Microplastics in FLOW: Seasonal patterns in major Latvian rivers

Rivers are considered key pathways for microplastics, transporting these pollutants from inland sources to marine environments. In this study, we investigated the seasonal fluctuations of microplastics in the surface waters of four major Latvian rivers (Daugava, Lielupe, Gauja, and Salaca) that flow into the Gulf of Riga. Sampling was conducted from spring 2022 to summer 2023 to represent distinct seasonal and environmental conditions. Using a Manta net (100 μm mesh size) samples were collected in triplicate and analysed for microplastic concentration, shape, size, colour, and polymer composition. Microplastic concentrations ranged from 0.63 to 132.88 particles/m 3 , with the highest levels observed in the River Salaca. Fibres and fragments were the predominant microplastic shapes, with polyethylene, polypropylene, and ethylene propylene diene monomer being the most abundant polymers. We found significant spatial and seasonal variations in microplastic concentrations in some rivers, with the River Salaca showing the most extreme seasonal fluctuations. However, overall, no significant correlation was observed between the suspended material and river discharge variables. Replicate sampling revealed variability between samples, highlighting the need to include replicas. These findings highlight the complexity of microplastic pollution dynamics and the need for careful consideration of seasonal factors when assessing environmental contamination. This article is the first to contribute data from Latvian rivers on the growing pool of information on microplastics contamination in waterways.