Eutrophication evolution of typical small and medium-sized lakes in Lake Taihu basin(1991-2010)

The eutrophication evolution process and characteristics of typical small and medium-sized lakes in Lake Taihu basin in the recent 20 years were investigated.Results showed that total nitrogen concentration exhibited a slow downward trend,while total phosphorus concentration showed upward trend during the past 20 years.In response to this,the algae biomass expressed as chlorophyll-a increased gradually and the average transparency(secchi depth) decreased totally.The average chlorophyll-a concentration had increased more than three times since 1991.Standard deviation of chlorophyll-a increased from 1.25 μg/L in 1991 to 19.06 μg/L in 2010,indicating that spatial heterogeneity of eutrophication and algae bloom occurred more frequently.Different types of lakes varied in the eutrophication process.It was not only controlled by nutrient input,but also by its own characteristics of the lake.The calculated Comprehensive Nutrition State Index showed that eutrophication status of Lake Dianshan and Lake Kuncheng were slightly eutrophic but more serious than Lake Shanghu and Lake Kuilei,which were mesotrophic.Designated use and water resource management model of lakes were the important factors in the lake eutrophication.Lake Shanghu and Lake Kuilei,which are under strict management as enclosed drinking water resource area,had stable lower nutrient concentrations.As a typical open lake,Lake Dianshan was classified as middle-eutrophicated TN and TP concentrations of which were increasingly high.As a typical fish-culture type lake,eutrophication problem was becoming more serious due to high-density fish culture in the past 20 years in Lake Changdang,with higher nutrient concentration,higher chlorophyll-a,and lower transparency.