| Polycyclic aromatic hydrocarbons (PAHs) are persistent and ubiquitous organicpollutants in the environment. It has the carcinogenic, teratogenic and mutageniccharacteristics which show a serious threat to living organisms and human health, As aresult, a lot of attention has been paid on the PAHs. But the research scholars have focusedtheir attention about the ecotoxicity of PAHs on a single population level, and ignored theeffect of mechanism research on biological communities and ecosystems. This paper tookthe aquatic algae, Daphnia magna and other aquatic populations as the research object,using the experimental basic method of ecological toxicology, combined with populationecology theory, studied the effect of PAHs under the exposure levels of acute and chronictoxicity. And different aquatic biological communites had been established by differentspecies to investigate the changes of the relationship between the species under the stressof PAHs. The study focused on the mechanism of toxic effect of population andcommunity level. Based on these experiment data, curve simulation had been made toanalyse and evaluate the ecological risk of PAHs on the single population and communitylevel. The main conclusions followed:(1) Studies about toxic effecte on algae of PAHs found, naphthalene,phenanthrene, fluoranthene, benzo[a] pyrene had a growth promote effect at lowconcentration and growth inhibits effect at high concentration on aquatic algae. The96h-EC50of PAHs on the growth of Scenedesmus obliquus were naphthalene15.178mg/L,phenanthrene1.821mg/L, fluoranthene0.281mg/L, benzo[a]pyrene0.00188mg/L.Thetoxicity of these four PAHs was in the order of naphthalene> phenanthrene> fluoranthene>benzo[a] pyrene. The96h-EC50of phenanthrene on the growth of three aglae wereMicrocystis aeruginosa0.924mg/L, Chlorella pyrenoidosa0.925mg/L, Scenedesmusobliquus1.821mg/L. The toxicity of phenanthree on the three aglae was in the order ofScenedesmus obliquus>Microcystis aeruginosa>Chlorella pyrenoidosa. Ecological riskassessment result on Scenedesmus obliquus was that naphthalene, phenanthrene,fluoranthene were free of risk, and benzo[a] pyrene was risk of irreversible.(2) Experiments made on the acute and chronic toxicity of PAHs to Daphnia magnaindicated that the24h-LC50of PAHs on Daphnia magna were naphthalene5.485mg/L,phenanthrene3.226mg/L, fluoranthene0.295mg/L, benzo [a] pyrene0.0041mg/L。The48h-LC50of PAHs on Daphnia magna were naphthalene4.158mg/L, phenanthrene2.488mg/L, fluoranthene0.237mg/L, benzo [a] pyrene0.0037mg/L. With time increasing,toxicity of PAHs had decreased. It indicated that the toxicity of PAHs would increase as thetime went by. The toxicity on Daphnia magna of the four PAHs was in the order of naphthalene <phenanthrene <fluoranthene <benzo[a]pyrene. Ecological risk assessmentresults on Daphnia magna population was that, naphthalene, phenanthrene andfluoranthene were free of risk, and benzo[a]pyrene had the lethal risk. In the chronictoxicity of21d test, compared with the control group, the total number of off springs, thelength, the days of first pregnancy, the days of breeding and the number of broods ofDaphnia magna in0.4mg/L phenanthrene stress were significant different. The totalnumber of off springs and the length seemed more sensitive. From the off spring index ofDaphnia magna,0.1mg/L was the critical point at which the stress of phenanthrenechanged to inhibition.(3) Futher studies about the interspecific competition between Scenedesmus obliquusand Microcystis aeruginosa found that the different initial cell density of Scenedesmusobliquus and Microcystis aeruginosa had the different response under the phenanthrenestress. In pure culture, phenanthrene had insignificant effect on Scenedesmus obliquusgrowth of different initial cell density. But the growth of Microcystis aeruginosa waslargely influenced. If the initial cell density was small and the growth inhibitory effectwould be large. In mixed culture, growth of two algae with different initial cell densitywere largely influenced in phenanthrene stress. In low concentration of phenanthrenestress, enhancing effect of growth stimulation of Scenedesmus obliquus appeared in2:1group. The growth of Microcystis aeruginosa was inhibited in the mixed culture, and theeffect would enhance with the increase of initial cell density and the increase ofphenanthrene concentration. With the increase of phenanthrene concentration,the interspecific competition would change and finally affected the final outcome of thecompetition. At the same time, intrinsic algae population growth rate were affected,which had a further influence on the ecological risk of the community stability.(4) The aquatic macrophyte is generally had effects on the growth of green algae. Inthis study, Canna planting root zone water has allelopathic effect on the growth ofScenedesmus obliquus and presented a "low promote high suppression" phenomenon. Atdifferent concentrations of phenanthrene stress, canna growing root zone water influenceof algae produce in different changes. In15%and30%the proportion of planting root zonetreatment group, with increasing concentrations of the phenanthrene, the growth ofScenedesmus obliquus role in promoting enhanced,100%planting root zone treatmentgroups, with increasing concentrations of the phenanthrene, on the ramp using algaegrowth inhibition enhanced. The rusult shows that phenanthrene stress so planting the rootzone to enhance the effect of water on the algae.(5) Daphnia magna have feeding behaviors on green algae, which represented byScenedesmus. In this study, Daphnia magna for Scenedesmus obliquus feeding effectmakes the Scenedesmus obliquus growth severely inhibition, and under the phenanthrene stress, Daphnia magna feeding ability shows "low promote high suppression" phenomenon.In0.05mg/L treatment group and0.1mg/L treatment groups Daphnia magna filtration rateand ingestion rate was higher than the control group, growth and reproduction are alsobetter than the control group;0.2mg/L treatment group Daphnia magna filtration rate andingestion rate was lower than the control group, feeding ability are decreased, growth andreproduction has been affected. At low concentrations phenanthrene stress Daphnia magnahas been enhanced either in feeding and the growth and reproduction. This phenomenonwill change the life history of Daphnia magna, could affect the entire aquatic ecosystemstructure and function of aquatic ecosystems with potential security threats.In this study, PAHs effect on the relationship between community in the kind ofconcentration is lower than the single population acute toxicity (half inhibition orsemi-lethal) the effect of concentration. In the ecological system, therefore, pollutants havetoxic effect on a population, has the structure of the biological community has irreversibleeffect. Aquatic ecosystems are complex and sensitive, but also the entire life system on thebasis of the food chain ring, understand pollutants provide strong evidence of ecologicalrisk populations of aquatic organisms directly for human health risk. Based on the actualmeasured data of population ecological risk assessment, a relatively complete and analysesthe influence of PAHs, Scenedesmus obliquus with large populations of Daphnia magnaecological risk, and try to analysis from the perspective of relationship between biologicalcommunities of PAHs, the impact on the ecosystem, provide reference for the riskassessment of aquatic ecosystems in the future. |
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