What influences the distribution of microplastics in the marine environment? An interdisciplinary study reveals key factors driving microplastic in the North Sea.

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Microplastics (MP) are known to be ubiquitous. The pathways and fate of these contaminants in the marine environment are receiving increasing attention, but still knowledge gaps exist. In particular, the link between mass-based MP quantification and oceanographic parameters is often lacking. In this study, we aim to interconnect different parameters for the first time through in-situ measurements with an autonomous surface vehicle in the German Bight. It simultaneously sampled air, sea surface microlayer, and underlying water for analysis of MP and additionally, extracellular polymeric substances (only in water). These compounds, secreted by microorganisms, can interact with particulate matter, influencing their transport dynamics and aggregation behavior in the environment. During the entire sampling, a weather station and conductivity, temperature, and depth sensors were installed on the vehicle. Depth profiles were taken with an accompanying research vessel to learn more about the stratification and horizontal processes of MP in the marine environment. Additionally, an acoustic Doppler current profiler recorded water current velocities and flow direction. A relationship was found between wind direction and the presence of MP in the atmosphere. Furthermore, wind speeds may seem to increase heterogeneity in both the composition and concentration of MP in the water. A tentative correlation between extracellular polymeric substances and MP was documented. Investigating horizontal and vertical velocities of currents within the surface and the water column helped to explain the distribution of MP. Up- and downwelling processes corresponded to the accumulation of MP along density fronts and across depth profiles.