Effects of Sediment Nutrients Release on Enclosure Ecosystems

Effects of sediment on lake ecosystem were studied by using enclosures. Eight enclosures were set up in Lake Donghu. Enclosure E1-E4 used sediment from eutrophic station I and enclosure E5-E8 used sediment from oligotrophic station Ⅲ. Lake water was pumped into enclosures E1,E2 and E5,E6,tap water was added into other four enclosures. The experiments lasted for about two and a half months. The results showed that pH,TN and TP were significantly higher in E1-E4 than those in E5-E8. Microsytis bloomed continuously in enclosures E1-E4 since August E1-E4 had much higher phytoplankton biomass than enclosures E7,E8 where no blooms were formed. E5,E6 had Microsystis blooms only for a short period of time.The index of ploytoplankton biodiversity was much lower in E1-E4 enclosure than that of E7,E8 enclosure. The above results indicated that the effect of sediments on shallow water ecosystem was much higher than water column,and the threshold TP value for Microsytis blooms was probably between 0.06-0.10 mg/L,i.e. between the average TP contents in E5,E6 and E7,E8.