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

The accumulation 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 (WFD) 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 5,862 vs Aguieira 1,658 MPs). Microplastic concentrations varied across sampling sites and periods in both reservoirs, with the Rabagão reservoir exhibiting greater spatial variation among sites within sampling periods and more pronounced seasonal fluctuations. In both study areas, the highest abundances were consistently recorded near the dams. In both reservoirs, the predominant colours were blue, black, and grey, and the most observed size ranged from 0.1 to 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 (according to the parameters evaluated following the WFD approach), Rabagão had higher microplastic contamination. On the contrary, Aguieira, which exhibited poorer ecological quality, had lower microplastic concentrations. This finding emphasizes that conventional water quality indicators may not adequately reflect the presence and influence of MPs, reinforcing the need to incorporate them into ecological assessment frameworks, especially in reservoirs used for human purposes.