Model uncertainties of a storm and their influence on microplastics / sediment transport in the Baltic Sea

Abstract. Microplastics (MP) are omnipresent in the aquatic environment where they pose a risk to ecosystem health and functioning. Little is, however, known about the concentration and transport patterns of this particulate contaminant. Measurement campaigns remain expensive and assessments of regional MP distributions need to rely on a limited number of samples. The prediction of potential MP sink regions in the sea would thus be beneficial for a better estimation of MP concentration levels and a better sampling design. Based on a sediment transport model, this study investigates the transport of different MP model particles, PET and PVC particles with sizes of 10 and 330 μm, under storm conditions. A storm event was chosen because extreme wave heights cause intense sediment erosion down to depths unaffected otherwise, and are therefore critical for determining accumulation regions. The calculation of metocean parameters for such extreme weather events is subject to uncertainties. This sensitivity study targets the propagation of uncertainty from the atmospheric conditions to MP erosion and deposition, on the basis of freely available models and data. We find that atmospheric conditions have a strong impact on the quantity of eroded and deposited material. Thus, even if the settling and resuspension properties of MP were known, a quantitative transport estimation by ocean models would still show considerable uncertainty due to the imperfect knowledge of atmospheric conditions. The uncertainty in the transport depends on the particle size and density, transport of the larger and denser plastic particles only takes place under storm conditions. Less uncertainty exists in the location of erosional and depositional areas, which seems to be mainly influenced by the bathymetry. We conclude, while quantitative model predictions of sedimentary MP concentrations in marine sediments are hampered by the uncertainty in the wind fields during storms, models can be a valuable tool to select sampling locations for sedimentary MP concentrations to support their empirical quantification.