Little evidence that dams in the Orange-Vaal River system trap floating microplastics or microfibres

Rivers can be major sources of plastics into coastal seas, but it is unclear whether dams retain floating plastics, thus reducing the amount reaching the sea. To test if dams trap microplastics, we collected bulk water and neuston net samples from five dams on the Orange-Vaal River system, South Africa. Most manufactured items were microfibres and densities of microplastics were modest (bulk water: 0.21 +/- 0.27 items.L-1; net: 0.04 +/- 0.16 items.m(-2)). There was an interaction between dam and season: dams on the densely populated Vaal River had higher microplastic concentrations during dry than wet conditions, whereas the opposite pattern occurred on the less industrialised Orange River. Overall there was no difference in microplastic concentration at sites above vs below dam walls nor was there a strong correlation between microplastic concentration and distance to the wall. Our results therefore suggest that dams do not trap floating microplastics or microfibres.