Glacier retreat reshapes nutrient dynamics in mountain streams of Norway

Climate change is driving rapid glacier retreat, facilitating the expansion of vegetation into previously unvegetated terrain, and is altering the nutrient dynamics of aquatic ecosystems downstream of glaciers. Here, we measured nutrient concentrations in catchments with varying degrees of glacier coverage in western Norway to determine the likely impacts of glacial retreat on nutrient cycling. Field investigations were conducted during 2024–2025 in five catchments (25–110 km²) draining the largest ice cap in mainland Europe, Jostedalsbreen, and alpine valley glaciers in the Jotunheimen mountain range. These glaciers have undergone sustained thinning and recession since the end of the Little Ice Age (c. 1750 CE). Water chemistry, including concentrations of nutrients, dissolved organic carbon, and suspended sediment, was measured along the longitudinal profiles of the studied streams, and incorporated subglacial, supraglacial, and proglacial waters. Regression analyses revealed that the concentrations of major ions (e.g., Ca²⁺, Na⁺, K⁺, HCO₃⁻, SO₄²⁻) increase as glacier cover within the catchment decreases. This pattern may suggest enhanced chemical denudation and intensified leaching of soil material in non-glaciated areas. In three of the investigated catchments, declining glacier coverage was associated with reduced concentrations of nitrogen compounds (NO₃⁻, NH₄⁺, and total N), indicating that glaciers might play a critical role in the delivery of nitrogen to downstream aquatic systems. Conversely, vegetation likely reduces nitrogen fluxes through biological uptake from soils. Positive Matrix Factorization (PMF) modelling corroborated these findings, identifying distinct sources affecting water chemistry. An atmospheric factor (dominated by Na⁺, Cl⁻, and total N), a biogenic factor (associated primarily with nitrogen compounds), and geogenic factors (linked to mineral weathering and chemical denudation) were distinguished. Biogenic and atmospheric contributions predominated in catchments with ≥60% glacier coverage, whereas geogenic contributions were more pronounced in catchments with <40% glacier cover. Overall, continued glacier retreat and vegetation encroachment are expected to increase the concentration of major ions in streamwaters while diminishing the export of nitrogen compounds, potentially reshaping the biogeochemical functioning of downstream aquatic ecosystems.