Transport And Transformation Of Mercury In Main Mangrove Wetlands Of China

In order to study transfer and transform of mercury in main mangrove wetlands of China, samples of plant, sediment, water, air were collected from thirteen mangrove wetlands which cover main mangrove areas of China. This paper focus on the following topics: (1) distribution of total Hg concentration and species in sediments of mangrove wetland; (2) total Hg concentrations in water of mangrove wetland; (3) total Hg concentrations in air and mercury exchange fluxes between air and sediment in mangrove wetland;(4)brief analysis of total Hg concentrations in overground part of different mangrove plants. Some conclusions were drawn as follows:1. Mercury pollution of different mangrove wetlands in China: Fujian is the highest, Guangxi is the lowest, Hainan and Guangdong are middle.2. Discrepancy of total Hg concentrations in mangrove wetlands is due to many environmental factors and human activities. pH value, grain size, organic matter, Fe/Hg, Mn/Hg are significantly related with total Hg respectively. Clay and Fe/Hg are obvious factors influencing total Hg concentration. Otherwise, different land use plays an important role in influencing total Hg concentration. Impact of environmental factors on mercury can be showed by follow regression equation:Y(total Hg) = 234.405 - 7.022 (pH value) - 19.313 (salinity) + 22.471 (organic matter) + 0.428 (powder) + 27.905 (clay) - 0.231 (Fe/Hg) - 5.518 (Mn/Hg).3. Among different mangrove areas, there are different trends of total Hg concentrations with depth change. The vertical distribution of mercury in sediment profile of Chinese magrove areas has three types: (1) bottom-enrichment, such as Da Guangsha, Fugong, Dong Zhaigang; (2) upper-enrichment, such as Shenzhen, Dong Zhaigang nursery; and (3) homogeneous type such as Gaoqiao. Properties of sediments and deposite rate change greatly in different mangrove areas. Economic level of local districts and human activities strongly affected pollution extent of mangrove wetlands. So content of total Hg varies greatly in spatial distribution. In addition, absorption and accumulation of mangrove plants are also important factors.4. Different distribution of mercury species wrere found in different mangrove wetlands. There are two trends: one is that mercury exist mianly in the form of volatile, such as Sanya, Dongzhai harbor, Gaoqiao, Daguansha, Beilun estuary, Yunxiao, Fugong. Another trend is that mercury exist dominatedly in residue state, such as Shenzhen.There are a significant positive correlation between total Hg concentration and volatile mercury (r=0.967, p<0.01), and there are not obvious relationship among other mercury species and total Hg concentrations. But the relationship of carbonate-bound mercury and iron-manganese oxide-bound mercury is strongly positive correlated (r=0.871, p<0.01), organic and sulfide-bound mercury is significant correlated with carbonates-bound mercury and iron-manganese oxide bound mercury (r₁=0.618, p₁<0.01; r₁=0.686, p₂<0.01) . And the relationship of organic matter and total Hg concentrations is significantly (r=0.526, p<0.05) ,and that of organic matter and residue state is positive correlated(r=0.590, p<0.01).5. Distribution of mercury in water of main mangrove wetland of China is irregular. Total mercury in water of Shenzhen averages 0.03ug/L. And the total mercury concentration in water of Da Guangsha is at a higher level, with an average mercury content of 0.95ug/L. The highest level of total mercury is in Gaoqiao, with an average mercury content of 1.10ug/L. And the average total mercury content in water of mangrove wetland of China is 0.60ug/L, which is higher than other wetlands of China, and far higher than other mangrove wetlands of the world6. It is generally higher of total mercury concentration in air of mangrove wetlands of China.And the total mercury concentrations ranges 168.6-310.7ng/m³, which is higher than other wetlands of the world. However, it is obviously different in different mangrove wetland areas. Otherwise, exchange flux between sediment-air in Fugong is 5.99-17.98 ng·m^-2·h^-1.7. It is a big different of total Hg concentrations among several major plants in mangrove wetland. Different mangrove plants have different absorption capacity of mercury, as the following order: Avicennia marina>Sonneratia apetala>Bruguiera gymnorrhiza>Aegiceras corniculatum>Excoecaria agallocha>Hibiscus tiliaceus> Kandelia obovata. Different flood conditon, source of mercury and plant physiology are important to total mercury concentrations of different kinds of plants. There is large difference among identical plants of different areas. There is also different mercury enrichiments among different organs of identical plant. Different organs absorb and cumulate mercury, which is related to mecury species. Volatile and solubility mercury can be absored easily by plants.8. Mercury can circle in air, water, sediment and organism in mangrove wetlands. Sediment of mangrove wetlands cumulates mercury from air and surface runoff of earth. Mercury in sediment of mangrove wetland could enter into air , water and organism. Mercury in water can be not only exchanged with mercury in seawater, but also deposited into sediment or absorbed by plants. And mercury migration path in plant is root-stem-leaf. One part of mercury in air can reenter into mangrove wetlands, another part of mercury in air can enter into aerosphere and transfer with atmosphere circumfluence.