Regional Variability and Drivers of Below Ice CO2 in Boreal and Subarctic Lakes

Northern lakes are ice-covered for considerable portions of the year, where carbon dioxide (CO 2 ) can accumulate below ice, subsequently leading to high CO 2 emissions at ice-melt. Current knowledge on the regional control and variability of below ice partial pressure of carbon dioxide ( p CO 2 ) is lacking, creating a gap in our understanding of how ice cover dynamics affect the CO 2 accumulation below ice and therefore CO 2 emissions from inland waters during the ice-melt period. To narrow this gap, we identified the drivers of below ice p CO 2 variation across 506 Swedish and Finnish lakes using water chemistry, lake morphometry, catchment characteristics, lake position, and climate variables. We found that lake depth and trophic status were the most important variables explaining variations in below ice p CO 2 across the 506 lakes . Together, lake morphometry and water chemistry explained 53% of the site-to-site variation in below ice p CO 2 . Regional climate (including ice cover duration) and latitude only explained 7% of the variation in below ice p CO 2 . Thus, our results suggest that on a regional scale a shortening of the ice cover period on lakes may not directly affect the accumulation of CO 2 below ice but rather indirectly through increased mobility of nutrients and carbon loading to lakes. Thus, given that climate-induced changes are most evident in northern ecosystems, adequately predicting the consequences of a changing climate on future CO 2 emission estimates from northern lakes involves monitoring changes not only to ice cover but also to changes in the trophic status of lakes.