The effect of planktivorous fish on the vertical flux of polystyrene microplastics

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.