Contamination, ecological risk, and nutrient coupling of heavy metals in surface sediments of a subtropical mariculture bay, northern Beibu Gulf.

Heavy metal contamination and eutrophication are widely recognized as co-occurring in stressors in mariculture-dominated coastal bays, however, the mechanistic linkages among sedimentary nutrients, hydrodynamic processes, and heavy metals dynamics remain poorly understood. This study investigated the spatialtemporal variations, nutrient-metal coupling relationships, contamination status, and ecological risks of heavy metals in surface sediments of the eutrophic Maowei Sea (MWS), a semi-enclosed subtropical bay in the northern Beibu Gulf. Significant spatial heterogeneity was observed for heavy metals (Cu, Zn, Pb, Cr, Cd, Co, Ni, Mn), total organic carbon (OC), total nitrogen (TN), and total phosphorus (TP), with elevated contents predominantly distributed in estuarine and inner-bay areas. Contamination assessment indicated overall low to moderate pollution levels, whereas Cd exhibited substantially higher enrichment than other metals and contributed more than 80 % of the integrated ecological risk index (RI), making it the dominant ecological risk factor. Pearson correlation and principal component analyses revealed strong associations between most heavy metals and OC, TN, and TP during the wet season (June 2020), indicating that nutrient enrichment and organic matter (OM) accumulation promoted metal retention via adsorption and complexation processes. In contrast, stronger relationships between heavy metals and fine-grained sediments (silt and clay) were observed during the dry season (December 2020), suggesting enhanced physical stabilization under relatively weak hydrodynamic conditions. Collectively, these results demonstrate that heavy metal distribution in the MWS is co-regulated by nutrient-OM interactions and hydrodynamic processes, with the dominant control shifting seasonally from nutrient-OM-mediated biogeochemical retention during the wet season to grain-size-driven stabilization under weaker hydrodynamic conditions during the dry season. This study establishes a nutrient-hydrodynamic co-regulation framework for understanding heavy metal accumulation and associated ecological risks in eutrophic mariculture bays.