Sandy beaches are being threatened by a changing climate. However, the effects of this changing environment, including warming, on these ecosystems, have hitherto been tentative and qualitative. Using concurrent long-term (1984−2007) observations on abundance and individual size, together with laboratory examinations of body abnormalities (morphological anomalies and epibionts), we provide evidence that the sandy beach yellow clam Mesodesma mactroides of the Uruguayan coast has responded to climate change. Regional sea surface temperature anomalies (SSTA) showed an increasing trend through time, with positive values after 1997. The position of the warm water front (20°C isotherm), a proxy of tropical waters, showed a long-term poleward shift rate of ca. 9 km yr −1. Clam abundance (total and discriminated by population component) decreased through time and was inversely related to variations in SSTA, with higher abundance during cold periods. Length frequency distributions (LFDs) showed polymodal size structures with fully represented clam population components in cool years. By contrast, LFDs showed fewer size classes and larger clams were virtually absent during warm years. Prevalence of body abnormalities in M. mactroides increased through time and was positively correlated with increasing SSTA, suggesting a link with climate stress. The population dynamics of M. mactroides seems to be driven by climatic forcing, mostly related with warming. Our results demonstrate the implications of climate change in the structure of sandy beach fauna, which could be particularly relevant in ectotherms with cold water affinities. Thus, climate change should be given a high priority in sandy beach conservation planning and management.

Several conceptual models relating biological and morphodynamic variables have been developed in sandy beach ecology. On estuarine sandy beaches, a major environmental factor influencing macrofaunal distribution is salinity. In this setting, the concurrent role of large-scale estuarine and morphodynamic gradients in shaping demographic patterns is still uncertain. To explain largescale variations in demographic aspects of the mole crab Emerita brasiliensis, 16 Uruguayan sandy beaches distributed along the estuarine gradient generated by the Rio de la Plata were characterized in terms of beach morphology, sediment traits and salinity over a 2 yr period. Abundance (total and discriminated by reproductive and recruitment components), biomass and body size of E. brasiliensis significantly increased from estuarine reflective to oceanic dissipative beaches. Further, generalized models (both additive and linear) showed that salinity, mean grain size and swash width were the most important explanatory factors of spatio-temporal variations in abundance. Sexually mature individuals and megalopae were almost absent in estuarine and oceanic reflective beaches, whereas oceanic dissipative beaches showed polymodal size structures with fully represented population components, together with extended reproductive and recruitment periods. We conclude that (1) morphodynamic and estuarine gradients are inextricably linked, affecting mole crab demography; and (2) estuarine and oceanic reflective beaches operate as sink habitats where populations do not prosper, whereas oceanic dissipative beaches act as source habitats. We thus provide empirical support to the source-sink hypothesis recently developed for sandy beaches. These long-term variations in demographic processes should be complemented by connectivity studies and genetic analyses in order to gain a deeper understanding of metapopulation dynamics in sandy beaches.KEY WORDS: Mole crab · Emerita · Demography · Salinity · Sandy beaches · Morphodynamics · Generalized additive models 398: 193-205, 2010 ing in lower abundance, fecundity, growth and survival rates than in dissipative beaches. In agreement with SEH and HHH predictions, studies conducted worldwide have shown a consistent decrease in community (McLachlan 1990, Defeo et al. 1992, McLachlan et al. 1993, Jaramillo et al. 2000, Soares 2003, Rodil & Lastra 2004, McLachlan & Dorvlo 2005 and population (truly intertidal species: Nel et al. 2001, de la Huz et al. 2002, Contreras et al. 2003 In relation to the patterns found at the population level, it has been postulated that post-settlement processes prevent intertidal species from establishing populations on reflective beaches. Moreover, it has been hypothesized that dissipative beaches can act as 'source' habitats, whereas reflective beaches would constitute 'sink' habitats where reproduction patterns of resident species are insufficient to balance mortality. This has been called the 'source-sink hypothesis' for intertidal sandy beach populations (Defeo & McLac...

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