不同季节升温条件下余氯对桡足类的毒性

TOXICITY OF RESIDUAL CHLORINE ON COPEPODS UNDER TEMPERATURE INCREASE IN DIFFERENT SEASONS

为探明桡足类对滨海电厂冷却水中余氯的忍受能力,在各季节于室内对采自乐清湾的12种桡足类进行30min不同升温幅度(ΔT)(0、4.0、8.0、12.0℃)下的余氯胁迫,观察桡足类在24h后的死亡率,用概率单位法计算其余氯半致死浓度(Median lethal concentration,LC50)。结果表明各季节桡足类余氯LC50随ΔT增大而显著降低(P<0.001),且多数种类呈显著线性回归关系(P<0.05)。对于哲水蚤目桡足类,余氯LC50随其粒径增大而升高。同一种桡足类在相同ΔT下,随适应水温升高,余氯LC50显著降低(P<0.01)。亚热带海区夏季自然水温较高,冷却系统轻微的升温即可加剧余氯对其的毒性。若以LC50×0.5来确定桡足类余氯安全浓度,则春、秋、冬季暴露时间≤30 min、ΔT≤12.0℃时,其余氯安全浓度分别为0.21—0.86 mg/L、0.26—0.86 mg/L、0.31—4.55 mg/L;夏季暴露时间≤30 min、ΔT≤8.0℃时,其余氯安全浓度为0.15—0.41 mg/L。当前电厂加氯浓度偏高对冷却水中桡足类造成较大影响,但排出水中余氯对其影响不大。基于生态安全考虑,根据对余氯较敏感的小型桡足类在不同季节下的余氯安全浓度,建议亚热带滨海电厂夏季余氯排放浓度不超过0.15 mg/L,其余季节不超过0.20 mg/L。

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 （△T） （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 △T （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 eopepod 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 △T ≤12.0℃ , while the safe concentration of various copepod species were 0. 15-0.37 mg/L as the exposure duration ≤30min and △T ≤ 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 smallsized 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. zooplank- ton, 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.