Spatial and temporal distribution of microplastics in water and sediments of a freshwater system (Antuã River, Portugal)

Highlights: • Antuã River is severely affected by microplastics, especially in water column. • Spatial and temporal distributions show different pattern of abundance of MPs. • Sampling sites were areas with intensive anthropogenic activities. • Methodologies, seasonal and hydrodynamic conditions cause these variations. • Polyethylene and polypropylene are the most common polymers detected. Microplastics (particles with a size < 5 mm), one of the most emerging aquatic pollutants, are of particular concern since they can reach high densities and interact with biotic and abiotic environment. The occurrence of microplastics in freshwater systems is less understood than in marine environment. Hence, the present study aims to provide new insights into microplastics abundances and distribution in Antuã River (Portugal) by applying the isolation method of wet peroxide oxidation with addition of zinc chloride to water and sediment samples collected in March and October 2016, in three sampling sites. The abundance of microplastics in water ranged from 5 to 8.3 mg m⁻³ or 58–193 items m⁻³ in March and from 5.8–51.7 mg m⁻³ or 71–1265 items m⁻³ in October. In sediments, the abundance ranged from 13.5–52.7 mg kg⁻¹ or 100–629 items kg⁻¹ in March and from 2.6–71.4 mg kg⁻¹ or 18–514 items kg⁻¹ in October. The water and sediment samples with the greatest abundances were from São João da Madeira and Aguincheira, respectively. Spatio-temporal distribution showed different pattern according to methodological approaches, seasonal and hydrodynamic conditions and the proximity to urban/industry areas. Analysis of plastics by Fourier transform infrared spectroscopy underline polyethylene and polypropylene as the most common polymer types identified in this work. The low medium high oxidation ratio was 56:22:22 (%) in March and 61:31:8 (%) in October. Foams and fibers were the most abundant type in São João da Madeira, while fibers and fragments were the most abundant in Aguincheira and Estarreja in water and sediment samples, respectively. This study emphasizes the importance of rivers as carriage systems of microplastics. Further studies should be performed to identify point sources in order to mitigate the microplastics contamination in aquatic systems.