Baltic plastic soup recipe: presence of paraffin increases micro- and mesoplastic contamination

Micro- (0.5-5mm) and meso-size (0.5-2.5cm) floating marine litter was collected in June 2022 in the southeastern part of the Baltic Sea using a manta trawl (mesh size 335µm). Plastic items, excluding fibers and paint flakes, accounted for approximately 90.1% of the total number of collected particles in fifteen surface water samples. The contamination level ranged from (0.33-23.90)×105 items/km2 (or 0.22-15.93 items/m3, or 5-1,779g/km2, or 0.03-11.86mg/m3). Foamed PS particles accounted for 49.2% of the total number of collected particles. The mass of paraffin in each sample was high, ranging from 1.4-3,750g/km2 (9.1×10-3-25.0mg/m3), which was comparable to or greater than the mass of plastic. This study suggests that plastic particles are retained at the water surface by paraffin stains left after washing tanks on transport ships. This finding highlights an unexpected aspect of the contribution of shipping to plastic pollution in the region.