Multidecadal water quality trends across 15 European river basins along a Mediterranean climate gradient

Water quality degradation is a major issue in Mediterranean regions, but identifying the key natural and human drivers remains challenging, requiring large-scale studies for meaningful synthesis and comparison. This study analyzed a vast Mediterranean dataset spanning 89,015 km2 across 15 Iberian river basins along a climate gradient (cooler-wetter north to warmer-drier south), 3 decades, 3441 stations, 19 parameters, and ∼15 million observations. It is the first study of this scale in the region, utilizing custom scripts for automated data compilation and processing. The study revealed an evident north-to-south water quality decline, with rising electric conductivity, pH, total suspended solids, nitrogen, phosphorus, organic carbon, and sulphate, alongside reduced dissolved oxygen and transparency. This pattern correlated with the latitudinal climate gradient and intensified agriculture in the south (Pearson correlation coefficient: 0.1 to 0.53; Spearman's rank correlation coefficient: 0.17 to 0.56), while increased forest cover had a mitigating effect (Pearson: -0.50 to -0.07; Spearman: -0.51 to -0.10). Multidecadal trend analysis revealed a shift around 2005 with most parameters decreasing, except for nitrate and phosphate, which rose likely due to the 2004/05 drought reducing river dilution and expanded irrigated agriculture, especially in Alentejo with the Alqueva reservoir. These findings are crucial for guiding future national management actions and pollution reduction initiatives in Mediterranean regions, aligning with the European Commission's "European Green Deal" that targets a 50 % reduction in nutrient losses to make agriculture more environmentally sustainable, and also provide a valuable reference to many other regions with similar challenges for water quality management.