Long-term hydrochemical dynamics and stoichiometric relationships in the boreal peatland-dominated Oulanka River system, Northeast Finland

Boreal catchments are undergoing significant hydrological and biogeochemical shifts due to climate and land-use changes. However, understanding these changes is hindered by a lack of long-term data. This study examines long-term (1966-2024), seasonal, and spatial trends in water quality in the Oulanka River system, a semi-natural catchment in Northeast Finland draining to the Arctic Ocean, using a 58-year time series and synoptic sampling (2020-2022). In contrast to many studies, we did not observe increases in dissolved organic carbon (DOC). The results identified a significant decline in total nitrogen (TotN), while total phosphorus (TotP) nitrite-nitrate (NO₂NO₃), and ammonium (NH4) remained stable, and phosphate (PO₄) increased. Shoulder seasons were identified as important transport periods, contributing higher loading compared to Winter and Growing seasons. Concentration-discharge (C-Q) relationships varied, with DOC showing mild chemodynamic behaviour, TotP maintaining a positive slope across seasons, and PO₄ exhibiting strong seasonal variation. Long-term trends revealed declining C-Q slopes for TotP, increasing slopes for NO₂NO₃, and decreasing CV ratios, particularly for PO₄. C:N:P stoichiometry was strongly carbon-dominated, with nitrogen increasingly limited over time. Surprisingly, seasonal and spatial variations in stoichiometry were minimal. The study reveals a surprising stability in long-term carbon and phosphorus nutrient dynamics despite significant warming in the Oulanka catchment. These findings underscore the importance of site-specific catchment studies to enhance understanding of boreal water quality and inform more effective management strategies.