Trophic transfer of heavy metals in a mangrove ecosystem: Bioaccumulation patterns and ecological risks in waterbirds

Mangrove ecosystems are vital biodiversity hotspots, but are increasingly threatened by anthropogenic pollution, especially heavy metal contamination. This study aimed to investigate the trophic transfer of heavy metals in a human-impacted mangrove system by linking contamination levels in abiotic compartments (sediment and water) with bioaccumulation patterns in prey organisms (macroinvertebrates and fish) and ecological risk in aquatic birds. We quantified concentrations of eight metals (As, Cd, Co, Cr, Cu, Ni, Pb, and Zn) using inductively coupled plasma mass spectrometry (ICP-MS), and assessed contamination through concentration factors (CFs) and risk indices, including the Potential Risk Index for birds (PRI_bird). Sediment acted as a sink for metals, with low CFs (<1), while water samples revealed severe contamination by Cd, Cr, Ni, and Pb (CF ≥ 6). The polychaete Arenicola marina showed the highest levels of Cd, Cr, Ni, and Pb, while mollusks and crustaceans accumulated Cu and Zn. The fish Sardinella fimbriata exhibited elevated arsenic levels. Among birds, Ardea alba showed the highest feather concentrations of As, Cr, Ni, and Pb, with PRI_bird ≥ 3, indicating high ecological risk. Ardea grayii and Chroicocephalus brunnicephalus displayed elevated Cu levels and species-specific accumulation patterns. Principal Component Analysis (PCA) revealed distinct metal transfer pathways, linking contamination in prey and environmental matrices to avian bioaccumulation. Our findings emphasize the need for long-term biomonitoring using aquatic birds as sentinels, and highlight the urgency of pollution control to preserve the ecological integrity of mangrove ecosystems under anthropogenic pressure.