Nitrogen Occurrence Characteristics and Reason Analysis in Different Trophic Status Freshwater Lakes

Based on one-year monitoring about the different trophic status freshwater regions of Lake Taihu, the temporal-spatial distribution and occurrence characteristics of nitrogen in the water, porewater, and sediments and their correlation with main aquatic environmental factors were analysed. The results showed that the concentrations of TN in overlying water and sediment ranged from 0.22 to 7.74 mg/L and 551.5 to 1542.8 mg/kg, respectively. The concentrations of NO3-N in overlying water, sediment, and porewater ranged from 0.04 to 3.86 mg/L, 14.3 to 42.5 mg/kg, and 0.01 to 0.72 mg/L, respectively. The concentrations of NH4 -N in overlying water, sediment, and porewater ranged from 0.03 to 0.25 mg/L, 17.7 to 78.2 mg/kg, and 0.41 to 7.03 mg/L, respectively. NH4 -N in overlying water had no significant spatial-temporal variation and the annual mean of NH4 -N in sediments was highest in Meiliang Bay. The annual mean of TN and NO3-N in overlying water and sediments was highest in Western Taihu Lake. The spatial and temporal distribution characteristics of NH4 -N and NO3-N in porewater were roughly consistent with those in sediments. Nitrogen in the sediment occurs in the form of organic nitrogen. Nitrogen in the overlying water was principally of NO3-N in Meiliang Bay and Gonghu Bay, and was principally of organic nitrogen in Xukou Bay. In the Western Taihu Lake, nitrogen in the overlying water was principally of NO3-N in summer and fall, and organic nitrogen in spring and winter. The results suggest that the type of organic matter in sediments was an important factor affecting the nitrogen occurrence characteristics and trophic status in aquatic environment. The correlation analysis showed that TN and NO3-N in overlying water was positively correlated with various forms of nitrogen in sediments, indicating that there was a strong exchange of nitrogen nutrients between water and sediments.