Occurrence of Microplastics in Coastal Sediments From the Inner Oslofjord, Norway

Globally, microplastics (MPs) are expected to accumulate at high concentrations in coastal sediments because these are readily linked with both land- and sea-based inputs, and changes in density and buoyancy facilitate sinking to the seabed. The Fjordic systems on the Norwegian coast have received some attention, reporting the presence of MPs in sediment samples, although explanations for the reported values are mostly lacking. This study presents records of MPs (< 1 mm) in coastal sediments sampled in the Vestfjord, the western part of the Inner Oslofjord, from 100 m of water depth. Our aim was to understand the scale of which the release of MPs in the Vestfjord was influenced by a WWTP system (VEAS) and how the redistribution of MPs is driven by oceanographic currents acting in the fjord. Thus, the upper 0— 5 cm of sediment cores were analysed from three areas of interest: two stations (ST4, ST5) in proximity of the VEAS diffuser and one (ST2) from the Vestfjord’s inlet. MPs were extracted from sediments using a NaI solution and were validated with μFT-IR. The distribution of MPs’ shape (fibre, fragment,film) and polymers in sediments was compared with grain-sizes (clay, silt, sand) via Canonical Correspondence Analysis (CCA).The most polluted area was ST4 with a total of 1.042 MPs/g dw, followed by ST5 with 0.524 MPs/g dw, both of which are close to the WWTP outlet. ST2 had the lowest concentration, 0.084 MPs/g dw. Fibres were the most common morphology found (76%), whilst fragments and films occurred with lower concentrations (18%, 6%). Common polymers were PES (50%), PP (18%), PMMA (9%), elastane (4%) and cellulosic materials (4%). MPs appear to preferentially accumulate according to their morphology and polymeric origin. Such that clays had preferential accumulation of PMMA, PA, PC, elastane and cellulosic materials; in silt PES, LD-PE, PS, PVC and films; in sand PP, fibres and fragments. The analyses on the vertical distribution of MPs showed that the occurrence of plastic particles decreased from 2002 and 2015 for ST2 and ST5 whilst increased for ST4. The input of plastic debris at ST4 and ST5 appears to derive from the release of treated waters from the WWTP diffuser with ST4 as the main accumulation zone - located down current from the diffuser. Due to the position of ST2 it is uncertain if the main input of MPs hails from the Inner or the Outer Oslofjord. Further analyses are needed to understand the main inputs of plastic debris in this area.