Microplastics in river estuaries – assessing seasonality and importance of sample replicas

Rivers serve as the primary pathways for microplastics to enter the sea, still, research on microplastic pollution in freshwater systems, including estuaries, remains limited. Estuaries play an important role in the amount of microplastic that enters the seas and oceans, as plastic can accumulate in these areas due to bi-directional flow, However, there is a lack of information regarding plastic waste transportation processes and seasonal variations in these areas. The aim of this study was to evaluate the role of surface water microplastic pollution seasonality in the estuaries of greatest Latvian rivers flowing in the Gulf of Riga (Daugava, Lielupe, Gauja, Salaca), and the significance of replicate sampling for results interpretation. A total of 42 surface water microplastic samples were collected using Manta net (mesh size 100 μm) during four seasons (autumn, winter, spring, summer), from October 2022 to July 2023 with sampling frequency once per two months. Results show that during the spring and summer seasons there are higher concentrations (1.53 – 34.37 particles/m3) of plastic particles found than during the autumn and winter seasons (1.03 – 3.62 particles/m3) indicating water flow and seasonal meteorological changes. We also concluded that performing surface water microplastic sampling with more than one replica can notably enhance result quality, since the amount of microplastic differs significantly between consecutively acquired samples at one sampling site. The study was funded by Latvian Environmental Protection Fund project No. 1-08/37/2022. Also see: https://micro2024.sciencesconf.org/553431/document