To evaluate effects of thermal effluent on the planktonic ecosystem,we carried out a mesocosm experiment according to the sea surface temperature gradient,near Guohua Ninghai Power Plant in Xiangshan Bay during October 5th to 11th of 2010,and determine biomass(B),respiration rate(R)and primary productivity(P)of marine planktons and the ratio of total primary production and total respiration(TPP/TR)and turnover rate in the planktonic ecosystem.The results show that:in the range of 25.20-26.40 ℃,effects of 0.25 ℃ warming on the B,R and P of all plankton were not obvious,0.52 ℃ warming can elevate the R,P of microplankton and nanophytoplankton.Effects of 0.52 ℃ warming on the B,R and P of picoplankton were not obvious.The TPP/TR and turnover rate in the planktonic ecosystem were increased 1.2 times and 1.5 times,respectively.A theoretical basis is provided for evaluating effects of thermal effluent on the marine planktonic ecosystem.