The impact of seasonal hydroclimatic variability and NDVI on long-term nutrient dynamics in a Finnish agricultural catchment draining to the Baltic Sea

Understanding how seasonal hydroclimatic variability and vegetation conditions influence the nutrient dynamics in agricultural catchments is crucial for sustainable management of surface water quality. In this study, we assessed the seasonal impact of hydroclimatic variability and shifts in vegetation conditions on >30 years of total nitrogen (TN) and total phosphorous (TP) in the Aurajoki catchment draining to the Baltic Sea, Southwest Finland. The datasets included long-term monitoring records of nutrient concentrations, discharge, temperature, precipitation, and the Landsat imagery series. Prior to the analyses, we characterized hydroclimatic variability using the Standardized Precipitation Index (SPI), Accumulated Winter Season Severity Index (AWSSI), and vegetation conditions were represented using Normalized Difference Vegetation Index (NDVI). We used Mann – Kendall statistics to assess seasonal trends in both TN and TP concentrations and loads, and generalized additive models (GAMs) to quantify the influence of NDVI and hydroclimatic conditions. Seasonal trend analysis revealed significant increases in both TN and TP during autumn and decreases during spring. Modeling results indicated that NDVI influenced nutrient concentrations but had no significant effect on nutrient loads. In contrast, the influence of hydroclimatic class was confined to nutrient loads and varied depending on the combination of hydroclimatic class and NDVI. These results highlight the relevance of accounting for seasonal hydroclimatic variability and NDVI-derived vegetation conditions when assessing nutrient dynamics in the agricultural catchments. The findings further support understanding of nutrient‑driven eutrophication in systems such as the Baltic Sea.