Page - 174 - in Freshwater Microplastics - Emerging Environmental Contaminants?

state of research,MP toxicity has been studied and in some cases demonstrated at relatively high concentrations. This has been criticized as lacking ecological rele- vance. However, the environmental concentrations of very small, biologically relevant MPs (<100 μm) remain unknown but may be higher than predicted based on analyzing larger MPs. In addition, species-specific responses may be incorrectly estimated by usingmicrospheres alone. The use of multiple polymer types, shapes, and sizes may establish that some species aremore sensitive than originally predicted. It is already established that high concentrations of suspended solids affect community structure through changes in growth, reproduction, and species inter- actions.Accordingly,evolutionaryadaptations(e.g.,peritrophicmembrane,mucus, avoidance)might explain the species-dependent resistance to high concentrations ofMPs (e.g.,D.magna,G. pulex).However,MPs can infiltrate habitats normally low in suspended solid and thereby affectmore sensitive species. ThecontinuingreleaseofMPsthroughthebreakdownoflitteredplastics thatare already present in the environmentmeans thatMPsmay become an increasingly important freshwaterpollutant in the future. Inaddition, thehighdemandofplastic materials/products will not decrease if continuing the business-as-usual mode. Accordingly, without rethinking and restructuring our resource production and use (e.g., within the framework of a circular economy, [100]), plastic waste will further accumulate in thebiosphere. Overall, traditional approaches for toxicity testingmaynot be appropriate for a multifaceted stressor such as MPs. The default assumption that standard model organisms act as appropriate surrogates for aquatic biocoenoses may ignore species-specific responses ofmore sensitive species. In addition, consideration of future scenarios may render vector-related impacts (e.g., biofilms, transfer of additives, andhydrophobic persistent pollutant)more prominent. Ourknowledge regarding the impactsofMPsonfreshwater species is limitedat thepresent time, althoughwearebeginning toappreciate someof thecomplexities asmore laboratoryandfielddatabecomesavailable.Firstandforemost,weneed to prioritizewhich physical and chemicalMPcharacteristics are toxicologically and ecologicallymost important. In this context, there is also a lot to learn fromother disciplineswith important data already abundant (e.g., ecological feeding studies, suspended solids, medicine, nanomaterials; see e.g., chapter “Freshwater Microplastics: Challenges for Regulation andManagement”). Ecological knowl- edge regarding theadaptationsof specificspecies aswell as factorsdrivingspecies compositionsmight help to identify especially sensitive biota. In addition, under- standing the role ofMPs relative to other stressorswill require amultidisciplinary approach.Overall, understanding thecomplex interactionsofplasticsand theenvi- ronment canonlybeachievedbya joint effort.Theupcomingchallengewill be to unravel the role thatMPsplay in amultiple-particleworld. 174 C.Scherer et al.