Floating microplastic debris in a rural river in Germany: Distribution, types and potential sources and sinks

Microplastic debris affects marine as well as freshwater ecosystems and an increasing number of studies have documented the contamination in aquatic environments worldwide. However, while the research focuses on oceans and larger rivers, little is known about the situation in smaller rivers within rural catchments. Since microplastics pose various risks to ecosystems, wildlife and human health, it is important to identify potential sources, sinks and transport patterns, which are probably different for small rivers. In this study, we investigate the contamination with microplastic debris of the river Ems, representing a smaller river in Northwest Germany with an agricultural catchment. We hypothesised that with increasing river length the plastic concentration increases, especially downstream of towns, waste water treatment plant (WWTP) effluents and major tributaries as they may be important point sources of microplastics. We collected 36 surface water samples at 18 sampling sites within the first 70 km using manual driftnets. We sampled every 7 km and upstream and downstream of three larger towns, four major tributaries and four WWTP effluents. Overall, we found 1.54 ± 1.54 items m^-3, which corresponds to the plastic concentrations in larger streams. However, the shape of the detected items differed as we did not find potential primary microplastic. Furthermore, the pattern contradicts our assumption, that the contamination increased with distance to the river's source. Downstream of towns, we found significantly less floating microplastic indicating possible sinks due to sedimentation at sites with slowing flow velocity caused by weirs in towns. Hence, the non-linear distribution pattern of microplastics indicates potential sinks of microplastics due to flow alterations on the river course. This should be considered in future studies modelling microplastic distribution and transport. Furthermore, studies especially in smaller rivers are urgently needed to identify and quantify sources and sinks and to find applicable solutions to reduce microplastic loads.