Page - 145 - in Freshwater Microplastics - Emerging Environmental Contaminants?

ecotoxicologically relevant concentration.Effect assessment and riskcharacteriza- tion are beyond the scope of this review,which focuses on freshwatermodels for fate andexposure (see [121] fordetails).However, inorder to framemodels in the context of ERA, herewe briefly touch upon thewide variety of adverse outcome pathways (AOPs) that exists for plastic debris. As plastic debris is a complex mixture of sizes, types, and shapes, which also can be associatedwith chemicals, there will be amultitude of ERMs. Each ERMcaptures the unique features of a particular type of debris present in a habitat in combinationwith specific traits of species in thathabitat, leadingtoanAOPdescribing thepresetecologicalorhuman health protectiongoals. Some reportedAOPs are entanglement, ingestion/suffoca- tion, blockage of the gastrointestinal tract, food dilution, chemical toxicity from associated chemicals, and a series of biomarker responses, which have been reviewed recently [6, 9, 122]. ERMs for physical effect of plastic can be defined in the formof amatrixwhere exposure and effect criteria such as habitat, species, life stage, mode of action, plastic size, plastic shape, and exposure duration are tabulated and scored.Using populationmodels, effects on individuals then can be integratedwith those from other stressors and habitat factors and, where needed, scaled to the population level similar to pesticide effectmodels (e.g., [100]). The ERMthenneeds tobeassessed inspaceand time,dependenton theprotectiongoal andtheaimof theERA.Ideally, fatemodelsasdescribed in this reviewshould thus be able to simulate or predict all relevantERMsemerging from the broad suite of species and particles that can be encountered in a habitat that has to be protected. Here, as mentioned before, for relatively simple site or material prioritizations, regional background concentrations as produced bymultimediamodelsmay suf- fice. Multimedia models can also be used in probabilistic ERA where spatial heterogeneity is accounted for by using a probability function that quantifies the spatial variation.For site-specificassessments,ERMsmaybepredictedbyexplicit models likeNanoDUFLOW[36, 96], INCA-contaminants [93], or similar particle models, as long as aggregation of nanosized particle fractions is accounted for. Exposure then canbe combinedwith effect thresholds in aPEC/NEC(NEC is the no-effect concentration) approach,where theNECmaycome fromdata for differ- ent dose response models dependent on the AOP (threshold model, log logistic, Weibull, binary).Due toconsiderableuncertaintycompared toERAfor traditional chemicals, probabilistic approachesare recommended,whichcanbeadopted from recent developments in theERAof engineerednanoparticles [118]. 6 ConcludingThoughts Contamination of the freshwater environment with plastic debris of all sizes has received increasing attention. In this reviewwe argue that in order to conduct a proper riskassessmentofplasticpollutantsandtheir sources,andgiventhescarcity ofdata,modelsareusefulcomplementarymethods forexposureassessment.These modelscanbuildonexisting transportmodels that simulateother typesofparticles, Modeling theFate andTransport of PlasticDebris inFreshwaters:Reviewand. . . 145