Microplastics in lakeshore and lakebed sediments – External influences and temporal and spatial variabilities of concentrations

Microplastics have been predominantly studied in marine environments compared to freshwater systems. However, the number of studies analyzing microplastic concentrations in water and sediment within lakes and rivers are increasing and are of utmost importance as freshwaters are major pathways for plastics to the oceans. To allow for an adequate risk assessment, detailed knowledge concerning plastic concentrations in different environmental compartments of freshwaters are necessary. Therefore, the major aim of this study was the quantification and analysis of temporal and spatial distribution of microplastics (<5 mm) in freshwater shore and bed sediments at Lake Tollense, Mecklenburg-Western Pomerania, Germany. Likewise, it addresses the hypothesis that lakes may serve as long-term storage basins for microplastics. Concentrations were investigated semi-annually over a two-year period at four sandy bank border segments representing different expositions and levels of anthropogenic influence. In addition, lakebed samples were taken along the longitudinal dimension of Lake Tollense. Mean microplastic abundances were 1,410 ± 822 particles/kg DW for lakeshore sediments and 10,476 ± 4,290 particles/kg DW for lakebed sediments. Fragments were more abundant compared to fibers in both sediment compartments. Spatial and temporal variation was especially recognized for lakeshore sediments whereas microplastic abundances in lakebed sediments did not differ significantly between sampling points and sampling campaigns. This can be related to long-term accumulation at the lakebed. Lower microplastic abundances were found within the intertidal zone at lake beaches where constant wave action reduces accumulation. Increased microplastic abundances were recognized at the beach with least anthropogenic influence but in proximity to a tributary, which may serve as microplastic input pathway into Lake Tollense due to its catchment comprising mainly agricultural areas. Furthermore, spatial variations in microplastic concentrations were related to the abundance of macroplastic items at beaches and correlated with pedologic sediment characteristics, namely the content of organic matter.