Estuarine front dynamics drive ichthyoplankton assemblage in the tidal freshwater zone of the Río de la Plata, South America

Abstract

Frontal zones are critical in the retention and dispersal of ichthyoplankton. This study evaluates the response of ichthyoplankton assemblage to frontal zone dynamics in one of the world's largest estuaries, the Río de la Plata, South America. Surveys were conducted between October 2014 and December 2017 in a nearshore habitat of the tidal freshwater zone of the estuary. Ichthyoplankton was collected using conical nets, and physico-chemical variables were registered in situ. The frontal zone dynamics were evaluated through the turbidity front (TF) location, determined by satellite imagery. Although most larval taxa belonged to the freshwater category, the estuarine category had the highest abundances. The highest species richness and abundance occurred during spring and summer, linked to the season and proximity of spawning areas. The dynamics of the TF emerged as the primary driver of interannual variability in ichthyoplankton assemblages within the tidal freshwater zone. A seaward shift of the TF correlated with a reduction in total abundance and the prevalence of freshwater larvae, whereas a retraction of the TF location corresponded to an increased abundance of marine larvae in the tidal freshwater zone. These interannual shifts in TF location and fish larvae assemblages were closely tied to significant variations in river discharges resulting from the ENSO events during the study period. The findings suggest that marine and estuarine fish eggs exhibit effective retention near the frontal zone and the outer regions of the estuary. In contrast, larvae from these categories disperse along the main axis of the estuary in response to the displacement of the frontal zone. Finally, we discuss into the role of the frontal zone in the Río de la Plata as a pivotal environmental cue for fish larvae, influencing their behavior to regulate dispersal patterns.