Abstract: With the drastic increase number of large-sized thermal (nuclear) power plants along the coastal areas,theamage of zoop lankton caused by chlorination in cooling water has become a serious marine ecological safety problem inhina1 Unfortunately,very little work has been carried out on the tolerance of copepods to the chlorination1 Moreover,here is no discharge criterion of the residual chlorine level in coastal power plants cooling systems in country1 The majorbjectives of this study are to 1) study the tolerances ofmarine copepods to residual chlorine under different thermal shockemperature,-) supp ly scientific basis for chlorination and residual chlorine risk assessment during environmental evalua-ion,and 3) provide suggestions for regulation about the drainage standard of residual chlorine concentration in coolingater1he experimental following twelve copepod species were collected from the Yueqing Bay in all the seasons: Calanusinicus,Labidocera euchaeta,Eucalanus subcrassus,Paracalanus aculeatus,Paracalanus crassirostris,Centropagesorsispinatus,Euchaeta concinna,S inocalanus tenellus,Calanopia thompsoni,Acartia pacifica,Acartia spinicauda andarpacticus unirem is. At each samp ling time (08/-006,11/-006,01/-007 and 05/-007),the natural water tempera-ures were -810,-110,1010 and 1610℃,respectively1 After two days’laboratory maintenance,healthy adult animalsere chlorine dosed for 30 min under different temperature increases (ΔT) (0,410,810 and 1-10℃) 1 The median le-hal concentration (LC50 ) of experimental species to residual chlorine was determined by theirmortalities after -4h accord-ng to the probit analysis1he results showed that the LC50 of experimental copepod species to residual chlorine decreased conspicuously withhe rising ofΔT (P e.g. P. aculeatus,P. crassirostris,A. pacifica and A. spinicauda) were more sensitive to theesidual chlorine than the larger ones (C. sinicus,L. euchaeta,E. subcrassus and E. concinna) 1 Thus,the copepod spe-ies diversity might reduce and the composition of copepod communities might simp lify due to the chlorination in coolingaters1f the safe concentration of residual chlorine on copepodswas determined by the formulation of LC50 ×015,the safeoncentration of various copepod species in spring,autumn and winter were 01-1-0186 mg/L,01-6-0186 mg/L and131-4155 mg/L,respectively,as the exposure duration ≤ 30 min andΔT ≤ 1-10℃,while the safe concentration ofarious copepod species were 0115-0137 mg/L as the exposure duration ≤ 30min andΔT ≤ 810℃ in summer1 Basedn these data,the current typical chlorination with high dosage (about - mg/L) affected the copepods in the coastal power lants cooling systems,but the common discharging residual chlorine (about 0115-01-5 mg/L) did not influence them1s far as the marine ecological protection was concerned,in subtropical area,the discharging residual chlorine levelhould not exceed 0115 mg/L in summer and 01-0 mg/L in the rest of seasons based on the safe concentration of small-sized copepod specieswhich were sensitive to the chlorination in different seasons1 However,during the criteria regulationbout the discharging level of residual chlorine,whether this residual chlorine at certain chlorine dosage could effectively revent the cooling systems from biofouling should be considered,achieving the op timal distribution of environmental risknd economic benefit1ore researches should be conducted to examine the LC50 of dominant copepods and other hydrobios (e.g. zoop lank-on,fish and shellfish) to residual chlorine in different regions,and it is also necessary to study the toxicity of residualhlorine on the marine organisms under different conditions,such as temperature,salinity,water pressure,pH and expo-ure duration1 Besides,further work should be required to assess the long-term response of the copepod community underhe chlorination stress of coastal power plants