Species-specific metal handling reveals the decoupling between environmental contamination and biological accumulation in Amazonian mangroves

Environmental assessments of metal contamination in mangrove ecosystems are commonly based on environmental concentrations, although these measurements do not necessarily reflect biological accumulation. Here, we investigated how sediment geochemistry and species-specific metal handling, encompassing metal uptake, retention/partitioning, and translocation, are associated with biological accumulation in mangrove seedlings growing along a gradient of port influence on the Brazilian Amazon coast. Metals were quantified in water, sediments, and seedlings of three dominant mangrove species across six sites representing contrasting levels of anthropogenic disturbance, together with sediment geochemical characterization (TOC, TN, δ 13 C, and δ 15 N). Contamination indices indicated higher metal contamination at sites influenced by port activities (PLI > 1, I geo class 2, and C deg up to 17.6). However, metal accumulation in seedlings did not consistently follow environmental contamination gradients. Instead, accumulation patterns varied among species and were associated with differences in sediment geochemistry and metal-handling responses. Avicennia germinans predominantly translocated metals to aerial tissues, Laguncularia racemosa exhibited context-dependent shifts between retention and translocation, whereas Rhizophora mangle maintained comparatively stable redistribution patterns. These contrasting responses indicate that biological accumulation cannot be inferred solely from environmental metal concentrations and highlight the importance of species-specific physiological responses for interpreting contamination patterns. Collectively, our findings support the Functional Triad as a conceptual framework based on correlative patterns in which sediment geochemistry, inferred metal availability, and species-specific metal handling interact to shape patterns of biological accumulation in mangrove ecosystems. This integrative perspective provides a basis for improving the interpretation of contamination gradients and may contribute to the development of biologically informed environmental assessment approaches for tropical mangroves.