Distribution, ecological risk, and sediment-influencing mechanisms of heavy metals in surface sediments along the intertidal gradient in typical mangroves in Hainan, China

The relative importance of each sediment physicochemical property to sediment heavy-metal (HM) contents has not yet been quantitatively evaluated. Differences in the HM contents of mangrove surface sediments among the high, middle, and low intertidal zones, and their quantitative relationships to sediment physicochemical properties, were investigated in Dongzhaigang and Qinglan Harbor reserves, Hainan, China. In both reserves, the Cu and Ni concentrations increased significantly from the low to high intertidal zones; the patterns of change in the Mn and Pb contents were opposite in the two reserves. The Cr concentration was significantly lower and the Pb concentration was significantly higher in the dry season than in the wet season. Ecological risks of HM were higher in Dongzhaigang than in Qinglan Harbor. Regression and redundancy (hierarchical partitioning) analyses showed that the sediment total sulfur, nitrogen and potassium contents and pH were key factors affecting the HM contents of mangrove surface sediments.