Studies On Distribution And Bioavailability Of Heavy Metals In Mangrove Wetland Sediments Of Northern Hainan Island

Mangrove ecosystem is a unique coastal transition ecosystem between the marine and terrestrial environments. With the global climate warming, sustained development of human on the shoreline erosion and various industrial activities, containing heavy metal pollutants continue to flock to the estuary and gulf region, and mangrove sediments had a high retention of tidal, capacity of heavy metals in fresh water rivers, heavy metals have become primary pollution source to the mangrove wetland ecosystem health threat in the intertidal zone.In this study, three typical mangrove wetlands in Hainan Island were selected as the research area. By on-the-spot investigation, data review and laboratory analysis, to research in the area of wetland of heavy metals in the sediments, the effective state content of the spatial distribution of summary, supplemented by physical and chemical properties of the correlation analysis and based on the content of the heavy metal content, the environmental quality of the sediments in the wetland was evaluated. At the same time, the enrichment ability of heavy metals in different mangrove plants tissues were measured. Finally, the paper discusses the bioavailability of heavy metals in mangrove wetland ecosystem. The following conclusions are obtained:(1) In the sediments of the study area, the content of heavy metals were different. The results showed that the relative enrichment of Cu elements appeared in the sediments of the Xinying Harbor. In this studying region sediments, the content of heavy metals, compared to all over the world typical mangrove wetlands are still in the low level, On the one hand, because of Hainan has limited the development of heavy industry with serious environmental pollution, in addition, the sediment sources of mangrove wetlands in the study area are mainly coastal erosion. By heavy metal safety level ERL and ERM values to assess the potential ecological risk of heavy metals in the sediments. The results show that only in Xinying harbor sediment, Cd may pose a potential risk to the health of the mangrove ecosystem.(2) The distribution of heavy metals in the Dongzhai harbor is relatively low, and the distribution of heavy metals in the Bamen bay and the Xinying harbor wetland are different, which dispersion are relatively high. Differences were found in content of heavy metals in the surface layer sediment of barren shoal, forest edge and in the forest. The heavy metals in sediments under the forests content in different depth are analyzed, the results show that, the target elements can be divided into 2 categories: Cr, Cu and Zn elements: the content of this three elements were increased with sediment depth increased significantly, especially in Xinying harbor Wetland. Ni, As, Cd, and Pb: four kinds of element content in Dongzhai harbor Mangrove Wetland showing a trend decreases with the increase of sediment depth, While in the other two wetlands, the depth of 15 cm was the highest, and decreased to the surface layer and deep layer.(3) The correlation analysis of physicochemical properties and heavy metal content of the sedimentary environment.The results showed that most of heavy metal between the significant positive correlation, indicating that their source are similar, and are accompanied by deposition phenomenon. Individual heavy metal elements were significantly correlated with pH and fine particles, and all elements was not significantly related to the TOC content of the sediment.(4) The effective content of Cu elements in the surface sediments accounted for the highest proportion of the total amount, and the biological efficiency was the highest. And in the vertical gradient and 7 kinds of heavy metals(except for Ni) effective state content accounted for the proportion of the total maximum values at the surface or the upper class and biological validity coefficients also showed with the increase of sediment depth and decreasing trend, reflecting the sedimentary environment of heavy metals exist barrier effect.(5) The physical and chemical properties of sediments can influence the available contents of heavy metal in sediments. The effective fraction of Cr and Cd content and coarser particles exist some adsorption, presented with the distribution trend; with increasing of pH, sediment heavy metals and organic matter are formed through complexation binding stability increases, reducing the moving and bioavailability of heavy metals.(6) Nemero comprehensive pollution index method and potential ecological risk index method were used to evaluate the environmental quality of mangrove sediments, which based on the contents of available forms of heavy metals in the study area. The results showed that, in the study area in the sediments of Mangrove Wetland in available forms of heavy metals content in still clean or low potential ecological risk state, three mangrove wetlands effective state of heavy metal pollution levels as follows: Dongzhai harbor Mangrove Wetland > Bamen bay Mangrove Wetland > Xinying harbor mangrove wetland.(7) Concentration of mangrove plants in different tissues(leaf, stem and root) with a metal elements are also different. The mangrove plants in other regions of the most heavy metal content compared mangrove in the study of heavy metal content is relatively low. Enrichment ability of three mangrove species to heavy metals are relatively small(BCF values in the range of 0.02-0.91) and different plant tissue enrichment coefficient were BCFroot > BCFstem > BCFleaf. Overall, Kandelia candel of 7 kinds of heavy metal enrichment of the strongest, and the trend to cumulate Cr, As, Cd and Pb, Aegiceras corniculatum enrichment ability second, mainly concentrated in the essential elements for plant survival, such as Cu and Zn, and Ceriops tagal on the elements of the weakest enrichment capability.(8) Cu and Zn in the mangrove plant tissue in biological transport coefficient(TF) is relatively high, Cd in insoluble forms exist in the plant roots, difficult to organize other migration, biological transport coefficient of the lowest. Three mangrove species, Three mangrove species, Ceriops tagal, stems and leaves of transfer coefficient relative to the maximum, which have the strongest migration ability of heavy metals, Aegiceras corniculatum migration ability in the second,and Kandelia candel was the weakest.