Heavy metal fractions and ecological risk assessments in sediments and seagrasses along the tropical coasts in Phang-Nga Bay, Andaman Sea

Seagrass meadows enhance sediment complexities that can affect geochemical metals in coastal sediments. However, these processes remain unclear along the coasts of Phang-Nga Bay, Andaman Sea. In this study, heavy metal (As, Cd, Cr, Cu, Ni, Pb, and Zn) fractions in coastal sediments and their accumulation in seagrasses were investigated to assess their ecological risks. Sequential extractions revealed that most of these heavy metals (except for Zn) are associated with crystalline phases, indicating their low metal bioavailability in these sediments; however, the elevated mean risk assessment code for Zn, Ni, and Cd exceeded 10%, indicating a medium risk caused by high potential mobility. The mean translocation factors for all metals in Enhalus acoroides exceeded 1.0, indicating preferential accumulation within leaf tissues rather than the rhizome-root tissues. Meanwhile, bioconcentration factors confirmed the high capacity of E. acoroides to sequester Cd, Zn, and Cu, which were likely driven by selective metabolic processes and essential physiological functions. Consequently, tissue metal concentrations may not correlate well with environmental levels, particularly under a low-pollution environment. This finding indicates that these internal regulatory mechanisms must be considered when using seagrass as a metal bioindicator.