Supplementary file 1_Spatio-temporal variations of heavy metal(loid)s and their influencing factors in surface sediments of Daya Bay, South China Sea.docx

To elucidate the spatiotemporal dynamics and driving mechanisms of heavy metal (loid)s (HMs) in coastal systems, this study investigated the surface sediments from Daya Bay across four seasons. Results indicated that the average HM concentrations (mg/kg) decreased in the order of Zn (65.35) > Pb (28.66) > Cu (13.19) > As (7.06) > Hg (0.073) > Cd (0.058), with Pb, As and Hg exhibiting significant seasonal heterogeneity (p < 0.05). The Geo-accumulation Index (Igeo) and Contamination Factor identified Hg as the dominant contaminant, registering the highest average Igeo value of 2.039. Overall, the surface sediments in the study area exhibited a moderate level of cumulative pollution with an average Pollution Load Index (PLI) of 2.48, following a seasonal decline of winter (2.76) > spring (2.49) > autumn (2.39) > summer (2.27). Crucially, Redundancy Analysis revealed that the distribution of Cu, Zn, Cd, and Pb was primarily governed by natural physicochemical binding to fine-grained, organic-rich matrices. In contrast, the accumulation of Hg and As exhibited a stronger dependency on dynamic biogeochemical factors, notably dissolved oxygen and anthropogenic nutrient inputs. These findings highlight the necessity of distinct management strategies that account for the coupling between sediment matrices and water column biogeochemistry.