Page - 265 - in Freshwater Microplastics - Emerging Environmental Contaminants?

information on particle number, shape, and size distribution to assess a potential ingestionbyorganismsandresultingadverseeffects.Table2givesanoverviewon the advantages anddisadvantages onboth sides. Apart fromanydiscussion about numbers versusmass balances, further discor- dances reduce thecomparabilityofmonitoringdata.Currently,massesornumbers are reported invariousunits as perm2water surface, perm3water volume, perm2 sediment surface, per liter sediment, or per kg sediment, to name just a few. For regulationpurpose, comparable units need to bedefined andgenerally applied. 3.7 AdaptationofEcotoxicologicalTest Systems Thepreviousparagraphsdiscuss thechallengesrelatedto theexposureassessmentof MPs. Furthermore, comprehensive hazard assessment is required to evaluate the environmentalriskofMPsandsubsequentlyformulatereasonableregulationstrategies. Asdescribedabove,particulatepollutantsbehavedifferentlythandissolvedchemicals, and thus,anadaptionof test systemsof toxicity tests fordissolvedchemicalscouldbe required.With regard to this, there is some experience from studies on engineered nanomaterials(see[72]).Researcherscanlearnfromthoseexperiences;however, they need toconsider that physicochemical propertiesofMPmight lead toagaindifferent behavior in test systems. High-density polymers will sink to sediments. Hence, sediment-living species are required– comparable to the testingof highly lipophilic, sediment-boundchemicals.Viceversa,low-densitypolymerswillfloatandwillonlybe available forsurface-feedingorganisms.Thosewillbeathigher risk inenvironmental systemsastheymightfeedselectivelyonfloatingmaterialsandaccumulatethemfrom thewaterphase.Chemical testing isusuallynot focusedon this feeding type. Furthermore,MPparticlescanimpactorganismsinvariousmanners–chemically andmechanically (see [50]). One particle can be seen as amultiple stressor itself. Hence, ecotoxicologists have to facedifferent challenges and to adapt common test systems and endpoints. Potentially, the chosen endpoints of mono-substance test designs simplyoverlook theeffectsofMP.Fundamental researchwouldbe required Table 2 Overview of the advantages and disadvantages of mass balance versus particle characterization as criteria for regulation Particle concentration Mass concentration Pro Contra Pro Contra • Informationabout size, shape, color ! Ecotoxicological relevance ! Distinguishprimary and secondaryMP ➢ Enables source tracking • Noexactmass quantification • Exactmass quantification • No further charac- terizationof particles• Snapshot ! Due to continu- ous fragmenta- tionunder environmental conditions • Fits to conven- tional regulation options ! Less information Appropriate for ecotoxicological questions Appropriate forwatermanagement FreshwaterMicroplastics:Challenges forRegulation andManagement 265