Microplastics in a deep, dimictic lake of the North German Plain with special regard to vertical distribution patterns

, Tamminga Matthias, Fischer Elke.

Investigations for microplastics within the aquatic phase mainly concentrated on the surface of the world's oceans. However, this surface layer comprises only a small percentage of the global water volume and is unlikely representative of the total water body. While the prevalence of microplastics in the oceans has been investigated for several decades, the global freshwater systems have only recently received rising attention. Considering that lakes constitute 87 % of the world's liquid surface freshwater, it is necessary to quantify microplastics in limnic systems. Knowing the distribution of microplastics along all three spatial dimensions, including depth profiles, is crucial in this regard. As for marine investigations, investigations on microplastic pollution in freshwater have almost exclusively dealt with the surface water layer inducing considerable uncertainties. Heterogeneous sampling, purification, and identification strategies add to this uncertainty. To close this gap, we investigated the vertical profile of microplastic pollution in a deep, dimictic lake of the North German Plain. The present investigation is part of the project ‘MICROLIM' that aims to budget microplastics within the model catchment area of Lake Tollense (Mecklenburg Western-Pomerania). Sampling was conducted at eight locations along a SW-NE transect (Fig. 1). At each location, samples comprising 1,000 l were taken at three depths (near-surface, 7 m and 10 m) by in situ pump-filtration including particles between 63-5,000 μm in size. Three field campaigns (Sep. 2018 and Mar. 2019 and 2020) were carried out to assess seasonal differences (total n=72). Moreover, water temperature and conductivity were recorded to investigate the influence of water stratification on microplastic abundance. Thereby, we estimated the inventory of microplastics within the lake's water column.

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