Effects of regional oceanography and climate variability on larval connectivity of the wedge clam Donax hanleyanus in South American beaches

The wedge clam, Donax hanleyanus, inhabits sandy beaches in the subtropical and temperate regions of the Atlantic coast of South America. Its distribution spans over 20 degrees of latitude from Brazil to Argentina, with the southernmost part of its range being influenced by the Rio de la Plata (RdlP) estuary, which limits the southward larval expansion. We used an individual-based model (IBM) to assess the larval connectivity patterns of the wedge clam during the period 2000–2012. The IBM combines a 3D hydrodynamic model with a biological sub-model that considers larval mortality due to low salinity (<7, and <9) and sea surface temperature range (high >30 °C or low <9 °C). The main larval connectivity patterns were observed near the release/recruitment areas, suggesting a high potential for self-recruitment. Based on the IBM and adult abundance data, we also identified the likely source and sink areas within this metapopulation. Source beach areas were Navegantes and Cassino in Brazil (from 26.3° S to 34.34°S), Arachania in Uruguay (34.56°S), and Santa Teresita in Argentina (37.15°S). A low probability of larval transport towards the poleward limit of the species' distribution was observed, supporting an irregular recruitment pattern typical of sink populations located at the edge of the distribution range of metapopulations. Larval mortality due to warm or cold waters did not affect connectivity patterns for this subtropical species. Southward larval transport across the RdlP estuary (from Uruguayan to Argentine beaches) only occurred for larvae released on early January 2011, concurrently with the strongest La Niña year observed during the study period. In light of a changing climate, marked by potential increases in extreme La Niña events and a poleward shift of atmospheric circulation patterns over the South Atlantic, we anticipate a strengthening of larval transport across the RdlP and a subsequent poleward expansion of the species’ distribution range.