Plastic waste is of increasing concern in marine ecosystems 1–3. Buoyant plastic particles accumulate in pelagic habitats whereas non-floating debris accumulates on the seafloor and in beach sediments, posing risk to the respective communities 1–4. Microplastic particles (<5 mm) are either directly introduced via sewage discharge or formed by biofouling and mechanical abrasion, making them more prone to consumption by aquatic organisms 2,3. As a consequence, they can accumulate in higher trophic levels 3–5. A variety of harmful effects of plastic and associated chemicals has been shown 2–4. Moreover, plastic debris can act as vector for alien species and diseases 2,6. A large portion of the plastic waste is produced onshore and reaches the marine environment, which is considered the main sink of plastic debris. There is, however, a considerable lack of knowledge on the contamination of freshwater ecosystems with plastic debris. We here show that freshwater ecosystems also act, at least temporarily, as a sink for plastic particles.