Microplastics Occurrence in Two Mountainous Rivers in the Lowland Area—A Case Study of the Central Pomeranian Region, Poland

Because of the increasing worldwide awareness concerning the occurrence of microplastics (MPs) in aquatic ecosystems, our goal was to analyze for the first time the quality and abundance of MPs and assess their seasonal variation in two unique rivers flowing through the low-land area in northern Poland. Their uniqueness is due to the fact they flow through landscape parks and urbanized zones, possess mountainous characteristics, and are aquatic habitats for sea trout (Salmo trutta m. trutta) and salmon (Salmo salar). In this pioneering study, the morphological types, geometric dimensions, and color of MP particles were analyzed by the use of an optical microscope. MPs particles were detected in 62.5% of the river water samples, while the average abundance was 3.6–4.2 items per sample. In terms of general seasonality, the sum of MPs items found in investigated river water samples decreased in the following order: spring (75 items) > summer (64 items) > autumn (52 items). Neither the total MPs abundance nor any morphological MPs types were statistically different between rivers according to single seasons. The quantity of MPs present in the river water was higher downstream of the wastewater treatment plant studied, which confirms that treated sewage effluent is a key source of MPs in an aquatic environment. Among the morphological types, fragments were prevalent among granules and fibers, while their average length not exceeding 1.0 mm enabled them to be classified as small. MPs were classified into nine colors, however, the bright colors were dominating only in the case of granule. In the case of the fragments and fibers, the dominating colors were transparent, white, blue, and black. Fourier transform infrared spectroscopy was performed on a small sample of microplastics (21.0%) due to their small size. Polymers containing polyethylene, polyvinyl chloride, polypropylene, polyester, and polystyrene were identified.