Abstract
<title>Abstract</title> <p>Arsenic (As) naturally occurs at elevated concentrations throughout the Pampean Plain (Argentina), where shallow eutrophic lakes are particularly susceptible to seasonal sediment–water exchanges that may alter As bioavailability. This study investigated the seasonal dynamics of dissolved and sediment-associated As in Los Padres Lake and evaluated its bioaccumulation in representative organisms from three trophic compartments: the bryophyte , the freshwater shrimp , and the silverside . Dissolved As concentrations increased markedly during summer, coinciding with higher dissolved Fe concentrations and a reduction in sedimentary labile As, indicating enhanced sediment As mobilization under warm-season conditions. Biological responses differed markedly among species. and closely tracked seasonal fluctuations in dissolved As, whereas showed comparatively stable tissue concentrations, with As detected only in liver and muscle but not in gills, suggesting predominantly trophic exposure and effective physiological regulation. Bioaccumulation factors (BAF) progressively declined from bryophytes to shrimp and fish, demonstrating a clear biodilution pattern across trophic compartments. These findings demonstrate that the biological response to seasonal sediment As mobilization becomes progressively attenuated with increasing trophic position owing to species-specific physiological regulation of As uptake, transformation, and elimination. This study provides an integrated framework linking seasonal geochemical processes with contaminant transfer through shallow-lake food webs and highlights the importance of incorporating temporal variability into monitoring programs, particularly under climate-change scenarios expected to intensify sediment As mobilization.</p>