Seite - 172 - in Freshwater Microplastics - Emerging Environmental Contaminants?

samples destroys hetero-aggregates andmakes it almost impossible to investigate them in their natural state. Overall,MP-associated risks aremultifaceted in their nature and the following must be considered:MP-biota interactions, toxicity of polymer-related leachates, adsorption-desorptionkinetics of co-occurringcompounds, biofilm-related effects, and the formation of hetero-aggregates. Thus, understanding the interaction of all these factors inreal-worldsituations isnecessary toevaluate theenvironmental risk ofMPexposures (Fig. 2). 4 Natural vs. SyntheticParticleToxicity Thesimilarities in the effects causedbyexposure tonatural fineparticles andMPs (see Sect. 3.1) provoke the legitimate question whether MPs have a different toxicological profile compared to natural solids. In general, organisms interact with a variety of particulatematter in freshwater ecosystems and possess adapta- tions to this potential stressor (e.g., peritrophicmembrane,mucus). Species occu- pying turbid waters might be less sensitive to high concentrations of SPM than species inhabiting clearwater. Species-specific effects of exposures to suspended solidswere highlighted innumerous studies investigating the anthropogenic intro- duction of particulate matter (e.g., arising from erosion, dredging; reviewed in [90, 91]). Suspendedparticles orfine sediments can reduce feeding rates, decrease reactiondistancetoprey, influenceembryodevelopment, increasemortality, reduce primary production, reduce species diversity, and decrease population size [90– 94]. Bilotta and Brazier [90] conclude that the magnitude of adverse effects depends on concentration, exposure duration, chemical composition, and particle size distribution. Tolerant species suffered moderately negative effects, while strong effectsmainly occurred in intolerant species (see a reviewonfish in [91]). These outcomes are also applicable for the effect studies withMPs and, thus, illustrate the importance of benchmarking the toxicity of MPs in comparison to naturally occurring particles. Considering the available literature, we can hypo- thesize a higher particle toxicity of MPs since adverse effects were observed at lowerconcentrationscomparedtofinesediments.However, studieswithsuspended solidshaveusedavarietyofunits (particlepervolume,masspervolume,partsper million), size classes, densities, and experimental conditions, which impedes a direct comparison.Accordingly, to answer the questionwhether the particle toxi- cityofMPsis indeeddifferent fromnaturalmaterials,ecotoxicologicalstudiesneed to include reference treatments with natural particles (e.g., minerals, charcoal). However, investigating particle toxicity necessitates a highly complex approach featuring multiple factors, e.g., concentration, material, size, shape, density, and surface characteristics. 172 C.Scherer et al.