Effects Of Environmental Factors On N Release In Sediment From Erhai Lake

Erhai is the second largest freshwater lake in the Yunnan-Guizhou Plateau.It is currently at the initial stage of eutrophication, and the water quality is relatively good, but the contents of nitrogen and phosphorus in the Erhai sediment are higher than that in most eutrophic lakes. Sediment conditions of high organic matter and high nitrogen and phosphorus are bound to have greater risk of releasing. In this study, three sediment core samples were chosen from different points of Erhai. Under in-situ simulation, we analyzed the coupling effects of pH value and dissolved oxygen on the concentration changes of ammonia, nitrate in the overlying water, and the chemical forms of nitrogen in the sediment. The results indicate that:(1) Under anaerobic conditions, ammonia concentrations in overlying water were higher and tiptop of aerobic conditions 5 times. However, there are little differences of overlying water nitrate concentrations between anaerobic conditions and aerobic conditions. Under different dissolved oxygen in overlying water of the three points nitrate concentrations are maintained at an average of 0.1mg /L to 0.12mg / L.(2) Nitrogen concentrations in the overlying water of Erhai have less variation under different pH in aerobic conditions. Erhai nitrogen concentrations under different pH values in anaerobic conditions have more obvious changes than aerobic conditions.(3)The nitrogen concentrations in overlying water in the culturing process increase in different culture systems of dissolved oxygen and pH. There ere differences among samples. The ammonia nitrogen concentration in the overlying water was highest when pH=6 in anaerobic condition. But for the other two points, the ammonia nitrogen concentration increased most when pH=10 in anaerobic condition. Besides, the nitrate nitrogen concentrations have less changes under different conditions.(4) Due to the differences in physical and chemical properties of sampled sediments from these three points, dissolved oxygen shows different levels of effect on the nitrogen releasing, leading to the variations in the nitrogen concentrations in cultured sediments. The contents of forms of nitrogen in cultured sediments with higher levels of nitrogen pollution were significantly lower than the less contaminated sediment nitrogen, the change trends of forms of nitrogen in cultured sediments are quite different.According to the results listed above, it can infer that due to the Erhai Lake in the aerobic phase currently, changes of pH will not have a great impact on nitrogen concentration in overlying water. But if the external pollution sources can not be controlled effectively around the Erhai Lake, once the outbreak of large blooms, water environmental conditions changed, dissolved oxygen content decreased, pH values increased,and the results of this study show that this can make the ammonia concentration in the overlying water increase, and this will lead the ammonia concentration in overlying water in regions which is less affected by external source contamination and less prone to bloom suffer a tremendous impact due to the increase of internal pollution. Therefore, we should strengthen the external source pollution control and avoid the pollution caused by exogenous changes in environmental conditions, which lead to pollution increasing, create a vicious cycle, increase eutrophication of Erhai Lake.