Examining the Vertical Distribution Characteristics and Pollution Levels of Total Nitrogen,Total Phosphorus,and Organic Matters in the Sediments of Nashina Lake

Associated with agricultural growth, industrial development, as well as urban population growth in the Nashina Lake region, pollutants from agricultural, industrial and domestic activities have led to the deterioration of water quality. Nashina Lake is in the center of the Lianhuan Lake, an important natural resource protection zone with a geographical area approximately 533 km2. In order to reveal the evolution process of eutrophication of the Nashina Lake, sediment core samples were collected from the lake using a gravity sampler in July2010. Contents of total nitrogen(TN), total phosphorus(TP) and organic matters(OM) of the sediments samples were determined and the vertical distribution characteristics of TN, TP, OM, and TOC/TN were analyzed.The results show that: TN concentrations in the sediments were ranged from 1 156 mg/kg to 4 191 mg/kg, with an average of 2 891 mg/kg. The TP concentrations in the sediments were ranged from 358 mg/kg to 509 mg/kg,with an average of 442 mg/kg. The concentrations of OM in the sediments were ranged from 1.29% to 3.64%,with an average of 2.03%. Comparatively, the concentrations of OM hada lower variation level, while those of TN and TP fluctuated significantly. The concentrations of TN, TP, and OM generally showed an ascendant trend, especially in1999-2009. The TOC/TN ratios in the lake sediments were ranged from 2.50 to 7.94, with an average of 4.35. Analysis of results also indicates that the OM and nutrients were derived from aquatic plants, zooplankton, and phytoplankton and alga. Based on the examinations of organic indices, we also found that the impacts of human activities were insignificant from 1831 to 1999, as the lake was continuously in the natural state, with organic matters mainly authigenicand the water belonging to the less clean category. With the rapid economic development from 1999 to 2009, associated with the intensified activities of agricultural, fisheries, animal husbandry and tourism, higher levels of feed and animal manure resulted in higher levels of OM and TN contents. As a result, the organic indices were much higher in 1999-2009. Finally, the organic nitrogen indices were very high for the whole period, indicating the high risk of organic pollution.