. Additional pressure on the lens exerted by
rising seas would compound intrusion. Our evidence
shows that some dune plants can uptake ocean water;
however more frequent periods of ocean water in the
terrestrial dune will cause prolonged osmotic challenges. The threshold of their tolerance will be important in order to determine what degree of ocean water
intrusion will result in changes to species survivorship,
community composition and the extent to which this
ecosystem can protect against land erosion by the rising
sea.
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Acknowledgements
The authors acknowledge this work would not have been
possible without funding from the EPA STAR program and the
University of Miami. We would like to thank Mike Storck, Hugo
Saltos-Romero, Hannah Thornton, Albert Greaver, Perry Tripp,
Linda Greaver and Kim Andrascik for their help conducting
field work. We are grateful to the Bahamian National Trust, H.
Fishbacher (the Buchaneer Club) and the Gerace Research station
for logistical assistance. This article was reviewed by the National Center for Environmental Assessment, U.S. EPA and
approved for publication. Approval does not signify that the
contents necessarily reflect the view and policies of the Agency
nor mention of trade names or commercial products constitute
endorsement or recommendation for use.
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