Plastic pollution of rivers : Citizen scientists investigate litter quantities, composition, and sources

The pollution of the environment with anthropogenic litter, especially plastics, has become a major global problem. Large quantities of litter pose a hazard to hundreds of species, infrastructure, and human health. While the ocean pollution of plastics has been the research focus of most studies to date, the majority of litter derives from land and rivers play a key role in the transport of litter from inland sources to the sea. As litter sources are very diverse, it is imperative to investigate a variety of environments to gain an overview of the extent of pollution and derive mitigation measures. Citizen science (involving the general public in research) is a promising approach to collect such data over large geographic areas. This thesis presents the results of the citizen science campaign Plastic Pirates, involving schoolchildren in the research of anthropogenic litter pollution of rivers. The project was conducted in Germany and Chile, involving (since 2016) over 15,000 schoolchildren and their teachers. The studies presented here analysed data from the campaigns of 2016 and 2017 of the Plastic Pirates. The results show that litter pollution of the riparian environment is ubiquitous in both countries: larger litter at the sampling sites occurred at almost all sampling sites (91% in Germany, 100% in Chile), including litter hazardous to humans (found at 89% of sampling sites in Germany, in Chile this analysis was not conducted). Litter quantities at the riverside averaged 0.5 items and 1.8 items per m2 in Germany and Chile, respectively. In both countries, plastics and cigarette butts were among the most frequently found litter items. The most important source of much of this litter are recreational visitors (i.e. people passing spare time at the riverside, consuming food). In addition, in Chile, residents and people illegally dumping litter have also been identified as important sources. Small plastic particles (meso- and microplastics, i.e. particles in the size range of 24.99 to 1 mm) have also frequently been found in the nets used to sample the river water (in 57% of samples in Germany, and 43% of samples in Chile). Considering that only particles larger than 1 mm were sampled, it can be assumed that the pollution by even smaller particles is common in the two countries. In Germany, some hotspots of pollution with small plastic particles were discovered, meaning sites where many particles were found (over 50 particles per hour). Potential sources of these particles were the plastic producing industry and wastewater treatment plants. The employed citizen science approach, involving schoolchildren and their teachers, proved valuable to collect many data in entire Germany and large sections of Chile. A wide variety of rivers were investigated, including smaller rivers, usually not in the focus of riparian litter studies. In addition to the scientific data collected by the schoolchildren, citizen science is a promising approach to raise the environmental awareness and scientific literacy of participants, and, in this process, plays an important role towards the opening of research processes to the general public.