Biogeochemical Cycling Characteristic Of Nutrients, Arsenic And Mercury In Lake Taihu Watershed

Lake Taihu watershed, one of the most developed regions in China, is in the lower reaches of the Yangtze River. However, in recent years, because the rapid development of the local economy and enhancement of the urbanization level, the lakes and rivers in Lake Taihu watershed have been seriously plagued by pollutions. These eutrophications of lakes and rivers are becoming one of the most important environmental problems in Lake Taihu watershed nowadays. These years, eutrophications problems in Lake Taihu and Lake Taihu watershed have already become one of the hot topics to the domestic and international research personnel. The aims of this dissertation were to investigate the temporal and spatial distribution characteristics of biogenic elements (carbon, nitrogen and phosphorus), discuss the response mechanism of nutrient distribution to cyanobacterical blooms, analysis the accumulation process and sedimentary characteristics of biogenic elements. The five selected research areas were Suzhou areas, Changzhou areas, Wuxi areas, Huzhou areas and Yixing areas in Lake Taihu watershed respectively. Otherwise, spatial variations of nutrients were investigated in surface sediments of the lakes in Lake Taihu watershed. At the same time, we evaluated the water quality conditions, eutrophication degree and pollution degree of As and Hg in surface sediments of the lakes in Lake Taihu watershed. The results are as follows:(1) The chemical factors, biogenic elements, phosphorus forms in Lake Taihu showed different characters and different variation in eight areas of the lake. The highest concentrations of TC、TN、TP and LOI appeared in Zhushan Bay, Meiliang Bay and Lake Dongtaihu. And the lowest concentrations of TC、TN、TP and LOI were in the middle area of the lake. These indicated that concentrations of TP、TC、TN and LOI were relatively higher in the northern part and eastern part of the lake. Minerals were the mainly composed in surface sediments of Lake Taihu. The results of the concentrations of phosphorus forms in surface sediments showed that phosphorus pool in surface sediment was mainly composed of inorganic phosphorus, which accounted for68%of TP. The relatively higher concentrations of Ex-P, Al-P and Fe-P appeared in Zhushan Bay and Meiliang Bay. And the second in the Lake Dongtaihu. The lowest concentrations of Ex-P, Al-P and Fe-P were in the middle area of the lake. On the contrary, the lowest concentrations of Oc-P and Ca-P were found in northern part of the lake, moreover, the spatial distributions of Ca-P showed an increasing trend from the northwest to southeast. The results showed higher nutrients in northern part of the lake were connected with anthropogenic activities. The causes of relatively higher concentrations of nutrients in Lake Dongtaihu were higher coverage of macrophytes. However, the reasons of relatively lower concentrations of nutrients in the middle area of the lake were less interference from anthropogenic activities and more sediment resuspension. The results of statistical analysis showed significantly higher relationship between TP, Al-P and Fe-P concentrations in surface sediments and TP concentrations in lake waters. However, the correlations between Ca-P concentrations in surface sediments and TP concentrations in lake waters were significantly negative. Moreover, significantly positive correlations were found between SRP concentrations in lake waters and TP、Ex-P、Al-P、Fe-P concentrations in surface sediments. There were significantly positive relationships between TP concentrations in surface sediments and biomass of Microcystin algae in lake waters.(2) The vertical distributions of TP、Ex-P、Al-P、Fe-P and Oc-P in three sediment cores taken from three typical lake areas showed an increasing trend from bottom to the surface sediments. On the contrary, the vertical distributions of Ca-P showed a reducing trend from bottom to the surface sediments. The increasing of TP、Ex-P、Al-P and Fe-P from the depth of18cm to the surface sediment in Meiliang Bay were associated withinputing huge amount of nutrients and sewage inputs originating from anthropogenic activities since1960s’. The results showed the concentrations of TP were significantly increasing from the depth of21cm to the surface sediment in the middle area of the lake. These results may related to the high nutrients in the northern area of the Lake Taihu since1990s’. The causes of increasing concentrations of TP、Ex-P、Al-P and Fe-P from bottom to the surface sediments in Lake Dongtaihu were the rapid development of fishing industry since the past two decades.(3) The average concentrations of nutrients in water column in lakes from Wuxi areas were the largest, and the average contents of nutrients in water column in lakes from Huzhou areas were minimum among the five researched areas in Lake Taihu watershed. The mean concentrations of total carbon, nitrogen and phosphorus in sediments of lakes from Huzhou areas were highest. The evaluations of water quality conditions showed the water quality of lakes in Wuxi, Yixing and Changzhou areas belong to five bad water quality. However, the water quality of lakes in Suzhou and Huzhou areas belong to five kinds standards. The evaluations of water eutrophication indicated the water eutrophication of lakes in Wuxi areas was the most seriously, however, the water eutrophication of lakes in Huzhou areas was the lightest among the five researched areas. Therefore, the water eutrophication degree of lakes in Lake Taihu Watershed was generally serious. Consequently, the water pollution of lakes in Wuxi area was the highest and the water pollution of lakes in Huzhou area was relatively light among the five researched areas respectively. Therefore, the spatial distribution differences of main nutrients in water, sediment and the eutrophication degree of lakes from Lake Taihu Watershed in this study would play an important role in ecological protection and pollution control.(4) The average concentrations of As in the sediments were relatively higher in Huzhou and Wuxi than other areas. The values exceeded environmental baseline in the basin of Lake Taihu. Average concentrations of Hg in the sediment from four areas (Huzhou, Suzhou, Wuxi and Yixing) were significantly higher than environmental baseline in the basin of Lake Taihu, except for Changzhou. The index Geoaccumulation (Igeo) values indicated that pollution intensity of As was unpolluted in all areas, but pollution intensity of Hg reached moderate level except for Changzhou. Therefore, the lakes from Lake Taihu watershed have been influenced by As and Hg, especially the heavy metal Hg pollution has been relatively serious.