Whether a non-tidal estuary produces microplastic maxima (case study of the Pregolya River, the Baltic Sea).

The microplastics (0.3-5 mm) and mesoplastics (5-25 mm) in the non-tidal estuary of the Pregolya River (south-eastern Baltic Sea) were investigated for the first time in order to trace the retention zone (estuarine microplastic maxima, EMPM) at the river-sea interface, which is characterised by a salinity gradient. The mean abundance of all plastics (0.3-25 mm) ranged from 7.7 ± 5.1 to 15.7 ± 4.9 items per liter. Fibers accounted for 98 % of all plastic particles found. The combination of polymer types (including polyurethane, various types of rubber, PVDF, PEEK, etc.) identified by Raman spectroscopy is a probable consequence of increased industrial activity in addition to intensive shipping traffic. The local maximum of water turbidity in the near bottom layer (while the location of the salt wedge was typical of the spring season) appeared to be a false estuarine turbidity maximum associated with the release of water from the dumping sites. Two maxima in number and mass of plastic debris were found along the estuary. As these two maxima were not exactly associated with the zone of the highest salinity gradient, it was hypothesized that the naturally occurring EMPM in the non-tidal estuary is much more widespread than the current location of the hydrofront. The deviation from the unimodality of the EMPM, especially in the Pregolya Estuary, may be caused by the dilution of the estuary waters with the cleaner waters of the Vistula Lagoon.