Difference in the fate of surface and subsurface microplastics: an example for open and coastal waters

Plastic is recognized as a threat to marine ecosystems and estimating the level of plastic and microplastics (MP) pollution of the World Ocean is nowadays the goal of many studies. However, the use of different methods for sampling and analysis of MPs leads to the problem of comparing the results obtained. Studies on surface MP pollution of the surface sea water are based on the application of the manta or neuston nets that collect water from the upper 10-20 cm layer (the “surface” MP) or submersible pumping systems, that collect water from the water layer 3-5 m below the sea surface (the “subsurface” MP). These two techniques allow to collect particles of different size fraction, i.e. >300 µm for the surface MP and >100 µm for subsurface MP. The aim of this work was to study the distribution of surface and subsurface MPs and to reveal an influence of oceanographic conditions on their spatial distribution as an example of the open ocean waters of the Central Atlantic and coastal water in Norwegian fjords. It was shown that microplastics found in the surface and subsurface layers differ not only in the size of the items found, but also in morphology, types of polymers, abundance, weight concentration and their spatial distribution. Different hydrodynamic processes affect the fate of plastic occurring exactly at the sea surface and several meters deeper. MPs inhabiting the subsurface waters (about 3-5 m depth) have buoyancy close to neutral and appear to be suspended in the surface mixed layer and are readily transported from the sources to the distant regions by ocean currents. Subsurface turbulence is probably the main process that maintains the MPs in the near-surface part of the water column. In contrast, the surface MPs (which are captured by sampling with surface nets) have positive buoyancy and its spatial distribution is significantly influenced by water dynamics, wind and waves, which led to a more scattered distribution on the ocean surface. It was shown that surface and subsurface microplastics differ significantly in a number of properties and, apparently, they should be considered as two independent groups that may have different sources, and their distribution is driven by different hydrophysical processes. Thus, MPs data collected using both methods simultaneously could provide additional information about MPs fate in the ocean.