Unseen contaminants in Portuguese reservoirs: linking microplastics to ecological potential and human pressures

The proliferation of microplastics (MPs) in aquatic environments is a contemporary concern of great relevance, however, freshwater ecosystems, particularly reservoirs, have received less attention. This study evaluates the MPs in Rabagão and Aguieira Portuguese reservoirs, and their role in ecological quality assessments. Along 2023, sub-surface water samples were collected to assess Ecological Potential, under Water Framework Directive metrics, and to characterize MPs by type, colour, size, and chemical composition. Reservoirs were also characterized by land use, soil occupation, and anthropogenic pressures. Results confirm MPs contamination in both reservoirs, predominantly fibres, with Rabagão exhibiting higher total abundance (Rabagão 5862 vs Aguieira 1658 MPs). In both reservoirs, the predominant colours were blue, black, and grey, and the most observed size ranged from 0.1–0.5 mm. ATR-FTIR analysis identified polyethylene, polyethylene terephthalate, polyester, nylon, polyvinyl chloride, and polyvinyl acrylate. Anthropogenic pressures including aquaculture, wastewater discharges, and recreational activities were identified as potential pollution sources. Despite fewer pressures and better ecological status, Rabagão had higher microplastic contamination, highlighting that better water quality does not necessarily reflect lower MPs pollution. This study emphasizes the importance of including MPs in water quality assessments, especially in reservoirs used for human purposes.