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Estuarine, Coastal and Shelf Science 154 (2015) 184e193 Contents lists available at ScienceDirect 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. 186 D. Lercari, O. Defeo / Estuarine, Coastal and Shelf Science 154 (2015) 184e193 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 188 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 190 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. 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