Atmospheric Wet Deposition Of Nutrient And It’s In Situ Enclosure Experiment In Lake Taihu Watershed

The input of nitrogen, phosphorus nutrients from atmospheric dry and wet deposition is one of the important process of lake eutrophication and also an important part of the biogeochemical material cycles. Currently, the most central problem in the security of lake ecological system and the function of lake social services in China is about lake entrophication, the frequent occurrence of cyanobacteria blooms and degradation of ecosystems. However, the attention has always been focused on non-point source pollution of lake basins, and the process of its emissions in contribution to the lake eutrophication, while little attention was paid on the understanding of effects of input of nitrogen, phosohorus from atmospheric wet and dry deposition.In the present study Taihu Lake basin was firstly as the research object to study pollutant characteristics of nutrients from the atmospheric wet deposition in order to provide the basic data in finding the methods to control the eutrophication pollution in Taihu Lake. Rainwater samples of5representative sampling sites in Taihu Lake basin were collected from May2012to December2012. The content of nutrients (TN、 NO3--N、NH4+-N、TP、DIP) in rainwater were measured, and the characteristics of chemicals in atmosphere were also analyzed. And then, to explore the effects of nitrogen, phosphorus nutrients from the wet deposition atmosphere on nutrient structure and growth of algae in lake water body, an in situ enclosure experiment at Meiliang Bay of Taihu Lake was conducted to compare parameters including the amount of dissolved oxygen, temperature, concentration of nutrients and chlorophyll a (Chl.a) before and after rainfalls. The results showed that:(1) The investigation of pollutant characteristics of nutrients from atmospheric wet deposition in Taihu Lake showed, during the period from May2012to December2012, the total rainfall of eight mouths was919.6mm, and rised by13%compared with the same period in previous year. Due to typhoon influence, the main rainfall concentrated in the summer, accounts for about53.0%of the total rainfall, and rainfall in August was up to275.3mm. If pH value of5.60was as acid rain standard, the frequency of acid rain in Lake Taihu Basin is74.7%. The lowest average pH appeared in August. Perhaps the reason for this was that the rainfall was heavy in August, and continuous cloud erosion lead to the clear of the various pollutants of the atmosphere under the cloud, and the alkaline coarse particle in atmospheric aerosol was washed away which greatly weakened the neutralization of atmospheric aerosol, and finally it greatly increased the risk of the occurrence of acid rain.In the present study, the contents of nutrients in rainwater were detected and calculated. The average concentrations of TN and TP from the atmospheric wet deposition were4.1mg/L and0.09mg/L, respectively, which suggested the atmospheric wet deposition in Taihu Lake basin was eutrophic. The wet deposition flux was5160.8kg/(km2·a) for TN,112.5kg/(km2·a) for TP. The annual atmospheric load of TN and TP were12062t/a and263t/a respectively. In Taihu Lake basin, the concentration of nutrients in wet deposition had a negative correlation with rainfall. The concentration of TN and TP in wet deposition was low in the months when rainfall was frequent. On the contrary, the concentration of nutrients was usually high in the months when rainfall was low. The reason of this was that nutrient materials in the air persisted for a long time and led to a high concentration of precipitation. There were significant differences in monthly average concentration of nutrient elements in the present study. Also there is significant increase of the atmospheric nutrient concentration in west pollution control region than that in other regions.(2) The results in situ enclosure experiment showed that TN and TP in enclosure group1which received wet deposition declined by59.9%and29.9%due to rainfall dilution respectively. TN and TP in enclosure group2which didn’t receive wet deposition declined by15.5%and13.3%, while TN and TP in lake water increased by42.7%and19.3%after raining. This result suggested that the main pollution source of Lake Taihu during the period of rain was not wet deposition but a runoff which is rich in nutrient.In enclosure group1(atmospheric wet deposition was the only nutrients source for algae growth and reproduction), chlorophyll a rised by21.6%and algae in water body of enclosure group1maintained a good condition of growth. The concentration of chlorophyll a in enclosure group2declined by16.0%. In water body of Taihu Lake, the concentration of chlorophyll a increased by28.6%, indicating eutrophication in northern Lake Taihu area was enhanced possibly by the input of nutrients from wet deposition during high water temperature period in autumn. That means cut or reduce the surrounding exogenous contamination is necessary to control the nutrient concentrations in Taihu Lake.