Spatial And Temporal Distribution Of Nitrogen And Phosphorus And Ecological Risk Assessment Of Dongting Lake

As an important natural ecosystem of the earth, lakes are the force of the society development and have many important functions of services. In recent decades, the intensification of human activities is causing lakes pollution, ecological environment deterioration, disasters and economic losses. Lakes have become one of the most sensitive areas in the interaction between man and nature, which needs more money and time to deal with. Eutrophication is one of the most important lakes pollution problems. The concentration of nitrogen and phosphorus are the key factors of the planktonic algae growth and eutrophication. Therefore, there is an important significance to explore the distribution of nitrogen and phosphorus contents, seasonal variation and nutritional characteristics of water for the prevention and treatment of lakes eutrophication.This paper made the Dongting Lake and four rivers as the research object, through collecting the history data, analyzing nitrogen and phosphorus contents of the overlying water and sediments, discussing the nutrient spatial and temporal distribution and nutritional characteristics, exploring the particle size and assessing the ecological risk, we draw the following main conclusions:(1) The concentration of total nitrogen (TN), total phosphorus (TP), ammonia (NH4+-N) and nitrate (NO3--N) in the overlying water were1.35～2.98mg/L,0.02～0.22mg/L,0.03～0.68mg/L and0.07～0.91mg/L, respectively. The average concentration of TN, TP, NH4+-N and NO3--N were2.34mg/L,0.06mg/L,0.27mg/L and0.54mg/L, respectively. The content of TN, TP, NH4+-N and NO3-N in sediments were558.082846.51mg/L,357.74～998.25mg/L,11.92～78.12mg/L and0.92～19.37mg/L, respectively. The average content of TN, TP, NH4+-N and NO3-N were1220.47mg/L,678.97mg/L,28.94mg/L and4.41mg/L, respectively.(2) The concentration of nitrogen and phosphorus in June generally are higher than that in January. Higher concentration of phosphorus and nitrogen occurred in the estuary of inflowing river. The average concentration of phosphorus and nitrogen in East Dongting Lake were highest compared with other regions of Dongting Lake. The comprehensive nutrition state index in June is higher than that in January.(3) Compared with other eutrophic lakes, alga blooms in large scale did not happen with relatively higher concentration of TN and TP in overlying water and sediments in Dongting Lake, which could be explained by the special conditions of hydrology and hydrodynamic force causing lower content of chlorophyl.(4) The particle size is increasing from east to west. The particle size of the Esat Dongting Lake is the smallest.(5) Ecological risk assessment showed that the standard index of TN in sediments in Dongting Lake is greater than1. It means the content of TN in sediments is excessive and the pollution of nitrogen is severely. The standard index of TP is equal or greater than1. It means the content of TP in sediments also is higher, especially in the estuary of tributaries and the East Dongting Lake. According to the criteria of sediment classification which was made by the U.S. EPA, the content of TN in sediments in Dongting Lake is between the lowest level of ecological risk and severity of effects. The content of TP has been ecological effects on the environment of certain hazards.