Authors: Abubaker Omer, Joy Bhattacharjee, Kati Anttila, Eliisa Lotsari & Harri Koivusalo
Publication type: A1
Abstract
Evidence is accumulating that climate change is driving fundamental shifts in hydrological regimes across northern catchments. Yet the biogeochemical consequences of these shifts and their implications for surface-water quality remain poorly understood. Here, we developed a process-based diagnostic framework that integrates hydrology–water-quality modeling, climate scenarios (SSP1-2.6, SSP2-4.5, SSP5-8.5), and statistical methods to identify the mechanisms by which climate forcing generates new hotspots of nitrogen and phosphorus mobilization in northern catchments. The framework was applied to the Vantaanjoki catchment, a well-characterized northern system with a documented history ofnutrient export to the Gulf of Finland and the Baltic Sea. We found statistically significant hydrological shifts under climate change scenarios, characterized by reduced snow storage and intensified winter–autumn runoff. Phenological mismatch between nutrient supply and plant uptake is projected to intensify under climate change, with nutrient release and mobilization increasingly occurring during periods of low biological demand. Combined with hydrological shifts, this leads to higher off-season losses and a transition in nutrient inputs to the Gulf of Finland from melt- to rainfall-driven pathways. Consequently, the seasonal share of annual nutrient export is projected to decrease in spring from 33% to 23–28%, while increasing in winter and autumn from 24% to 27–31% and from 30% to 36–40%, respectively, under SSP scenarios relative to the current condition. Moreover, climate change is projected to alter the emergence and expansion of nutrient-mobilization hotspots in vulnerable cropland and urban–agricultural transition zones through hydrological shifts and weakened nutrient–biological coupling. Based on the study’s findings, we present a transferable, mechanism-guided nutrient management framework to address climate-driven biogeochemical risks in comparable systems worldwide.