| 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 sediment loads are higher which lead to a big risk releasing under certain conditions. Therefore, study the distribution of phosphorus in the sediments, the release characteristics and influence factors on the control of eutrophication of great significance to the Erhai Lake. This study is the part of "project of national water issues Erhai special". The characteristics of phosphorus forms, releasing characteristics and its influencing factors are studied in order to provide scientific evidence for the prevention and treatment of Erhai Lake in early eutrophication. The main results are summarized as follows:(1) The higher OM contents of surface sediments were distributed in the north or south areas of Erhai Lake, ranged from 2.50%to 11.72%. However, the higher TP contents ranged from 710.29 to 1961.22 mg/kg, and mainly distributed in the central part of the lake. The average concentration of OP in the sediments from Erhai Lake was from 212.71 to 526.23 mg/kg and the distribution trend was consistent with that of TP. The content of organic phosphorus with different extraction fractions ranked as following:Residual-Po> Fulvic-P> HCl-Po> NaHCO3-Po> Humic-P.(2) In the sediment, OM was positively associated with OP/TP, Fulvic-P and Humic-P(P<0.01), and negatively associated with Residual-Po(P<0.01). However, OM has no obvious relativity with NaHCO3 and HCl-Po(P>0.05). This showed that the organic matter might be greater impact on the lower activity of organic phosphorus than the higher activity of organic phosphorus in the sediments of Erhai Lake.(3) The main form of OP in the sediments from Erhai Lake was nonlabile organic phosphorus (the average is 58.74%) and the percentage of Fulvic-P to OP was higher than HCl-Po(except for E7 point). The characters of organic phosphorus forms from Erhai Lake resulted in its lower bioavailability. Accordingly, the distribution characters of organic phosphorus may be one of the reasons that lower concentrations of the phosphorus existed in the overlying water in Erhai Lake. However, TOC/OP<200 indicated that the risk of organic phosphorus's latent release is much bigger.(4) pH had great effect on phosphorus releasing in the sediment under both aerobic conditions and anaerobic conditions, but the law was not obvious. This may be related to the nature and sediment itself. The content of phosphorus releasing in the sediment was also affected by dissolved oxygen concentration in overlying water obviously. Due to the Erhai Lake in the aerobic phase currently, dissolved oxygen concentration in overlying water might be the main factor in Erhai sediment.(5) In the thermo-chemical treatment process, the content of phosphrus releasing from sediment in Erhai Lake was higher than in Taihu Lake. And the phosphorus form influenced on the content of phosphorus releasing in the Erhai Lake was Ca-P while in the Taihu Lake was OP. With the elevateing temperature, content of organic phosphorus decreased while NH4Cl increased in both Erhai Lake and Taihu Lake, this indicated that organic phosphorus might mineralize into active phosphorus and release into the overlying water with the humification in lake sediments. Thereby increasing the degree of lake eutrophication.
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