| Microplastics are widespread pollutants in water environments,which have brought many negative effects to aquatic organisms.In aquatic organisms,plankton orgniams are important model species and they are ususlly very sensitive to various environmental pollutants.Threfore,plankton species has been widely used as the target organism to evaluate the ecological consequences of microplastics.However,most studies on plankton have been carried out at the individual level using lethal experiments,with lethal effects or changes in individual characteristics indicative of microplastic hazards.The effecitive concentrations that having direct lethal effects are often much higher than their actual concentration in natural environments,and therefore,the negative impacts of microplastics on aquatic orgniams and eocystems will be underestimated by using these prediction methods.Recent studies have suggested that community-level indicators are more sensitive to environemtal pollutants than the individual-level indicators,however,less is known about the community-level biological consequences of microplastics on plankton ecosystems.Therefore,this study evaluates the negative impacts of microplastic concentration(relatively low concentration)and size from the aspects of system structure and function.By using the Chlorella vulgaris,Ankistrodesmus angustus,Scenedesmus quadricauda and Daphnia magna(filter feeders)and larvae damselfly(zooplanktivorous species)were used as experimental organisms.The influence of microplastic concentration on the life history characteristics of Daphnia magna was evaluated with the system containing only daphnia magna as the control,and the mechanism of the effect of microplastic concentration on the daphnia magna magna damselia larva system was investigated.By comparing the system containing only D.magna,the influence of microplastic concentration on life history characteristics of D.magna was determined,meanwhile,the influence of microplastic concentration on the stability and persistence of the D.magna-larval damselfly system was studied.So as to reveal the mechanism of the effects of microplastic concentration on D.magna-larval damselfly dynamics.But microplastics are not only harmful to organisms because of their concentration,but also their size in environments.Thus,two co-culture systems of C.vulgaris-A.angustus and S.quadricauda-A.angustuswhen and two apparent competition systems(C.vulgaris-A.angustus and S.quadricauda-A.angustuswhen adding D.magna as a filter feeder)were conducted to explore the influences of microplastic concentration and size on the relative abundance of phytoplankton in the plankton ecosystem.The results indicate that:(1)In the experiment of system stability of D.magna-larval damselfly dynamics,the reproductive capacity of D.magna decreased along with the increase of microplastic concentration,the total number of offspring per female sharply decreased by 1.83%,43.23%,and 60.95% in treatments with 5,20,and 40 mg/L of polyethylene(PE)microplastics compared with the control treatment.The reproductive capacity of D.magna was decreased by the influence of microplastics,so the stability and persistence of the D.magna-larval damselfly system significantly decreased along with the increase of microplastic concentration,all D.magna individuals were consumed by larval damselflies after 11 days of exposure to 5 mg/L,8 days of exposure to 20 mg/L,and 7days of exposure to 40 mg/L of PE microplastics.(2)In the coexistence system in the absence of grazers,the ratio of A.angustus to C.vulgaris and the ratio of A.angustus to S.quadricauda increased significantly with the increase of microplastic concentration,while the change of size had no significant effect on the ratio.In the coexistence system in the absence of grazers,the interspecific interaction was mainly affected by microplastic concentration,and the increase of microplastics concentration promoted the growth of A.angustus.But in two apparent competition system,the exposure to 5–20 μm microplastic led to significantly reduced the ratio of A.angustus to C.vulgaris and the ratio of A.angustus to S.quadricauda along with increasing microplastic concentrations from the whole experimental period;In contrast,when exposed to microplastics of 20–27 μm and 27–32 μm microplastic,the ratio of A.angustus to C.vulgaris increased significantly with the increase of microplastics concentrations,and the ratio of A.angustus to S.quadricauda decreased at the early stage and increased at the later stage with the increase of microplastics concentration throughout the experimental cycle.It clear that in apparent competition system,with the addition of D.magna,microplastics with small size promoted the relative abundance of C.vulgaris and S.quadricauda,while microplastics with large size promoted the relative abundance of A.angustus.In the presence of D.magna,the mechanism of microplastics changing the relative abundance of algae was related to the change of the filtration efficiency of D.magna to the co-cultured algae.Because in algae-grazer systems,the filter feeding efficiency of D.magna on three kinds of phytoplankton decreased with the increase of microplastic concentration,but the decrease amplitude was related to microplastic size.When the microplastic size was5–20 μm,the filtration efficiency of D.magna to C.vulgaris and S.quadricauda decreased significantly.However,when exposed to microplastics of 20–27 μm and27–32 μm,the filter feeding efficiency of A.angustus decreased significantly.In other words,Microplastic tended to interfer the filtering process of grazers on similar-sized algae cells to help protect the algae species.Therefore,in apparent competition system,the relative abundance of algae will change due to the change of microplastic size.This study proved that microplastic concentration and size are important factors affecting the structure and function of the planktonic ecosystem.The increased concentration of microplastics decreased the stability of the D.magna-larval damselfly dynamics and eventually led to system collapse.Microplastics increased the relative abundance of algal species with similar size when in the presence of D.magna.Therefore,community indicators are indeed more sensitive than individual indicators and can more accurately predict future ecological risks caused by microplastic pollution. |
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