Effect Of Polyethylene Microplastics On The Interspecies Interaction Among Plankton Organisms And The Trophic Cascade Effect In A Planktonic Food Chain

Microplastic is a new type of environmental pollutant,which exists in large quantities in water environment and brings great harm to aquatic organisms.Plankton is the foundation of aquatic food chain and plays a key role in the material circulation and energy flow of aquatic ecosystem.Besides,plankton are very sensitive to various environmental pollutants.Therefore,the harm effects of microplastics to plankton ecosystems has gradually attracted people’s attention.But so far,these studies have mainly focused on the individual responses,the consequences of these biological changes in terms of interspecific interaction and ecosystem function remain poorly understood,especially in freshwater ecosystems.Therefore,by using the green alga Chlorella vulgaris,cladoceran grazer Daphnia magna,and larval damselfly（zooplanktivorous species）,we quantified algal growth,and grazer responses in heart-beat rate,swimming behavior,and grazing rate at higher levels of polyethylene（PE）microplastics(0,5,40,160 mg·L^-1);then,variation in trophic cascade strength of the algae-grazer-larval damselfly food chain to lower levels of PE microplastics(0,5,20,40 mg·L^-1)were investigated.Results showed that:（1）The density of C.vulgaris was not affected by PE microplastics throughout the experiment.However,D.magna behavior was significantly affected by PE microplastics.Specifically,after 1h of exposure to 5,40 and 160 mg L^-1 of PE microplastics,heart-beat rate reduced by 5.88%,9.26%and 11.18%,while,hopping frequency decreased by 11.18%,and 4.02%and 10.26,compared to the control treatments.（2）With increasing PE microplastic concentration,the grazing rate of D.magna exhibited an overall downward trend,and there was a significant difference among the treatments after 3 h of exposure.Compared with the control treatment,the grazing rate of D.magna decreased by 20.74%,51.06%and 64.89%after 3 h of exposure to 5,40and 160 mg L^-1 of PE microplastics compared with the control treatment.（3）With increasing PE microplastic concentration,the hopping frequency of D.magna decreased,while,the capturing frequency of larval damselfly on D.magna individuals increased in the food chain experiment.As a result,the survival rate of D.magna consistently decreased with increasing PE concentration,which finally promoted the growth of C.vulgaris cells,i.e.,promoted the positive effects of larval damselfly on algal growth（the trophic cascade effect was promoted）.In summary,microplastics can not only alter interspecific interaction,but also trophic cascade strength of food chain through behavior-mediated interspecies interaction.Therefore,the present study demonstrated the negative effects of microplastics on aquatic ecosystem at community level,which provides a reference for comprehensively evaluating the ecological risks of microplastics.