Page - 173 - in Freshwater Microplastics - Emerging Environmental Contaminants?

In principle, aquatic species interact withMPs through a variety of pathways featuringdirector indirect ingestion, respiration,orattachment to thebodysurface. Therefore, a single stressor (e.g., inert particle) influences life-cycleparameters on multiplelevels.For instance, thepresenceofMPscanlimit thenutrientassimilation byreducing theproportionofavailable foodparticlesorbyinterferingwith feeding mechanisms and locomotion, influencing digestion efficiency, and driving behav- ioral adaptations (e.g., avoidance, foraging). This implies that effect studies with MPs should focus on multiple endpoints including typical life-cycle parameters (e.g., reproduction, growth, nutritional state), histological analyses, andbiomarker responses. Furthermore, the implementation of time-course, chronic, and multi- generational test designsmight helpuncover adaptive responses aswell as cascad- ing effects in populations. Only the simultaneous investigation and direct comparisonof the toxicityof natural andpolymeric particleswill enable discover- ingspecificMP-associatedrisks inthediversityofparticulatematter. In theabsence of this reference, adverse effects of MPs observed in the laboratory could be nothingbut a representationof the (normal) biological response andphysiological condition inducedbynatural particles.However, species in freshwater systemsare adapted to naturally occurring particles, and it remains relatively unclearwhether polymer particles act differently or have the potential to bypass protective adaptations. 5 Implications forFreshwaterEcosystems Although plastics have been released into the environment for many years, researchers have barely begun to understand the extent of MP distribution in freshwater systems. As such, the environmental impacts of MPs have not been thoroughly evaluated. Importantly, the term “microplastics” encompasses a tre- mendousvariety of polymers that in turn spans averywide rangeof sizes, shapes, andchemicalcompositions. In this sense,MPsdonot representonestressor,whose impacts canbeevaluated relativelyeasily, butavery largenumberof stressors that potentially act jointly. The use of copolymers, product-specificmixtures of addi- tives, and source- and pathway-specific sorbed pollutants further complicates the situation. In physical terms,MPs can influencewater (e.g., translucency [42]), sedimen- tation (e.g., feces [95]), and sediment (e.g., thermal conductivity [96]) characteris- tics. In ecological terms,MPs can affect the aquatic biocoenosis on a large scale (Fig. 2), for instance, as vectors for invasive species and pathogens [97–99]. The existing toxicological studiesmostly focus on the interaction ofMPs and biota in simplifiedexposure regimes, commonlyusingsphericalmicrobeadscomposedofa singlepolymer.Here, there isa tendency forelevatedadverseeffects (e.g., reduced reproduction, inflammatory response)withdecreasingparticle sizes.At thecurrent Interactions ofMicroplasticswithFreshwaterBiota 173