Estuarine turbidity maximum in non-tidal conditions and relations to microplastic retention

The estuarine turbidity maximum (ETM) is typically found in tidal estuaries. The research on the ETM in non-tidal estuaries is comparatively limited. The presentation is focused on the ETM in the context of non-tidal Baltic Sea estuaries, particularly that of the Pregolya River, and is the first attempt to estimate the presence of ETM in the Baltic Sea river mouths environment. The study presents field measurement results examining the spatial distribution of temperature, salinity, turbidity, and suspended matter content along the Pregolya River estuary (South-Eastern Baltic). Measurements were taken monthly in 2024 to determine seasonal changes in these characteristics. It was found that ETM did not appear during the spring increase in river runoff, but formed during the period of low water, especially during the wind surges in the autumn. The biogenic component of suspended matter prevailed during the warm season. Lithogenic component prevailed at other times of the year (except during the spring runoff period). Results showed that ETM can occur under non-tidal conditions, exhibiting seasonal cyclicity and being driven by processes associated with the gravitational, colloidal-sorption, and biogenic stages. The microplastics (0.3-5 mm) and mesoplastics (5-25 mm) were investigated for the first time. The mean abundance of all plastics (0.3-25 mm) ranged from 7.7±5.1 to 15.7±4.9 items per litre. Fibres accounted for 98% of all plastic particles found. Two maxima in number and mass of plastic debris were found along the estuary, and it was hypothesised that the microplastic maximum is gradually formed by seasonal dynamics.