Estuarine Front Dynamics: A Key Driver of Fish Larvae Dispersion in a Large Subtropical Estuary

Abstract

Abstract Estuarine fronts are critical in the retention and dispersion of ichthyoplankton. This study evaluates the response of ichthyoplankton assemblage to front dynamics in one of the world's largest estuaries, the Río de la Plata. Surveys were conducted between October 2014 and December 2017, using the turbidity front (TF) location determined by satellite imagery as a proxy for the estuarine front. Although most larval taxa belonged to the freshwater category, the highest abundances were found in the estuarine category. Contrary to most estuaries, the incidence of pelagic egg spawners was high in the studied estuary. The highest species richness and abundance occurred during spring and summer, linked to the spawning seasons and locations. TF dynamics was the primary driver of interannual variability in ichthyoplankton assemblages, revealing opposite trends with ENSO (El Niño-Southern Oscillation) events. A strong El Niño event (2015–2016) led to decreased total abundance and dominance of freshwater larvae with a seaward shift of the TF, while a weak La Niña event (2016–2017) resulted in increased abundance of marine larvae with the retraction of the TF. Based on current climatic predictions, it is likely that the estuarine frontal zones will be affected in the future, which could have a significant impact on ichthyoplankton assemblage. Therefore, coastal conservation is critical and should be integrated into management plans to preserve ichthyoplankton dynamics and fish populations.