Two decades of nutrient dynamics in Polish rivers: Long-term trends in nitrogen and phosphorus

• Nationwide trends in riverine NO 3 -N, TN, and TP in Poland were assessed (2005–2022). • WRTDS modeling with EGRET enabled flow-normalized trend analysis across 52 sites. • Phosphorus concentrations declined strongly; nitrogen trends were spatially mixed. • Municipal sewage improvements drove TP decline; diffuse agriculture fueled NO 3 -N rise. • Poland’s trends reflect broader European nutrient patterns but with regional disparities. Understanding long-term changes in nutrient concentrations in rivers is essential for effective water management and environmental protection, especially in the context of international policies such as the EU Water Framework Directive. This study presents a comprehensive national assessment of the trends in nitrate nitrogen (NO 3 -N), total nitrogen (TN) and total phosphorus (TP) concentrations in Polish rivers over the period 2005–2022. Long-term water quality data from 52 monitoring sites with an average annual sampling frequency of 13.4 samples per year, divided into eight geographical groups, were combined with simulated river discharge data and analyzed using the Weighted Regressions on Time, Discharge, and Season (WRTDS) method. The results revealed a spatially heterogeneous but nationally coherent pattern of decreasing TP concentrations in most regions, in contrast to the increasing NO 3 -N and TN trends observed in several catchments, particularly in central, eastern and northern Poland. The observed decreases in phosphorus are consistent with substantial reductions in TP loads from municipal wastewater treatment plants, while the increasing nitrogen trends are consistent with increased use of mineral fertilizers. Particular attention was paid to the Lower Odra and Vistula rivers, which integrate nutrient inputs over large areas and directly impact the Baltic Sea. Substantial declines in TP concentrations were observed at both sites (−40 % and −30 %, respectively), while nitrogen trends were mixed, with stable or slightly increasing NO 3 -N and modest declines in TN. Comparisons with trends reported in Northern and Central-Eastern Europe reveal similar trajectories, highlighting common successes in phosphorus management and persistent challenges in controlling diffuse nitrogen pollution. This study addresses a significant knowledge gap regarding the dynamics of nutrient concentrations in Polish rivers, offering valuable insights for the development of more targeted and effective nutrient management policies.