Climate Change‐Induced Landscape Alterations Increase Nutrient Sequestration and Cause Severe Oligotrophication of Subarctic Lakes

We combined decadal data (23-35 y) on nutrient concentrations for nine subarctic lakes with satellite imagery of vegetation (NDVI) to link the ongoing nutrient declines to the climate change-induced greening of landscapes. Total phosphorus water concentrations (Total-P) showed declining trends for all nine lakes, ranging from 1.5%-3.6%/y over the last decades. For most of the lakes' drainage areas, NDVI showed a dramatic increase during the 1990s and leveled off between 2001 and 2020. P sequestration in the lakes' drainage areas generally increased by 12%-30% between 1983-1994 and 2001-2020, with an exception of one high-elevation lake for which P sequestration more than doubled. Area-specific P-sequestration estimates for 1983-1994 averaged 1.04 ± 0.10 tons P/km² among all lakes but increased by 12%-33% for eight of the nine lakes during 2001-2020. Similar trends were found for nitrogen (N) sequestration, although these were an order of magnitude higher. These estimates illustrate long-term changes in the sequestration of N and P by terrestrial vegetation in the region. Total-P and DIN water concentrations showed negative correlations with both the NDVI_max of their drainage areas and plot-scale measurements of tundra dwarf shrub cover. These correlations explained 51.8%-75.4% of the variability in declining nutrient water concentrations and showed the strong links between terrestrial vegetation development and declines in nutrient inputs to downstream lakes. Similar processes are likely ongoing in other parts of the Arctic where vegetation development is progressing, but are either not detected due to the lack of long-term monitoring data or compensated for by nutrients released from thawing permafrost and/or thermokarst slumps. Upscaling our P- and N-sequestration estimates for the nine lakes to the entire Arctic/alpine ecoregion in Sweden showed an average increase of 12.0 ± 1.7 Mtons P and 122.6 ± 17.5 Mtons N between the periods 1982-1994 and 2001-2020.