Temporal variability of litter pollution of rivers in Germany - A long-term assessment by schoolchildren as citizen scientists.

Understanding litter dynamics on riversides is challenging due to the need for extensive data. A seven-year citizen science study with schoolchildren showed no major changes in litter density, composition, and sources on German riversides. The study categorized 26,757 macrolitter items (> 2.5 cm in size), mainly consisting of plastic and cigarette butts, with a median litter density of 0.28 items m. We found that wide rivers (> 51 m) had significantly higher median litter densities (0.42 items m) than narrow rivers (< 10 m, 0.14 items m), indicating greater litter accumulation in downstream areas. A zero-inflated binomial hurdle model showed that litter density variations were influenced by the interaction of the river system with sampling campaign and river width. Schoolchildren identified visitors as the most likely source of riverine litter. These findings highlight the value of citizen science in understanding long-term trends and show the urgent need for political measures.