| Mangrove swamps are vital ecosystems in tropical and subtropical intertidal zones, playing an important role in the maintenance of ecological integrity and biodiversity, water resource conservation and weather regulation in coastal cities. Heavy metal contamination in mangrove swamps has become a matter of concern. In order to protect the mangrove swamp in Zhanjiang City and support local economic development and environmental conservation, we investigated the concentration, spatial distribution, sources and factors influencing heavy metal pollution in the mangrove swamp districts of Zhanjiang. Thirty-eight sampling sites were located in ten mangrove swamp districts; six sites in Lianjiang, four sites in Suixi, two sites on the west coast of Leizhou, four sites on the east coast of Leizhou, four sites in Wuli of Xuwen and five sites in Hean of Xuwen, four sites in Mazhang and three sites in Potou, Techeng island and Donghai island. Three water samples were collected at 0.5m below the surface at each site and then mixed to give a composite sample for determination of eight heavy metals (Fe, Mn, Cu, Cr, Cd, Pb, Ni and Hg), as well as total organic carbon (TOC) and total nitrogen (TN). The pH, DO, water temperature, salinity and conductivity of the water were measured in situ. The comprehensive heavy metal pollution level was calculated using the water quality index method. Spatial differences of heavy metals and the physicochemical properties of water in the different areas were analyzed using least significant difference. Principal component analysis (PCA) was used to identify sources of heavy metals and redundancy analysis (RDA) was used to identify factors influencing heavy metals in the mangrove swamp. Concentrations of Fe, Mn, Cu, Ni, Pb, Cr, Cd and Hg were low, with mean concentrations respectively of (91.14±10.88), (78.88±10.65), (8.07±0.59), (4.96±0.44), (0.77±0.07), (0.72±0.05), (0.16±0.03), (0.03±0.01) μg/L. Single-factor and integrated pollution indices of the heavy metals both indicate light heavy metal pollution in the waters of Zhanjiang mangrove swamp, except for a few individual sites in the Techeng island and Leizhou districts, where Cu and Ni were the metals of concern. Principal component analysis (PCA) indicated that Fe and Cr content is controlled by weathering of parent rock, Hg, Pb and Cu are closely related to agricultural activities and sewage discharge, and Mn, Ni and Cd are contributed primarily by mining and heavy industry. Redundancy analysis (RDA) showed that TOC and pH are the primary environmental factors associated with heavy metal pollution, displaying significant positive correlations with Mn and Ni and positive correlations with the other heavy metals. |
