Spatial Distribution, Seasonal Variation, and Ecological Risk Assessment of Heavy Metals in Tropical Mangrove Wetlands Across Hainan Island, China

Mangrove wetlands are important coastal ecosystems and are increasingly vulnerable to heavy metal contamination. The accumulation of heavy metals in man-grove ecosystems is well studied; however, studies on the seasonal variations of heavy metals in mangrove wetlands are scarce. This study investigated heavy metal (Cd, Cr, Cu, As, Pb, and Zn) accumulation in surface sediments of six typical mangrove wet-lands (DZG, QLH, XCP, SYR, SBW, and XY) in Hainan Island, China, during wet and dry seasons. In addition, potential ecological concerns and relationships between sedimentary physicochemical parameters and metal accumulation were assessed. The findings demonstrated significant spatial differences in heavy metal accumulation, with higher concentrations in the northern localities and lower concentrations in the southern areas. There were notable seasonal fluctuations in heavy metal concentrations, with higher levels in the dry season. Risk assessment models exhibited that Cadmium (Cd) and Arsenic (As) were the principal contaminants of concern in most research sites with moderate levels of contamination and posed at least moderate ecological concerns in both wet and dry seasons. The overall ecological risk index indicated a moderate risk to the environment, especially in the dry season. The principal component analysis (PCA) and correlation analysis results indicated that the physicochemical properties of sediments, mainly total organic carbon (TOC), total phosphorus (TP), total nitrogen (TN), and salinity, had significant effects on the heavy metals accumulation in the mangrove sediments. The present study helps raise awareness of seasonal fluctuations in heavy metal pollutants and provides strategies for the prevention and monitoring of metal pollution in mangrove wetlands.