Macroalgal blooms,which can dramatically modify nitrogen (N) and phosphorus (P) cycle in water column and sediments, have frequently occurred in coastal areas in recent decades. Rongcheng Swan Lake,a natural coastal lagoon in the eastern Shandong Peninsula in China,has suffered from extensive macroalgal blooms dominated by Chaetomorpha sp. over the past ten years. In the present study,Chaetomorpha sp. and the sediments from different areas of Swan Lake were collected to study the influence of algal decomposition on N and P levels in the overlying water. In addition,the changes of physio-chemical parameters in water were detected during the algal decomposition,and the release potential of N and P from sediments in different lake areas was estimated. The results showed that the concentrations of total nitrogen (TN) and NH_4 ~+ -N in water increased obviously during the first 16 days due to the decomposition of Chaetomorpha sp.,and reached 12.40 mg /L and 7.98 mg /L,respectively. However,the concentrations of total phosphorous (TP) and soluble reactive phosphorous (SRP) changed slightly at the early stage,and increased significantly after 19 days. During the experiment, the concentrations of TP and SRP ranged from 0.02-1.14 mg /L and 0.01-0.40 mg /L, respectively. After 16 days of the incubation experiment,N and P concentrations in water at different sediment treatments both followed the order of: treatment with sediments and algae > treatment with only algae > treatment with only sediments. There were great differences in the release ability of N and P from the sediments among different lake regions. Under the condition of algal decomposition,the order of the release amount of N and P from the sediments was: center > northwest > south and northwest > center > south. During the degradation phase of green tide,the concentrations of TN,NH_4 ~+ -N,TP and SRP in water all increased significantly due to the nutrient release from the decomposing Chaetomorpha sp. and the indirect effect on promoting nutrient release from the sediments,which aggravated the level of water eutrophication in Swan Lake,China. Therefore,the massive decomposing Chaetomorpha sp. should be salvaged quickly to avoid the deterioration of water quality in Swan Lake
