Variability in microplastic sediment concentrations in the Upper Okavango Delta

River pollution with microplastic particles (MP) has become a global scale problem. Recent research has shown that MP can remain in riverbed sediment for several decades, where they interact with the sediment and biota, potentially impacting food webs, while slowly releasing chemical additives. Thus, a good understanding of the MP distribution within riverbed sediment is critical to quantify potential risks to riverine ecosystems. Here we present first results of MP sediment concentrations for the upper Okavango Delta (Panhandle) in Northern Botswana, which home to about 90,000 people. The Panhandle comprises meandering channels, oxbow lakes and a wide alluvial plain. River flows in the Okavango Basin are dominated by a strong seasonal pulse. At the upstream end (Mohembo), discharge varies between <100 m3/s during low flow and 700 m3/s during peak flow (March-April). Sediment samples were collected at 7 sites between Mohembo and Sepupa by scooping. A 15 cm-long push core was also collected to assess MP distribution in depth. Organic matter was digested using either Fenton’s reagent or only H2O2. Each digested sample was split; part analysed with Raman spectroscopy to identify main polymers and part stained with Nile Red for fast MP quantification using fluorescent microscopy. Sediment grain size analysis was conducted using laser diffraction. Fluorescent microscopy showed MP sediment concentrations from around 57 MP/kg dry weight in some of the oxbows to 400 MP/kg in the main channel. About 92% of the identified MP were classified as fragments, varying in length between 70-1065 μm (lower detection limit was 64 μm). The remainder of the particles were identified as fibres. Raman spectroscopy showed PET, PP and PVC as the main polymer types. Future studies will focus on determining basin-wide MP concentrations and their relation to sediment transport patterns.