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.