Microplastics and fibers from three areas under different anthropogenic pressures in Douro river

Highlights: • Microplastic concentrations in the sampled areas were 231 MP L⁻¹ • Concentrations of 46 F L⁻¹ for total fibers and 6 F L⁻¹ for synthetic fibers • Wastewater treatment plant and boat dock and maintenance are potential sources. • High concentrations are a factor of the detection down to a lower size limit of 2 μm. Sources contributing to specific concentration of microplastics and fibers are still not completely understood. This study aimed at assessing the concentrations of microplastics (2–5000 μm) and fibers (18–5667 μm) in three areas of distinct influences in the Douro river, Porto, Portugal: (i) a countryside area; (ii) a wastewater treatment effluent release zone; and (iii) an area in proximity to a boat dock and maintenance station. Nile Red staining coupled with microscopy allowed the identification of small microplastics (≥2 μm) with a median concentration of the three areas of 231 MP L⁻¹. Most were fragments (69%). Sizes −1 and 6 F L⁻¹, respectively. High concentrations of microplastics and fiber contamination suggest that the wastewater treatment plant effluent and boat dock/maintenance are the likely sources originating hotspot areas.