Microplastic contamination in reservoirs: an invisible threat to ecosystem and water quality integrity

Reservoirs are essential aquatic ecosystems that provide drinking water and support various human activities. However, they are increasingly threatened by plastic pollution, posing risks to both environmental and human health. Despite this, the evaluation of the Ecological Potential (EP) of reservoirs under the Water Framework Directive (WFD) does not yet account for microplastics (MPs). This study aimed to quantify and characterize MPs in two Portuguese reservoirs (Rabagão and Aguieira), focused on their relation to the water quality and anthropogenic pressures. For this purpose, throughout 2023, sub-surface water samples were collected to assess the EP of the reservoirs, based on WFD metrics, and to identify MPs by size, colour, type, and chemical composition. Furthermore, the study areas were characterized by land use, soil occupation, and anthropogenic pressures to help identify potential sources of MPs. A total of 5862 MPs were observed in Rabagão and 1658 in Aguieira, with the highest concentrations recorded near the dam, in both reservoirs. For both reservoirs, the predominance of fibres between 0.1 and 0.5 mm was observed, and blue, black, and grey were the most common colours. Chemical composition analysis using ATR-FTIR identified polyethylene as the predominant polymer present in the samples from both reservoirs. Rabagão reservoir exhibited aquaculture as the main anthropogenic pressure, and a Good EP classification, while Aguieira exhibited greater anthropogenic impact (e.g. wastewater discharges, recreational activities, intensive forestry) and a Moderate EP classification. Overall, an apparent discrepancy was observed between the water quality of the reservoirs and the concentration of MPs, suggesting that a good ecological classification, based on existing metrics, does not necessarily indicate low microplastic pollution. This emphasizes that incorporating MPs monitoring into water quality assessments can provide important complementary information, particularly for reservoirs of notable environmental and social significance.