Spatial distribution of microplastics in North Sea surface waters and sediments

It is well known that microplastics (<5 mm in size) are omnipresent in the marine environment and will hardly degrade but merely fragment over time. Furthermore, standardized and reliable methods to securely detect microplastics are yet to be set. Determining the abundance and identity of microplastics is a crucial objective in microplastic research, especially with regard to requested monitoring. Nevertheless, information on the abundance and composition of microplastics in the North Sea is still scarce particularly concerning the lower-micrometer range (<500 µm). Hence, we analysed samples from two complex environmental matrices at 24 stations in the southern North Sea. Surface water samples were collected with a 100 µm net attached to a neuston catamaran and sublittoral sediments were taken with a Van Veen grab. To isolate microplastics (11–5000 µm) sediments were first subjected to a density separation performed with the MicroPlastic Sediment Separator (MPSS) and a zinc chloride solution (ρ = 1.7 g cm-³). Subsequently, a highly promising enzymatic-oxidative purification was applied to all samples using newly developed microplastic reactors. This was followed by a state-of-the-art analysis via Fourier transform infrared spectroscopy (FTIR) imaging. This provides information on polymer quantities, types and sizes as well as spatial distribution of microplastics in North Sea surface waters and sediments. Results show that microplastics are present in all of the analysed North Sea samples exhibiting a variety of polymer types, dominated by rubbers, polyethylene, polypropylene and acrylates/polyurethane. Concentrations ranged from 0.1 to 2.1×10² particles m-3 in surface waters and from 2 to 1.5×10³ particles kg-1 of dry sediment. Concerning size, the vast majority of the detected microplastic particles (95%) was less than 75 µm in length. Finally, this study aims at contributing to a basis for future monitoring measures and stresses the need to include microplastics in the lower-micrometer range into these approaches.