Seite - 191 - in Freshwater Microplastics - Emerging Environmental Contaminants?

[71, 72], plastisphere communitiesmay alternatively be hypothesized to facilitate transport of pollutants between ecosystems and to biota (Sect. 3.2). 3 KnowledgeGapsandResearchNeeds 3.1 Sources andTransportBetweenHabitats Processescontributing tomicroplastictransportdifferbetweenfreshwaterandmarine ecosystems(Sect.2.1).ConditionsencounteredwithinWWTPandunmanagedfresh- waters alsodiffer fromoneanother.Apriority for research involves determining the extenttowhichplastic-colonizingtaxaassociatedwithwastewaterandothersourcesof plastic (such as landfills) are transported downstream along rivers and streams and whethertheyremainviableandactiveuponenteringmarinehabitats[12,40].Aspartof thiswork, research is required tocharacterize theresidence timesofpolymerparticles within several environments, including different stages of thewastewater treatment process. Most WWTPs are based on three main treatment stages, although slight differencesintheirconfigurationcanbefound.Duringprimarytreatment, largedebris fragments are removed byusing a 6mm(or larger) screenmesh.During secondary treatment, large aeration tanks are used to remove suspended and dissolved organic materialandnutrientsthroughmicrobialactivity.Subsequently,flocculatesandsettling tanks are used to facilitate separation of sewage sludge from the post-processing effluentpriortoapotentialdisinfectionstep,alsoknownasadvancedtertiarytreatment. Studies reporting pathways ofmicroplastics through different wastewater treatment stagesareonlybeginning toemerge[73–75],andlittle is still knownabouthowthese stages influence thedevelopmentofplastispheremicrobialcommunities. Overall, studies of microplastic movement and associated biofilms should be basedonwell-establishedprinciplesofecosystemandcommunityecology[39]and are prerequisite to estimating the spatial scales overwhich plastics are distributed within awatershed. This approachwill best informhowplastic-associatedmicro- bial communities can be expected to change with movement from freshwater to marine habitats. There is also a need to compare plastisphere communities in managed and natural environments, within several locations along thewater col- umn, as well as between pelagic and benthic habitats. Research into plastic- associated biofilms has focused on surfacewaters (despite the long-term accumu- lation of microplastics in sediments; [8, 27]), and investigations of benthic plastisphereassemblageshavebeenrestricted tomarinehabitats [21,47]. Inseveral environments, no information is available on plastic-associatedmicrobial assem- blages. For example, no data have yet been published on plastisphere consortia withinWWTP,andalthoughthebuildupofplasticdebris indeep-seaenvironments has been reported [76], biofilms associatedwith this debris have not been studied. Microplastic-AssociatedBiofilms:AComparisonofFreshwater andMarine. . . 191