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Numerous studies have analyzed the distribution and concentration of microplastic particles (MPs) in various aquatic habitats, and estimated how they affect residual biota. Decidedly less research concerns how organisms (e.g. ichthyofauna) affect the quantitative and qualitative characteristics of MPs in the water. Therefore, the aim of our study was to test, if fish can increase the vertical flux of MPs to sediments. This would be the result of the combined effect of increased sedimentation rate and the active transport resulting from vertical migration of the fish in the simultaneous presence of a thermal gradient (and in turn water density gradient). We conducted eight 5-day-long experiments in thermally stratified twin vertical columns filled with a suspension of polystyrene beads of 25 and 250 µm in size, with initial density of 1000 and 200 × L−1, respectively. In each of the experiments, we placed sediment traps at three different depths in each of the columns, and four planktivorous fish in one of them. During the experiments, we monitored the distribution of fish. At the end of the experiment, we determined the number of MPs and the amount of detritus in both sediment traps and water. We found that the presence of planktivorous fish significantly increased the vertical flux of MPs to sediments and decreased the density of MPs in the water column. These effects seem to be more likely due to the increased sedimentation rate of MPs due to the fish activity, rather than to their active transport, as MPs in the experimental variant were covered with detritus to a greater extent, than in the control, and the fish remained in the subsurface layer in the water column. Our study seems to be the first one showcasing the effect of fish on the concentration and distribution of MPs in aquatic ecosystems. Fish foraging activity increased the sedimentation of PS MPs. Fish foraging activity decreased the PS MP concentration in the water column. The study seems to provide the first experimental evidence on the effect of fish on the distribution of MPs in aquatic environments. [ABSTRACT FROM AUTHOR]

Microplastic pollution is currently one of the most intensely studied ecological issues. Numerous studies have estimated the distribution and concentration of microplastics in various environments and determine how they affect their inhabitants. Much less effort has been place on assessing the possible effects of microplastics on interactions between organisms, including interspecific competition. Our aim was to test the hypothesis that the presence of microplastics affects the proportion of individuals of coexisting species and the elimination rate of the inferior competitor. The hypothesis was tested in competitive experiments done in the absence and presence of spherical non-biodegradable polystyrene and polyethylene and biodegradable polyhydroxybutyrate in environmentally relevant densities. In each of the experiments, we used three different pairs of closely related planktonic species of the genus Daphnia composed of the superior and inferior competitor: D. pulex and D. magna, D. magna and D. galeata, D. pulex and D. galeata. The results support our hypothesis and demonstrate each microplastic type had a different effect on the density of the competing species. The presence of polystyrene and polyethylene lowered the density of the superior competitor in each of the three pairs, at least partially due to a reduction in the number of gravid females, but not their fecundity. The presence of the polyhydroxybutyrate, in turn, increased the population density of D. magna in the variants with each of the two remaining species. Moreover, the presence of microplastics affected the elimination rate of the inferior competitor, i.e. polystyrene expedited the exclusion of D. magna by D. pulex, and polyhydroxybutyrate hampered the exclusion of D. magna by D. pulex. Our results suggest that long-term exposure to environmentally relevant densities of both non-biodegradable and biodegradable microplastics may affect the relative abundance of co-occurring species in zooplankton communities, and thus the functioning of aquatic ecosystems., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:Piotr Maszczyk reports financial support was provided by Polish National Science Centre. Ewa Babkiewicz reports financial support was provided by Polish National Science Centre., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)