Hydrodynamic controls on the spatial distribution of heavy metals in surface sediments of the Beibu Gulf, China

Semi-enclosed bays are particularly vulnerable to heavy metal accumulation because water exchange is limited and particle-bound contaminants are efficiently retained. In this study, six heavy metals (Cr, Cu, Zn, As, Pb, Cd) and key biogeochemical indicators, including total organic carbon (TOC), total nitrogen (TN), the C/N ratio, δ¹³C_TOC, δ¹⁵N, black carbon (BC), δ¹³Cʙᴄ, and the BC/TOC ratio, were measured in surface sediments from the Beibu Gulf to investigate the sources, distribution, and controlling factors of heavy metals. The results indicate that As is primarily derived from natural weathering, whereas Cd reflects mixed natural and anthropogenic inputs; Cr, Cu, Zn, and Pb are predominantly associated with anthropogenic activities. The spatial variability of heavy metals is pronounced, with elevated concentrations in the northeastern Beibu Gulf and lower levels in the southeastern region. Hydrodynamic sorting under monsoon-driven circulation regulates grain size, TOC, and BC distribution, thereby governing metal retention, while localized inputs further modulate the formation of metal hotspots. Our findings underscore the significant influence of hydrodynamic processes on heavy metal distribution in the Beibu Gulf. Although overall contamination levels are generally low, localized ecological risks linked to Cu and Cd persist in low-energy nearshore convergence zones, highlighting the need for spatially targeted monitoring and management.