Distribution characteristics and pollution risk evaluation of the nitrogen and phosphorus species in the sediments of Lake Erhai, Southwest China.

Erhai is a famous tectonic rift lake in China. In this study, the distribution of nitrogen and phosphorus species in Erhai sediment cores and their ecology risk were evaluated. The total nitrogen (TN) in the sediment cores ranged from 1583.3 to 8018.5 mg/kg. Nitrogen (N) was still accumulating in the sediment. For depths of 0 to 25 cm, the TN decreased dramatically and for deeper depths the TN got stabilized. The proportions of various N fractions in the sediments of the study areas ranked as follows: the strong oxidation extractable N (SOEF-N) > the weak acid extractable N (WAEF-N) > the strong alkali extractable N (SAEF-N) > the ion-exchangeable N (IEF-N). The total phosphorus (TP) ranged from 814.9 to 1442.3 mg/kg. The vertical distribution of each fraction of phosphorus showed that there were different sources of sediment phosphorus around the Erhai Lake. The results of nitrogen and phosphorus pollution evaluation in sediments by single pollution standard index method showed that the standard index of the TN (STN) ranged from 4.29 to 14.01, and the standard index of the TP (STP) ranged from 1.69 to 2.18. It illustrated that N and P in the sediments were the serious ecological pollution risks in Erhai Lake.