Responses Of Lake Water Calcium And Magnesium To Regional Acid Deposition And Algae Blooms In Taihu Lake

Taihu Lake is the third largest freshwater lake in China, located in the downstream of the Yangtze River. Along with the rapid economic development in Taihu watershed and the increasing energy consumption, acidic substance emission is huge, acid rain and its frequency are still intensive in the basin. Taihu Lake is one of the most severely eutrophic freshwater lakes in China. Based on the studies of the natural lake water chemistry and regional acid deposition, with calcium and magnesium as the study objects, we analyzed the behavior of Ca2+, Mg2+ in the lake water under the effects of both the regional acid deposition and cyanobacteria biomineralization. The results showed that compared with the natural condition, acid deposition enhanced carbonate-weathering process of Ca2+, Mg2+ in the soils of the basin and thus resulted in the significant increasing of Ca2+, Mg2+ in the lake water. Acid deposition inputs more and more acids into the lake water resulting in the decreasing of pH value and the saturation index of CaCO3 and MgC03. While, cyanobacteria biomineralization in the lake water promotes saturation precipitation of carbonate (CaCO3, MgCO3), contributing to the removal of Ca2+, Mg2+ from the water phase to the sediment phase in Taihu Lake. However, compared with the MgCO3 precipitation, CaCO3 was the dominance precipitation in the process of biomineralization.Through analyzing the sediment physicochemical properties, the sediment Ca and Mg species, the results showed that the acid-extracted Ca in the sediment accounted for 17-30% of the total content Ca, and acid-extracted Mg accounted for 9-14%. Due to the strong buffering capacity of carbonate minerals to acid deposition, the sediment is still in a neutral or weak alkaline environment. The organic matters and the conductivity were higher in the east lake surface sediment (0-10 cm) than that in the northwest lake. During the stronger acid deposition, the more inputs of acids caused the dissolution of acid-extracted Ca and Mg in the surface sediment (0-10 cm) in the northwest lake, while for the east lake, it was probably the phosphorus-bound calcium dissolution. On the contrary, cyanobacteria biomineralization in the lake water promoted carbonate (CaCO3, MgCO3) accumulation in the surface sediment, which explains that acid-extracted Ca and Mg in the surface sediment in the northwest lake (algae dominant area) were higher than that in the deeper sediment (>10 cm). Simultaneously, they were also higher than that in the east lake (hydrophytes dominant area). Statistical analysis showed that acid deposition exerted a stronger impact on the variation of acid-extracted Ca and Mg in the surface sediment than the bio-mineralization in Taihu Lake. Whereas, no significant change of the total Ca and Mg concentration between the surface and deeper sediment in the northwest lake indicates that the carbonate precipitation via biomineralization and the carbonate dissolution via acid deposition are in a dynamic balance.Both the acid deposition and biomineralization in Taihu Lake play significant role in the circulation of Ca2+ and Mg2+ in the lake water-sediment phase. Currently, the lake water Ca2+ and Mg2+ are still increasing, the economic value and ecological value of the lake water will be damaged due to the rising of the lake water total hardness (Ca2+ and Mg2+). We recommend that during the control of eutrophication in Taihu Lake, acid deposition needs to gain sufficient attention and this condition should be improved effectively at the same time.