The widespread presence of microplastics in multiple environmental compartments has largely been demonstrated. Assessing the ecological risk that microplastics pose is, at the present stage, hindered due to methodical differences. Moreover, different methods hamper meaningful comparisons between studies and data on microplastics <300 μm is scarce. Therefore, we focused on microplastics >20 μm in freshwater and sampling-related aspects in this concern. Sampling was conducted between 2018 and 2020 in the Tollense catchment in northeastern Germany and was carried out by in situ pump filtration. Two different sampling systems (cutoff sizes 20 μm and 63 μm) were applied to filter water volumes of 0.075–1.836 m³. Retained particles were analyzed by a combination of Nile red staining and micro-Raman spectroscopy. Thereby, we found microplastic concentrations between 123 and 1728 particles m⁻³ using the 63-μm cut-off size and between 1357 and 2146 particles m⁻³ using the 20-μm cut-off size. Local hydrodynamics (discharge and flow velocity) and land cover are likely influencing the observed microplastic concentrations and fluxes. The variability between both sampling systems cannot fully be explained by the different mesh sizes used. We argue that differentiation between a theoretical cut-off size (finest mesh) and a factual cut-off size (reliable quantification) can help to understand sampling related differences between studies.

微塑料在多个环境隔间中的广泛存在已在很大程度上得到证实。由于方法上的差异，在现阶段评估微塑料造成的生态风险受到阻碍。此外，不同的方法阻碍了研究之间进行有意义的比较，并且关于 <300 μm 的微塑料的数据很少。因此，我们关注淡水中 >20 μm 的微塑料和与采样相关的问题。采样于 2018 年至 2020 年间在德国东北部的 Tollense 集水区进行，并通过原位泵过滤进行。^{应用两种不同的采样系统（截止尺寸 20 μm 和 63 μm）来过滤 0.075–1.836 m 3}的水量. 通过尼罗红染色和显微拉曼光谱的组合分析保留的颗粒。因此，我们发现使用 63 μm 截止尺寸时，微塑料浓度在 123 到 1728 个颗粒 m ^-3之间，使用 20 μm 截止尺寸时，微塑料浓度在 1357 到 2146 个颗粒 m ^-3之间。当地流体动力学（流量和流速）和土地覆盖可能会影响观察到的微塑料浓度和通量。两种采样系统之间的可变性不能完全用所使用的不同网格大小来解释。我们认为，理论截止大小（最细网格）和实际截止大小（可靠量化）之间的差异有助于理解研究之间与抽样相关的差异。