Occurrence of Heavy Metals in Sediments and Bioaccumulation Factor in Rangia cuneata Clams from a Protected Coastal Lagoon in Mexico

This study evaluated the seasonal variability, origin, and ecological risk of heavy metals in the Pom–Atasta lagoon system, a tropical estuary in southeastern Mexico subject to increasing anthropogenic pressure. The main objective was to determine how seasonal changes influence the distribution, bioavailability, and risk of metals in sediments and benthic organisms. Thirty sampling stations were monitored during dry, rainy, and north wind seasons. Sediment concentrations of As, Cd, Cr, Ni, Pb, and V were measured, and bioaccumulation was assessed in the bivalve Rangia cuneata. Ecotoxicological risk was evaluated using the Adverse Effects Index (AEI), Toxic Risk Index (TRI), and potential ecological risk index (ERI). The results showed higher metal concentrations during the rainy and north wind seasons, likely due to increased runoff and sediment resuspension. Cr and Ni exhibited the highest enrichment, with values from 115.0 to 130.4 µg g−1 and from 60.5 to 75.9 µg g−1, respectively. Ni showed the highest bioaccumulation factor (BSAF > 1.51) in R. cuneata, indicating high mobility and environmental availability. Weak correlations among some metals (As, Cr, and Pb) suggest mixed natural and anthropogenic sources. TRI values indicated low to moderate toxic risk, and ERI classified most sites as low risk (ERI < 60) at several stations. Organic carbon levels remained within tolerable limits (<10%) for benthic fauna. These findings highlight the role of seasonal dynamics in metal distribution and confirm R. cuneata as a suitable bioindicator for monitoring ecological health in tropical estuarine systems.