Martyanov S.D., Ryabchenko V.A., Ershova A.A., Eremina T.R., Martin G.

To study the propagation characteristics of microplastic particles coming with the Neva river waters, in the Neva Bay and in the eastern part of the Gulf of Finland, a three-dimensional numerical hydrodynamic model based on the Princeton Ocean Model is used. The model is implemented on a uniform quasi-orthogonal horizontal grid with a step of 100 m, in the vertical direction 7 uniformly distributed sigma levels are used. The marine initial conditions and conditions at the western boundary for water level, temperature and salinity were taken from the Baltic Sea operational model HIROMB-BOOS of the Danish Meteorological Institute with discreteness of 1 hour. On the eastern boundary at the mouth of the Neva the average monthly climatic river discharge and temperature of the Neva were set. Atmospheric forcing was taken from the results of the ECMWF ERA-Interim reanalysis with 6-hour temporal resolution and with a spatial resolution of 0.125 × 0.125°. Two types of suspension were considered that simulated the propagation of microplastic particles in the water: admixture of neutral buoyancy and a sinking suspension with a sinking velocity of 0.2 m/day. Both types of suspension come from the Neva River with a constant volume concentration of 10 −6 . To calculate the thickness of the layer of the settling fraction at the bottom the simplified Exner equation is used. The calculations were performed for the period May—August 2018 when the quantity and composition of plastic litter was monitored on the coast of the Neva Bay and the eastern part of the Gulf of Finland.
According to model results, the spatial distribution of the sinking particles, in general, repeats the distribution of the admixture of neutral buoyancy, with the only difference being that the farther from the particle source to the west, the lower the concentration of the sinking particles. An essential feature of the distribution is that during most time of the considered period the concentrations of both suspensions in the northern part of the model domain is higher than those found in its southern part. The change in the thickness of the bottom layer of the particles of the settling fraction at the end of the period on August 31, 2018, i.e. the accumulation of microplastic particles in bottom sediments for the period under consideration, is characterized by the same feature as the space distribution of the admixture of neutral buoyancy in water: the accumulation of microplastic in bottom sediments in the northern part of the model area outside the Neva Bay was noticeably greater than in the southern part, especially in the coastal zone.
The data on monitoring the coastal pollution by plastic litter indirectly confirm the model results: there was practically no plastic litter on the southern coast of the eastern Gulf of Finland outside the Neva Bay between June and August 2018, while it was found on the northern coast in significant quantities. Thus, model estimates of the distribution of microplastic particles in water and its accumulation in bottom sediments can be used to select areas for future work on monitoring plastic litter pollution on the coast of the Gulf of Finland.

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