[Variation of water quality and trophic state of Lake Tianmu, China].

Water quality survey was carried out in 2006 and 2007 in Lake Tianmu (Shahe Reservoir) of China in order to make clear the eutrophication reasons of urban lake-type reservoirs in the middle and lower reaches of Yangtze River, and then put forward the corresponding control measures for such reservoirs. The survey frequency was once a month in 2006 and once two months from May 2007 to Dec. 2007. The results indicate that total nitrogen (TN) and total phosphorus (TP) average concentrations were 1.49 mg/L and 0.06 mg/L respectively in 2006, which are 2.7 and 2 times those of 2001-2002. The average permanganate index was 3.68 mg/L in 2006 and it is a little higher than that of 2001-2002. Secchi disk depth (SD) was 1.2 m in 2006 and it is 25 cm lower than that of 2001-2002. The average algal biomass of 2006 was 46.39 mg/L. All of these show that Lake Tianmu has been situated meso-eutrophication in 2006 from the meso to meso-eutrophication in 2001-2002. Control measures of Lake Tianmu were put forward according to the joint survey results of water quality, sediments, fishery, plankton, benthos etc. in 2006, which considered exogenous, endogenesis and substance cycle of the lake ecosystem in together. The measures pointed out that it was needed to control the pollution of agriculture, river, tourism, soil and water erosion as well as fishery etc. in order to increase SD of the reservoirs. TN and TP average concentrations in 2007 were 1.25 mg/L and 0.05 mg/L respectively, and the average algal biomass of 2007 was 22.56 mg/L. Water quality variation between 2006 and 2007 showed that the control measures had effect on reducing nutrients contents and phytoplankton biomass of the lake.