Sedimentary diatom flora changes and eutrophication reconstruction for the last 200 years in Lake Chaohu

The evolution of diatom flora and the dynamic process of total phosphorus concentration (TP),were reconstructed for the last 200 years in Lake Chaohu based on the sedimentary diatom analysis and radionuclide dating for a 40cm long core from the centre of West Lake Chaohu,combining with an established diatom-TP transfer function in Yangtze floodplain lakes. The results showed that the diatom flora experienced two important stages from the Aulacoseira granulata predominant assemblages (1826-1978AD) to the Cyclostephanos dubius predominant flora (since 1978AD). Before 1978AD TP remained around 60-75μg/L and afterward increased rapidly to 80-100μg/L,which reflected an obvious nutrient accumulation. Since 2000AD TP had increased to 110μg/L and C. dubius became dominant species. The relationship between main changes of diatom flora and TP reflected that diatom flora changes mostly attributed to the nutrient enrichment. However,hydrodynamic conditions also had certain influence on diatom composition in the historical period. Since 1970s industrial,agricultural and domestic sewage had become major cause of the eutrophication and diatom flora changes,while shift of hydrodynamic conditions resulted from the establishment of the Chaohu Gate was also an incentive of nutrient accumulation. From the beginning of the 21st Century the deterioration of eutrophication may be related to domestic sewage discharges,internal nutrient releases and the global warming.