Contextualizing microplastic pollution in different river habitats through rapid habitat assessment in South Africa

Abstract There has been a notable increase in research into microplastics in rivers over the last decade. However, no clear or concise standardized monitoring method exists for the sampling of microplastics in rivers. Many guidelines exist for the sampling and analysis of microplastics from rivers, but researchers continue to use methods and tools that may contaminate samples, such as plankton nets, or collect and filter bulk water samples with differing volumes and numbers of replicates. This can result in the same river system being shown to have notably varying levels of microplastics between sites. The aim of this study was to determine microplastic abundances in water and sediment, together with a critical evaluation of the sampling sites, to ascertain which environmental factors impacted microplastic distribution. This was achieved by applying the rapid habitat assessment method and simultaneously sampling microplastics. The results indicated that factors such as water velocity, physical obstructions to water flow and, most importantly, discharge, can change the context of a site with respect to its microplastic content, which can impact niche-specific organisms. The results were used to develop a new measurement unit, the microplastic discharge unit. The MDU provides a contextualized representation of the microplastics moving through a river, and can be used as an indicator of microplastic pollution at a site and its impact on biota. The results will be used further to produce a guide for the accurate sampling of microplastics in rivers.