| Located in the Yangtze River Delta region, Taihu Lake is one of the districts with highest regional population density and most developed economy in China. The rapid development of industry, agriculture and the intensified urbanization have led to the increasing serious air pollution in Lake Taihu and made it becoming one of the most serious eutrophic lakes.Compared with deep lake, the unit volume of water in shallow lake has a larger area for water and sediment. While the distinct features "source-confluence" of sediment not just helps to adopt and reserve the pollutant, but can lead to the second pollution of water. Taihu Lake is a typical shallow lake and has reserved large quantity of nutrients and organics in the sediment. When the sediment is disturbed by wind wave and the activities of creature, the nutrients and organics will come into the water again which forms the pollution load in endogenous lake, and intensifies the eutrophication of the lake and increase the possibility for the outbreak of blue algae bloom. Thus, the research on the source of organic in water has great practical significance on the evaluation of quality condition in water, and the research on the nutrition quality change of the lake. Moreover, it also provides a theory basis for governing Taihu and shallow lake.Taihu is divided into five parts in this paper:North Lake District (Zhushan Bay, Meiliang Bay and Gong Lake); West Lake District (west coastal zone); South Lake District (north coastal zone); East Lake District (East Taihu and Xu Lake) and Huxin Lake District. The sampling of water and the sediment on the surface have been collected in the five lake districts. The survey of water environment status is conducted in Taihu and the source of organics are also estimated which are in surface layer sediment of Taihu.Based on the index of total nitrogen (TN), total phosphorus (TP), total organic carbon (TOC, dissolved oxygen (DO), transparency (SD), PH value (pH) in water samples, the water quality can be evaluated by the single factor assessment method and comprehensive pollution index method. The evaluation results show that the overall water quality of Taihu reaches to the IV water standard on the earth surface, appears to be the moderately polluted state, and the main pollution factors are TN and TP; the major polluting regions are Zhushan Bay, Meiliang Lake and western lakeshore. ANOVA analysis shows that the TN in north Lake District is significantly higher than that in the central Lake District (P<0.05), TP is significantly higher than that in east Lake District (P<0.05), TOC is significantly higher than that in east and south Lake District(P<0.05); TOC in east Lake District is significantly lower than that in north and west Lake District (P<0.05); TP in West Lake District is significantly higher than that in south and easte Lake district (P<0.05) and TOC is significantly higher than that in south Lakes district (P<0.05); DO in the five lake districts has no significant difference. The results indicate that the north and west Lake Districts are effected by the rivers which flow into the lake, and their nutrient loading is high. Meanwhile, under the influence of storms and hydrodynamic force, the pollutants are prone to gather; East and south Lake Districts have flourishing higher aquatic plants with good quality self-cleaning function, and the water environment is in good condition.Based on the contents of stable carbon isotope (δ13C), stable nitrogen isotope (δ15N), total organic carbon (TOC) and total nitrogen (TN) in the sediments distributed in the whole lake, the spatial distribution of each element and the difference of lake districts in the five districts are discussed. The δ13C and δ15N values in the eastern Lake District are significantly lower than those in other lake districts (P< 0.05). The rate of C and N is not significantly different from other districts. The TOC has a positive correlation to TN in Taihu. The result shows that the sediment of δ13C is heavier and the δ15N is lighter due to the wider distribution of water vascular plants in the eastern Lake District than others. The northern Lake District is influenced by the algae blooms, so that the sediment of δ13C is lighter. The sediment of C/N accords with the characteristics f alga and phytoplankton, which indicates the main sources of the organic matter are alga and phytoplankton in the five lake districts.In addition, the dyadic analysis method of rate of the 13C and C/N is adopted to qualitatively determine the organic matter source of the sediment with the help of the model of isotope to calculate the contribution ratio (f).The results show that the main sources of the sediment organic are the lake freshwater algae and aquatic plants, while the terrigenous origin is not obvious. The contribution rate of the freshwater algae to the organic in the lake accounts to 65%. Among them, the contribution rate (fa) of the freshwater algae to the organic matter accounts to 100% in Zhushan Bay. The northern and the western lake districts respectively are 72.1% and 72.7%. The contribution rate of fa is equivalent to that of the aquatic plant of sediment organ (fv) in the eastern Lake District. In the nearshore of East Taihu which is covered by aquatic vegetation, the fv reaches to 93.1%.The characteristics of water environment and the source identification of organic show that there are obvious differences in the characteristics of water environment and the water environment quality is influenced by the inflow rivers. The spatial distribution of the C and N elements in the sediment matches that of the lake vegetation. The organic in the lake sediment mainly comes from the interior source. The distribution rate of the freshwater algae to the organic is 65%. |
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