With the drastic increase number of large-sized thermal(nuclear) power plants along the coastal areas,the damage of zooplankton caused by chlorination in cooling water has become a serious marine ecological safety problem in China.Unfortunately,very little work has been carried out on the tolerance of copepods to the chlorination.Moreover,there is no discharge criterion of the residual chlorine level in coastal power plants cooling systems in country.The major objectives of this study are to 1) study the tolerances of marine copepods to residual chlorine under different thermal shock temperature,2) supply scientific basis for chlorination and residual chlorine risk assessment during environmental evaluation,and 3) provide suggestions for regulation about the drainage standard of residual chlorine concentration in cooling water.The experimental following twelve copepod species were collected from the Yueqing Bay in all the seasons: Calanus sinicus,Labidocera euchaeta,Eucalanus subcrassus,Paracalanus aculeatus,Paracalanus crassirostris,Centropages dorsispinatus,Euchaeta concinna,Sinocalanus tenellus,Calanopia thompsoni,Acartia pacifica,Acartia spinicauda and Harpacticus uniremis.At each sampling time(08/2006,11/2006,01/2007 and 05/2007),the natural water temperatures were 28.0,21.0,10.0 and 16.0℃,respectively.After two days' laboratory maintenance,healthy adult animals were chlorine dosed for 30 min under different temperature increases(ΔT)(0,4.0,8.0 and 12.0℃).The median lethal concentration(LC50) of experimental species to residual chlorine was determined by their mortalities after 24h according to the probit analysis.The results showed that the LC50 of experimental copepod species to residual chlorine decreased conspicuously with the rising of ΔT(P 0.001),and most of them represented significant regression(P 0.05).Tolerance of the same species to residual chlorine decreased markedly(P 0.01) with the elevation of acclimation temperatures in different seasons.Thus,in subtropical sea area,any further increase in temperature of the cooling water could severely enhance the toxicity of residual chlorine on copepods in summer.As to the eleven calanoid copepod species,it seemed that the copepods with small body size(e.g.P.aculeatus,P.crassirostris,A.pacifica and A.spinicauda) were more sensitive to the residual chlorine than the larger ones(C.sinicus,L.euchaeta,E.subcrassus and E.concinna).Thus,the copepod species diversity might reduce and the composition of copepod communities might simplify due to the chlorination in cooling waters.If the safe concentration of residual chlorine on copepods was determined by the formulation of LC50 × 0.5,the safe concentration of various copepod species in spring,autumn and winter were 0.21— 0.86 mg/L,0.26— 0.86 mg/L and 0.31— 4.55 mg/L,respectively,as the exposure duration ≤ 30 min and ΔT ≤ 12.0℃,while the safe concentration of various copepod species were 0.15— 0.37 mg/L as the exposure duration ≤ 30min and ΔT ≤ 8.0℃ in summer.Based on these data,the current typical chlorination with high dosage(about 2 mg/L) affected the copepods in the coastal power plants cooling systems,but the common discharging residual chlorine(about 0.15— 0.25 mg/L) did not influence them.As far as the marine ecological protection was concerned,in subtropical area,the discharging residual chlorine level should not exceed 0.15 mg/L in summer and 0.20 mg/L in the rest of seasons based on the safe concentration of small-sized copepod species which were sensitive to the chlorination in different seasons.However,during the criteria regulation about the discharging level of residual chlorine,whether this residual chlorine at certain chlorine dosage could effectively prevent the cooling systems from biofouling should be considered,achieving the optimal distribution of environmental risk and economic benefit.More researches should be conducted to examine the LC50 of dominant copepods and other hydrobios(e.g.zooplankton,fish and shellfish) to residual chlorine in different regions,and it is also necessary to study the toxicity of residual chlorine on the marine organisms under different conditions,such as temperature,salinity,water pressure,pH and exposure duration.Besides,further work should be required to assess the long-term response of the copepod community under the chlorination stress of coastal power plants.