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Estuarine, Coastal and Shelf Science 154 (2015) 184e193
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Estuarine, Coastal and Shelf Science
journal homepage: www.elsevier.com/locate/ecss
Large-scale dynamics of sandy beach ecosystems in transitional
waters of the Southwestern Atlantic Ocean: Species turnover, stability
and spatial synchrony
Diego Lercari a, b, *, Omar Defeo a, b
a
b
4225, Montevideo, 11400, Uruguay
UNDECIMAR, Facultad de Ciencias, Igua
n Este, Ruta nacional N 9 interseccio
n con Ruta N 15, Rocha, Uruguay
GEPEIA, Centro Universitario de la Regio
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 July 2014
Accepted 4 January 2015
Available online 10 January 2015
Transitional waters (TW) are interfaces between the terrestrial and freshwater environments and the sea.
These ecotones are characterized by highly dynamic physico-chemical and hydro-morphologic conditions, resulting in a mosaic of habitats in which species are particularly well adapted to variability.
However, sandy beach ecotones occurring along estuarine gradients are rarely addressed from the TW
perspective. We conducted a 2-yr study to assess the seasonal dynamics of environmental and macrofaunal descriptors in 16 sandy beaches of the Uruguayan coast in TW defined by the widest estuary of the
world (Rio de la Plata). A strong variability in environmental conditions was found at inner estuarine
beaches, reflecting the seasonal dynamics of the estuarine discharge. The greatest abundance and species
richness found in dissipative oceanic beaches were also characterized by their lowest temporal variability, indicating that macrofaunal communities were more stable towards oceanic conditions, where
environmental variability was also lowest. Spatial synchrony was reflected in changes across seasons in
the species richness in the TW system. A high turnover of species along spatio-temporal gradients
occurring within the TW ecotone was observed. Mollusca and Polychaeta were absent in highly-variable
estuarine beaches, irrespective of the morphodynamic state. A functional equivalence between species
was found at the extremes of the salinity gradient. The environmental variables that best explained
community patterns differed among seasons: in summer and autumn, salinity, wave period and beach
width were the main explanatory factors, whereas temperature had a primary influence in winter and
morphodynamic variables exerted a major influence in autumn. We highlight the need to consider
concurrent variations in estuarine and morphodynamic variables when assessing the spatial distribution
of macrofaunal species richness and abundance in sandy beaches occurring along TW.
© 2015 Elsevier Ltd. All rights reserved.
Keywords:
Uruguay
transitional waters
Río de la Plata
sandy beaches
morphodynamics
benthic macrofauna
species richness
seasonal dynamics
1. Introduction
Populations of plants and animals gradually change along gradients of physical conditions, shaping distinct ecological communities within broadly defined ecosystems, such as forests,
grasslands and estuaries (Ricklefs, 1990; Gaston, 2000). Every
coastal habitat exists within a multi-dimensional mesh of environmental gradients, notably related to four main sources of variation: parameters that change across-shore (e.g. temperature,
humidity), wave exposure, sand particle size and salinity (Raffaelli
and Hawkins, 1996).
4225, Mon* Corresponding author. UNDECIMAR, Facultad de Ciencias, Igua
tevideo, 11400, Uruguay.
E-mail address: [email protected] (D. Lercari).
http://dx.doi.org/10.1016/j.ecss.2015.01.011
0272-7714/© 2015 Elsevier Ltd. All rights reserved.
Transitional waters (TW) are ‘bodies of surface water in the vicinity of river mouths which are partially saline in character as a
result of their proximity to coastal waters but which are substantially influenced by freshwater flows’ (Directive, 2000/60/EC). They
comprise estuaries, deltas and coastal lagoons, defining ecosystems
with unique functional and structural characteristics (Cognetti and
Maltagliati, 2008; Basset et al., 2013). Indeed, TW are ecotones
between land, sea and freshwater, characterized by highly dynamic
physical, chemical and hydro-morphologic conditions, resulting in
a mosaic of habitats in which species are particularly well adapted
to variability (Elliott and Whitfield, 2011). Ecotone dimensions
correspond to major discontinuity boundaries between different
ecosystem types, shaping TW as multi-dimensional, hierarchically
organised, ecotones. The hydrodynamic ecotone dimension (1st
order) mainly reflects a spatial or temporal salinity gradient that
D. Lercari, O. Defeo / Estuarine, Coastal and Shelf Science 154 (2015) 184e193
limits the local number of species (a diversity) for some groups by a
filtering process from the regional species pool (Zobel, 1997;
Mouillot et al., 2007). The spatio-temporal patchiness found in
TW is likely to support high taxonomic turnover among locations (b
diversity) and then a high regional g diversity (Barbone and Basset,
2010). Additional ecotone dimensions occur at other TW interfaces,
including the across-shore axis from the supralittoral to the sublittoral margins (Basset et al., 2013). The relative size (spatial
dimension) and the temporal presence of these ecotones vary with
locality and type of TW.
The dynamics of exposed sandy beach ecotones occurring along
estuarine gradients has been rarely studied from the TW perspective. Exposed sandy beaches are relatively uncommon in TW such as
estuaries, and large-scale macrofauna variations along estuarine
gradients have been scarcely documented. Recent reviews in sandy
beach ecology highlighted the need to consider a multiscale
approach for ecosystem modelling, including the underlying processes and mechanisms (Defeo and McLachlan, 2005, 2011; Fujii,
2007; Schlacher et al., 2008). Alongshore patterns in a single beach
affected by a salinity gradient showed an exponential decrease in
species richness and abundance towards low salinities (Lercari et al.,
2002; Lercari and Defeo, 2003). Temporal variations in freshwater
discharges resulted in local short-term changes in community attributes (Defeo and Lercari, 2004). At the macroscale (hundreds of
km), salinity range become an ecological master factor governing
macrofaunal distribution patterns in sandy beaches located along an
estuarine gradient (Lercari and Defeo, 2006). Tolerance to salinity
changes varies among species according to the beach zone inhabited
(intertidal species have lower tolerance than supralittoral ones) and
development mode (species with planktonic larvae have lower
tolerance than direct developers) (Barboza et al., 2012).
Another source of variability in sandy beach ecotones is defined by
their morphodynamic characteristics. Beach morphodynamics play a
key role in determining the structure of sandy beach communities,
with species richness increasing from reflective (steep slope, coarse
grains) to dissipative (gentle slopes, fine grains) beaches (reviewed in
Defeo and McLachlan, 2013). In this context, sandy beaches occurring
in TW are affected by the highly dynamic hydro-morphologic conditions that characterize these systems, resulting in weaker wave
action and a decrease in wave period and swash width at inner
estuarine conditions (Lercari and Defeo, 2006). However, the dynamics of these intertidal ecotones in TW has yet to be resolved. This
uncertainty arises partly from two main issues: firstly, the traditional
comparison of only one pair of sites in sandy beach studies (e.g. high
and low salinities, one dissipative vs. one reflective beach) cannot
distinguish between linear and nonlinear changes in increasing
severity of the gradients. Secondly, although salinity is a major
explanatory driver of diversity patterns, little is known about how
temporal variations in complex TW systems affect structural and
functional changes in sandy beach communities occurring inside TW.
This paper describes a 2-yr study to assess the spatial and
temporal dynamics of the environment and the macrobenthic
community on sandy beach ecotones occurring along the Rio de la
Plata, the widest estuary of the world that defines a TW system in
the Southwestern Atlantic Ocean. According to the theoretical
frameworks developed for estuaries (Attrill, 2002; Basset et al.,
2013) and sandy beaches (Defeo and McLachlan, 2005), we predict: 1) a lower number of species and abundance in sandy beaches
affected by high salinity variability, and in those with reflective
characteristics; 2) a higher community stability in less variable
environments (both in terms of salinity and beach morphodynamics); 3) a replacement (turnover) of species along the TW
gradient maintaining functional redundancy, and 4) changes in
community structure across seasons occurring synchronically
(spatial synchrony) among beaches.
185
2. Materials and methods
The Rio de la Plata estuary, located at 35 S on the Atlantic coast
of South America, constitutes a major TW system with unique
characteristics. It drains the second largest basin of the continent
with an average freshwater flow of 22,000 m3 s1 (Simionato et al.,
2001). This large and microtidal estuary covers more than 400 km
from the head to the mouth (200 km wide) with a broad permanent
connection to the sea. The estuary is characterized by seasonal
variations in river inputs, being highly sensitive to the atmospheric
forcing, especially to wind-induced turbulence. The strong seasonal
variability is controlled by the balance between onshore and
offshore winds, the river discharge, the tides and the Coriolis force
(Simionato et al., 2001).
The study covered 400 km in the TW of the Uruguayan coast,
where we selected eight beaches along 150 km of the Río de la Plata
estuary (inner beaches) and eight beaches along 250 km of oceanic
shores (outer beaches) outside the mouth of the estuary. Sampling
sites were selected considering, whenever possible, close pairs of
beaches with the same salinity range but markedly different morphodynamic features. Biological samples on each beach were taken
every 2 months from July 1999 to May 2001, along three transects
perpendicular to the shoreline, spaced 8 m apart, from the base of
the dunes to the lower limit of the swash zone. Sampling units
(SUs) on each transect were done every 4 m with a sheet metal
cylinder, 27 cm in diameter and 40 cm deep, and the material was
sieved through a 0.5 mm mesh. In order to cover the entire beach
habitat, the number of samples taken on each beach and sampling
event varied according to the beach width, which was measured as
the distance between the base of the dunes and the lower limit of
the swash zone, where water moves over the beach face after a
broken wave collapses on the sand (Lercari and Defeo, 2006). The
organisms retained were fixed in 5% formalin, counted and identified to the species level (whenever possible). Sampling on each
beach was made at the same time of the day in order to minimize
daily effects associated with sampling conditions. Following Defeo
(1996), macrofaunal abundance was expressed as individuals per
strip transect (ind m1). The total number of species recorded on
each beach was considered as a measure of a diversity (Barboza
et al., 2012).
At each beach, salinity and water temperature were measured at
the surf zone with an YSI 33 thermosalinometer, wave height was
visually estimated and wave period was determined with a stopwatch. At each SU, we measured temperature at the surface and
15 cm depth, sand compaction (kg cm2), and beach slope (Emery,
1961). Sediment samples were also collected at each SU to estimate
granulometric variables (Folk, 1974), sediment moisture and
organic matter content (weight differences between wet, dry and
incinerated samples, respectively). The amount of wrack or carcasses deposited on all the beaches can be considered negligible.
Swash width was measured as the distance between upper and
lower swash limits at sampling time. Mean estimates for each
beach and sampling date were obtained by averaging individual SU
estimates.
Alongshore patterns in salinity were modelled by season, and
the best model was selected according to the coefficient of determination (r2) and statistical significance. Beach morphodynamics
were assessed by Dean's parameter U (Short, 1996):
U¼
Hb$100
Ws$T
where Hb is breaker height (m), Ws is sand fall velocity (cm s1) and
T is wave period (sec). Annual mean values of U were employed to
classify beaches according to their morphodynamic features.
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Between-season differences in the number of species and total
abundance were tested by ANCOVA, using the distance from the
innermost site as a covariate. We tested for assumptions of
normality
(KolmogoroveSmirnov
test),
heteroscedasticity
(Cochran's test) and parallelism, to assess whether the continuous
(distance) and categorical (season) predictors interact in influencing responses (abundance and richness) (Underwood, 1997).
Changes across seasons in environmental and community descriptors was measured by the coefficient of variation
(CV ¼ standard deviation/mean) for each beach across all seasons.
As CV standardizes for the mean, it is less dependent on the mean
than the standard deviation, and provides an index of temporal
variation relative to the mean (Tilman, 1996; McCann, 2000). CVs
were calculated on a beach-by-beach basis, using data gathered on
each beach for all sampling dates. Intra-annual variability in environmental and biotic variables was modeled by the relationship
between the CV and the distance from the innermost beach. Models
were adjusted by nonlinear least squares, using the LevenbergeMarquardt algorithm. Community stability was also evaluated using the CV of total abundance with respect to species
richness. Increases in CV correspond to decreases in stability, and
vice versa. When increasing species richness was associated with
decreasing (increasing) CV, we referred to this as a positive
(negative) effect of richness on stability.
Temporal coherence or spatial synchrony refers to the tendency
of population, community or ecosystem to behave similarly among
locations through time as a result of spatially-correlated environmental stochasticity (Huttunen et al., 2014). In order to assess the
strength of coherence in changes across seasons in abiotic (salinity,
temperature) and community (species richness, total abundance)
descriptors along the estuarine gradient, we used a simple linear
regression approach. Thus, we tested the importance of changes
across seasons through linear regressions between all possible
season pairs. Perfect coherence (i.e. concurrent changes in the
variable occurring in all beaches from one season to another) was
reflected by a significant linear regression between two contrasted
seasons, suggesting that locations behave similarly through time,
i.e., the biotic or abiotic variable changed at a same proportion from
one season to the other.
Changes in environmental conditions across beaches and seasons were assessed by 2-way Permutational Analysis of Variance
(PERMANOVA; Anderson, 2001) included in the PRIMER 6.0 software package (Clarke and Gorley, 2006). This analysis was based on
the Euclidean distance matrix of log-transformed standardized
environmental variables. Two fixed factors (beach and season) and
999 permutations of residuals under a reduced model were
considered in the design, which also assessed the beach season
interaction. A 2-way PERMANOVA was also performed to assess
differences in community structure using rooteroot transformed
data from each site and the BrayeCurtis similarity index, and
following the same design as in environmental variables. To assess
which combination of environmental variables best explained
spatio-temporal variations in macrofaunal abundance, we used the
routine BIOENV (Clarke and Ainsworth, 1993). The degree of significance between matrices was tested with the RELATE routine of
PRIMER 6.0 (Clarke and Gorley, 2006).
Table 1
Parameter estimates and associated statistics of the nonlinear models that relate
salinity (S) and distance from innermost site (D) in each season in TW of the
Southwestern Atlantic Ocean. All models and their parameters were highly significant (p << 0.001).
a
b
c
R2
1.03
4.06
32.71
27.56
0.019
0.021
0.983
0.982
0.81
5.70
28.44
24.49
0.9911
0.9888
0.996
0.987
b
S ¼ ð1eðac$DÞ Þ
Summer
Autumn
S ¼ a þ b$ð1 C D Þ
Winter
Spring
best explained by an asymptotic model, and by a logistic function in
autumn and summer (Table 1; Fig. 1a). Seawater temperature
showed two distinct patterns (Fig. 1b): 1) estuarine beaches presented a warmer period (spring and summer) and a colder one
(autumnewinter), and 2) oceanic beaches showed marked changes
across seasons, being highest in summer and lowest in winter,
whereas intermediate values were registered in spring and
autumn.
Alongshore trends in morphodynamic variables reflected the
selection of beaches during the sampling design (Supplementary
material, Fig. S1 and Table S1). Mean grain size was remarkably
constant in all beaches throughout the period, whereas slope was
steepest on most beaches during autumn (erosion period) and
3. Results
3.1. Environmental factors
Salinity increased from west to east in all seasons and beaches,
and from autumn to summer. The salinity gradient was steeper over
the first 150 km from the innermost site than along the oceanic
sector. In winter and spring, large-scale changes in salinity were
Fig. 1. Seasonal large-scale variations in a) mean salinity (nonlinear models are
shown) and b) swash water temperature from the innermost beach site (0 km) to
oceanic conditions in 16 Uruguayan sandy beaches.
D. Lercari, O. Defeo / Estuarine, Coastal and Shelf Science 154 (2015) 184e193
187
Fig. 2. Large-scale variations in the coefficient of variation of biotic (number of species, abundance) and environmental (temperature, salinity, beach and swash width) variables in
16 Uruguayan sandy beaches located in TW of the Southwestern Atlantic Ocean. All models are highly significant (p < 0.001).
flattest in summer (accretion period), particularly at oceanic beaches. Beaches with coarser grain size had steeper slopes (Fig. S2).
Beach width tended to be greater at beaches with fine grains and
gentle slopes and did not show evident changes across seasons.
Swash width steadily increased from inner estuarine to oceanic
conditions at all seasons. Sediment moisture was higher in wide
beaches with fine grains and gentle slopes throughout the study
period. Dean's parameter U was higher in fine grain and gentle
slope beaches than in coarse-grained beaches with steep slopes,
particularly in oceanic beaches, but no changes across seasons were
detected. The large-scale patterns depicted above were reflected in
higher CV values towards the inner estuary in temperature, salinity,
beach width and swash width (Fig. 2 aed ). By contrast, mean grain
size, beach slope, sediment organic matter and Dean's parameter
did not show evident seasonal variability.
Coherence in changes across seasons varied strongly among
abiotic and biotic response variables. Salinity change on sandy
beaches was remarkably coherent when considering winter, spring
and summer, but this was not the case when comparing autumn
with the remaining seasons (Supplementary material Fig. S3).
Temperature coherence was very low among all seasons
(Supplementary material Fig. S4). Concerning biotic variables,
species richness along the study area showed a remarkable spatial
synchrony (Fig. 3), while total abundance showed departures from
the coherence pattern when considering the summer season
(Supplementary material Fig. S5).
Environmental variables significantly differed between beaches
(2-way PERMANOVA: pseudo-F ¼ 7.63, p ¼ 0.001) and seasons
(pseudo-F ¼ 11.61, p ¼ 0.001), while the beach season interaction
was not (pseudo-F ¼ 0.78, p ¼ 0.96). This means independence of
the effect of the factor ‘beach’ on the level of the factor ‘season’,
reflecting again coherence in environmental changes for all beaches across seasons. Widespread differences between pair of beaches occurred in the four seasons, mainly reflecting unique abiotic
characteristics for each beach occurring all along the year
(Table S2a, b).
3.2. Macrobenthic community
Total species richness was highest in summer and lowest in
winter and increased towards oceanic beaches, being highest in
Beach 16 (Barra del Chuy) throughout the study period (Table S1;
Fig. 4a). However, the number of species in the two innermost
estuarine Beaches 1 and 2 was higher than those found in inner
estuarine Beaches 3 and 4, which showed the lowest species richness during the four seasons. Considering contiguous beach pairs,
dissipative or intermediate beaches harboured more species than
reflective ones. In particular, estuarine beaches presented lower
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D. Lercari, O. Defeo / Estuarine, Coastal and Shelf Science 154 (2015) 184e193
Fig. 3. Spatial synchrony analysis by pairwise linear regressions between the number of species recorded on each beach on every season in 16 Uruguayan sandy beaches.
***p < 0.001, **p < 0.01, ns p > 0.05.
species richness during autumn and winter (Fig. 4a). The number of
species significantly differed between seasons (ANCOVA: Table S3):
for a same distance from the innermost beach (covariate), winter
had significantly lower number of species than the remaining three
seasons (LSD test: p < 0.01). The number of species was significantly lower in spring than in summer, whereas autumn only
differed with winter.
Total macrofauna abundance followed the same pattern
observed for species richness (Table S1; Fig. 4b), increasing towards
both extremes of the TW system. The highest abundance occurred
on the innermost Beach 2 in winter and autumn, decreasing in
more than an order of magnitude from winter (105,000 ind m1) to
summer (8000 ind m1). The outermost Beach 16 showed the
highest abundance among oceanic beaches throughout the 2-yr
study period, particularly in spring (26,000 ind m1). The high
variability in abundance precluded the fulfilment of basic ANCOVA
assumptions (e.g. homogeneity of variances). Nonlinear modelling
of the CV showed a higher intra-annual variability of species
richness and total abundance towards the inner estuary (Fig. 2eef).
The CV of both biological descriptors exponentially decreased towards oceanic conditions, irrespective of the beach morphodynamic state. Moreover, a negative exponential trend was found
between abundance variability (as denoted by its CV) and species
richness, meaning less variable communities in oceanic sandy
beaches (Fig. 5).
A high species turnover was found along spatio-temporal gradients occurring within the TW ecotone. Changes across seasons in
species dominance in the 16 sandy beaches showed that (Fig. 6),
firstly, species dominance was lowest at both extremes of the
studied area, whereas Mollusca and Polychaeta were absent in
outer estuarine beaches, irrespective of the morphodynamic state.
These Phyla were always found in all oceanic beaches, and in the
inner estuary (Beaches 1 and 2). However, different species characterized both extremes of the TW system: a) concerning Mollusca,
Donax hanleyanus and Mesodesma mactroides were dominant in
oceanic beaches, while Erodona mactroides (Bivalvia) and Heleobia
D. Lercari, O. Defeo / Estuarine, Coastal and Shelf Science 154 (2015) 184e193
189
sp. (Gastropoda) dominated at inner estuarine beaches; b) concerning Polychaeta, Euzonus furciferus, Scololepis gaucha and Hemipodus olivieri occurred in oceanic beaches, whereas Laeonereis
acuta and Nephtys fluviatilis only appeared in Beaches 1 and 2.
Secondly, the cirolanid isopod Excirolana armata dominated estuarine and oceanic dissipative beaches, excepting the two innermost
Beaches 1 and 2. E. armata was the only species found in inner
estuarine beaches during autumn and spring, and showed a clear
dominance in oceanic beaches during autumn, winter and spring.
Thirdly, the isopod Excirolana braziliensis and the talitrid amphipod
Atlantorchestoidea brasiliensis were dominant all year round in
outer estuarine and oceanic reflective beaches. Fourthly, the mole
crab Emerita brasiliensis was a typical inhabitant of oceanic beaches
in all seasons. On inner estuarine beaches, this crab was only found
during summer (Fig. 6a) in the form of recently settled megalopae.
Differences between beach pairs occurred in all the four seasons, highlighting distinctive benthic communities for each beach.
Results of PERMANOVA Pair-wise tests are shown in Table S4b.
3.3. Biotic e abiotic matching
Multivariate linking between macrofauna and its habitat (BIOENV analysis) was highly significant for the four seasons
(0.797 < Rho < 0.915, Table 2). The analysis highlighted salinity as
the main explanatory variable of the observed large-scale trends in
summer, autumn and spring, where wave period and beach and
swash width also played a role as explanatory variables. In winter,
temperature was the most important abiotic variable, followed by
salinity and beach width.
4. Discussion
Fig. 4. Seasonal large-scale distribution of: a) number of species and b) total abundance in 16 Uruguayan sandy beaches located in TW of the Southwestern Atlantic
Ocean.
This study demonstrated important changes across seasons in
sandy beach communities in the TW system defined by the Río de
la Plata, as well as the role of the morphodynamic and estuarine
gradients in structuring these communities. Sandy beaches occurring along the Uruguayan coast showed environmental and biological similarities with other TW, notably the presence of strong
abiotic gradients producing environmental filtering that result in
heterogeneity of species composition (Barbone and Basset, 2010).
Our analysis illustrates how environmental gradients may drive the
dynamics of the sandy beach communities, as reflected by the
strong species turnover among sites. Community stability was
positively related with species richness and negatively with environmental variability. Moreover, spatial synchrony in salinity and
species richness was observed along the study area.
4.1. The environment
Fig. 5. Nonlinear model between the coefficient of variation (CV) in community
abundance and the mean number of species found in each Uruguayan beach
(p < 0.001). Beaches are numbered from west to east: 1: Pascual (km 0); 2: Penino (km
2); 3: Honda (km 51); 4: Verde (km 52); 5: Costa Azul (km 108); 6: Baguala (km 112);
nica (km 197); 10: J Ignacio (km 203);
7: Hermosa (km 125); 8: Negra (km 139); 9: S Mo
11: Aguada (km 257); 12: Arachania (km 260); 13: S Isabel (km 267); 14: P Diablo (km
350); 15: Achiras (km 356); 16: B Chuy (km 378).
The estuarine discharge showed sharp intra-annual oscillations.
Indeed, salinity was highest in all beaches during the warm period
and lowest during the cold season. Moreover, the influence of
freshwater discharge from Uruguay and Paran
a rivers into the Río
de la Plata increased in autumn, generating a decrease in salinity in
estuarine beaches (Fig. 1a). In this system, salinity is controlled by a
number of variables interacting in a big and shallow basin (Mianzan
et al., 2001). Seasonally, the surface horizontal salinity gradient is
mainly controlled by winds, which determine two major seasons:
springesummer (winds from the NE) and autumnewinter (winds
from the SW) (Guerrero et al., 1997). However, in intertidal environments, the horizontal salinity gradient may be dominated by
river runoff, wind patterns, tidal range, the proximity of secondary
freshwater discharges and the local shore configuration (e.g.
orientation, closeness to rocky heads). The interaction among these
factors at a local scale is hardly known, but our results highlight a
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D. Lercari, O. Defeo / Estuarine, Coastal and Shelf Science 154 (2015) 184e193
Fig. 6. Relative contribution in abundance of the main macrofaunal species recorded in 16 Uruguayan sandy beaches: a) summer; b) autumn; c) winter; d) spring.
higher intra-annual salinity variability in estuarine than in oceanic
beaches.
Seawater temperature also showed strong changes across seasons, which markedly differed along the coast. Inside the estuary
we found two main periods (cold autumnewinter and warm
Table 2
Results of the BIOENV analysis showing the group of environmental variables that
best explained variations in sandy beach community structure in each season, in 16
Uruguayan sandy beaches located in TW of the Southwestern Atlantic Ocean.
Season
r
Variable combination
Summer
Autumn
0.797
0.848
Winter
0.828
Spring
0.822
Salinity; wave period; sediment compaction
Wave period; grain size; sediment temperature; sediment
water content
Water temperature; salinity; compaction; sediment water
content
Salinity; beach width; sediment sorting; sediment organic
matter
r ¼ Spearman rank correlation coefficient.
springesummer), whereas three periods were found in the oceanic
coast (cold winter, temperate autumnespring and warm summer).
This is in agreement with the trends observed by satellite imagery
in this coastal zone, where two periods (autumnewinter and
springesummer) were identified in relation to the solar radiation
cycle, dominant wind patterns and the influence of the freshwater
plume (Simionato et al., 2010). Intra-annual variations in temperature were also higher under estuarine conditions and could
represent an additional stressor to the macrofaunal species
inhabiting the inner estuary. Patterns of water salinity and temperature variation along the study area showed that the salinity
gradient was the most important descriptive component of TW,
with the widest variation occurring at the freshwater e brackish
water and brackish wateremarine interfaces (Beaches 8 and 9).
This area may correspond to the actual discontinuity between
freshwater and marine ecosystems (Basset et al., 2013).
Large-scale variations in beach morphodynamics largely reflected the sampling design (Lercari and Defeo, 2006). Grain size
D. Lercari, O. Defeo / Estuarine, Coastal and Shelf Science 154 (2015) 184e193
and Dean's parameter displayed blur changes across seasons,
without showing seasonal trends along the coast. Grain size has
been previously defined as an enduring variable in sandy beaches
(Ortega et al., 2013) because it does not exhibit substantial changes
over time (Valesini et al., 2010). By contrast, beach and swash width
showed a well-defined exponential decrease in the CV towards
oceanic beaches, while slope showed variations according to
erosion/accretion processes. These variables have been mentioned
as key factors influencing the macrofauna, where beach squeeze
from marine to estuarine regions constitutes a direct evidence of
habitat reduction (Lercari and Defeo, 2006). Therefore, in addition
to salinity and temperature variability, species inhabiting estuarine
beaches have to cope with a wide variation in habitat expansion/
reduction cycles.
4.2. Abundance and species richness
Independently of the season of the year, species richness and
abundance increased from reflective to dissipative domains
(McLachlan, 1990; Defeo et al., 1992; Brazeiro, 1999; Defeo and
McLachlan, 2005). Species richness on most beaches rose sharply
in spring, peaked in summer and then strikingly declined during
autumn and winter (excepting Beach 2). Therefore, differences
among seasons in the number of species were found when
removing the effect of the position of beaches along the estuary.
Total abundance did not show a consistent seasonal pattern in
all beaches. Different beaches attained their highest abundance in
different seasons, regardless their location along the estuary. Variations among seasons in sandy beach communities are common in
temperate and subtropical sandy beaches (Dexter, 1979, 1984;
Haynes and Quinn, 1995; Jaramillo et al., 1996, 2001), and have
been mainly ascribed to the clear timing in recruitment periods
that mainly occurred during warm seasons (Silva et al., 2008). In
addition, temporal variability in abundance was highest when
species richness was lowest, particularly in inner estuarine beaches, whereas dissipative oceanic beaches with the highest species
richness (Beaches 13, 15 and 16) had the lowest abundance variability. These results suggest that macrofaunal communities are
more stable at the ocean edge of the TW system, where environmental variability is also lowest.
4.3. Species turnover
Species turnover, i.e., the replacement of species along spatiotemporal gradients occurring within the TW ecotone, was very
high and substantially higher than those found in large-scale
studies conducted on exposed ocean beaches with different morphodynamics, meaning an unambiguous effect of TW system
characteristics in structuring macrofaunal communities. Moreover,
our patterns were similar to those depicted for other TW systems,
including estuaries, deltas and coastal lagoons (Basset et al., 2013).
Mollusca and Polychaeta were absent in sandy beaches located in
the middle of the main estuarine axis, whereas they were found in
oceanic beaches at all morphodynamic conditions, and in the
innermost estuarine Beaches 1 and 2. By contrast, crustaceans were
conspicuous components that inhabited the whole salinity gradient
in TW. Excirolana armata was systematically recorded in estuarine
beaches, suggesting a high tolerance to estuarine conditions (Lozoya
and Defeo, 2006; Lozoya et al., 2010). It was dominant in all fine
grain size and gentle slope beaches throughout the coast, and
therefore could be defined as a high substrate-specific species (Defeo
et al., 1997; Petracco et al., 2010). The congeneric isopod Excirolana
braziliensis and the talitrid amphipod Atlantorchestoidea brasiliensis
were always dominant in outer estuarine and oceanic reflective
beaches, which are considered as more stable and safer
191
mez, 2005).
environments for supralittoral species (Defeo and Go
Estuarine reflective beaches may also prevent these supratidal species to be submerged into waters with highly fluctuating salinity,
mez and
causing an osmotic stress and potential lethal effects (Go
Defeo, 2012). The mole crab Emerita brasiliensis was a typical
inhabitant of oceanic beaches during all seasons, but it was also
registered in inner estuarine beaches in late summer and autumn,
because of the occurrence of recently settled megalopae. This could
be explained by the highly-dispersive larval phase of the species,
which in the summer of 2000 was favoured by a marked decrease in
the freshwater runoff of the Río de la Plata (Piola et al., 2005),
together with a higher influence of the warm Brazil current. This
environmental scenario disappeared in autumn, and no E. brasiliensis
individuals overwintered inner estuarine beaches. Therefore, inner
estuarine beaches act as sink habitats in the metapopulation dynamics of this species (Celentano et al., 2010). Regardless of this
punctual observation, no settling organisms of other marine species
with pelagic larvae (e.g. Donax hanleyanus, Mesodesma mactroides)
were observed in inner beaches. This suggests an important role of
small to mesoscale oceanographic features in larval accumulation,
recruitment and circulation patterns close to reproduction areas
(Pineda et al., 2007; Porri et al., 2014). Further surveys including
plankton sampling, coastal currents studies and genetic aspects will
clarify the processes and mechanisms involved in the connectivity of
sandy beach populations in this TW system.
Although high turnover may affect the structural properties of
beach communities, a functional equivalence between species were
found at the extremes of the salinity gradient. For example, the
oceanic filter feeder bivalves (e.g. Donax hanleyanus and Mesodesma
mactroides) were replaced by brackish species as Erodona mactroides in the inner estuary; similarly, the marine deposit-feeding
polychaete Euzonus furciferus was replaced in brackish waters by
its functional equivalent Laeonereis acuta. Thus, different species
sustained similar functions and therefore fill equivalent niches,
reinforcing the concept of functional redundancy (Basset et al.,
2013). The capacity of these species to perform the same function
in markedly different parts of the TW system (i.e., in sandy beaches
located in freshwater and marine waters) substantially increases
the level of redundancy within functional groups (Basset et al.,
2013). This functional redundancy provides some sort of ecological insurance against environmental fluctuations (Yachi and
Loreau, 1999), increasing the ecological resilience of ecosystems
occurring in TW.
An ecotone may be defined as a narrow ecological zone dividing
two different and relatively homogeneous community types (Attrill
and Rundle, 2002). The fact that different species of Mollusca and
Polychaeta were only registered at both extremes of the TW system
analysed here reinforces the concept that an ecotone model could
be used to describe the variation of species composition along the
Uruguayan coast. Indeed, the two innermost Beaches 1 and 2 (inner
estuary) showed unique faunal components, which presented
marked differences in species composition among seasons. The
temporal dynamics of species dominance also behaved quite
differently than in the remaining beaches. For example, higher
species dominance was attained during winter and spring. In
addition, the distribution of most species recorded in inner estuarine beaches goes beyond the intertidal fringe, having an extensive
subtidal distribution on the Río de la Plata estuary (Cortelezzi et al.,
2007). In contrast, the habitat of all species that occurred in outer
estuarine and oceanic beaches is restricted to a narrow intertidal
fringe of sand between terrestrial and marine systems. These
trends suggest an ecological shift in sandy beaches towards the
inner estuary, thus developing a narrow ecotone, which is in
contrast to the expected gradual transition defined by an ecocline
model (Attrill and Rundle, 2002; Elliott and Whitfield, 2011).
192
D. Lercari, O. Defeo / Estuarine, Coastal and Shelf Science 154 (2015) 184e193
4.4. Matching environmental and faunal dynamics
The strong correlations between the biotic and abiotic multivariate matrices in all seasons remarked the outstanding role of the
environment in controlling sandy beach macrofaunal assemblages
(Defeo and McLachlan, 2005; Ortega Cisneros et al., 2011). However, the set of environmental variables that best explained the
community structure differed between seasons, suggesting that the
sandy beach fauna could be controlled by a different suit of conditions depending on the time of the year. In warmer periods
(summer and autumn), salinity, wave period and beach width were
the main environmental factors influencing sandy beach assemblages, whereas temperature had a primary influence in winter and
morphodynamic variables exerted a major influence in autumn.
This highlights the need to consider morphodynamic information
when treating sandy beaches occurring in TW systems. These
variables (e.g. wave period) may act in addition to the stress produced by salinity variability to filtering the regional pool of species
and limiting local diversity values. Our study demonstrated that in
sandy beaches occurring inside TW, spatio-temporal patchiness
(discreteness) is likely to support high taxonomic turnover among
locations (b diversity) and then high regional diversity (g) (Barbone
and Basset, 2010).
In summary, environmental changes across seasons in sandy
beaches occurring in TW plays a fundamental role in structuring
macrofaunal communities. Beaches located towards freshwater
conditions showed a major environmental dynamism, which
generated high variability in species richness and abundance
mainly driven by a higher intra-annual salinity variability in estuarine than in oceanic beaches. Decreasing abiotic and biotic variability towards oceanic conditions led to more stable communities,
which could explain the spatial synchrony found in species richness
in the TW system. Taxonomic turnover was observed among beaches with contrasting morphodynamics and also along the salinity
gradient, but a functional equivalence between species were found
at the extremes of the salinity gradient. Long-term studies will
elucidate the importance of climatic factors in deciphering the
strength of coherence through time in biotic and abiotic variables of
these dynamic ecosystems occurring in TW.
Acknowledgements
We would like to express our gratitude to all the people that
helped in sampling and laboratory analyses. We are grateful for the
financial support provided by CONICYT (FCE 4034), GEF-FAODINARA (GCP/URU/030/GFF), PEDEClBA and ANII.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.ecss.2015.01.011.
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