Abstract: Effects of low water level on water quality and ecology of the Tianmuhu Reservoir,Liyang,China were evaluated by means of high-frequency in situ monitoring of the water quality and plankton in the reservoir duting the period from September 2008 till August 2009,when the water level of the reservoir was lowered down by 1.73 m below the mean water level of the years 2004-2008 for reinforcement.It was found that the reservoir weakened its buffering capacity of incoming exogenous nutrients in that year of low water level operation.Peaks of TN concentration was closely related to heavy rainfall.One early spring heavy shower(Feb.17-27,2009)immediately led to a sharp rise of the ρ(TN)of waterbody from 0.92 to 2.09 mg·L^-1 ,which returned to normal only after the end of the summer storm season.TP concentration in the waterbody was mainly affected by the biomass of plankton and the content of supspended substances therein and littleby the low water level.In the summer of 2009,the proportion of Cyanophyta among the phytoplankton was obviously higher than that in the summer of 2008,suggesting that the entire alga community structure tended towards that under eutrophication.Compared with that in June August,2008,the mean abundance of protozoa was 1730 L^-1 and of rotaria 4188 L^-1,in June August,2009,almost doubled,that of copepod decreased by 21% but of cladocera fluctuated sharply.Moreover,the Chl-a nutrition index of the waterbody increased.In general,low water level makes the whole ecological system more sharply,and the waterbody more susceptible to changes in environmental conditions.It is,therefore,suggested that it is essential to keep a suitable water level in management of reservoirs in terms of water quality and ecology.

Key words: low water level, reservoir, aquatic ecology, eutrophication