Microplastics in the Water Column of the Rhine River Near Basel: 22 Months of Sampling

Measured microplastic concentrations in river surface waters fluctuate greatly. This variability is affected by season and is codriven by factors, such as sampling methodologies, sampling site, or sampling position within site. Unfortunately, most studies comprise single-instance measurements, whereas extended sampling periods are better suited to assessing the relevance of such factors. Moreover, microplastic concentrations in riverine water column remain underexplored. Similar to the oceans, however, this compartment likely holds significant amounts of microplastics. By representatively sampling the entire Rhine River cross-section near Basel through five sampling points over 22 months, we found a median microplastic (50-3000 μm) concentration of 4.48 n m^-3, and estimated a widely ranging load between 4.04 × 10² n s^-1 and 3.57 × 10⁵ n s^-1. We also show that the microplastic concentration in the water column was not well explained by river discharge. This suggests that although high discharge events as observed here can over short time periods lead to peak microplastic concentrations (e.g., 1.23 × 10² n m^-3), microplastic load variance was not dominated by discharge in the study area.