Drivers and variability of CO₂:O₂ saturation along a gradient from boreal to Arctic lakes

Lakes are significant players for the global climate since they sequester terrestrially derived dissolved organic carbon (DOC), and emit greenhouse gases like CO₂ to the atmosphere. However, the differences in environmental drivers of CO₂ concentrations are not well constrained along latitudinal and thus climate gradients. Our aim here is to provide a better understanding of net heterotrophy and gas balance at the catchment scale in a set of boreal, sub-Arctic and high-Arctic lakes. We assessed water chemistry and concentrations of dissolved O₂ and CO₂, as well as the CO₂:O₂ ratio in three groups of lakes separated by steps of approximately 10 degrees latitude in South-Eastern Norway (near 60° N), sub-Arctic lakes in the northernmost part of the Norwegian mainland (near 70° N) and high-Arctic lakes on Svalbard (near 80° N). Across all regions, CO₂ saturation levels varied more (6-1374%) than O₂ saturation levels (85-148%) and hence CO₂ saturation governed the CO₂:O₂ ratio. The boreal lakes were generally undersaturated with O₂, while the sub-Arctic and high-Arctic lakes ranged from O₂ saturated to oversaturated. Regardless of location, the majority of the lakes were CO₂ supersaturated. In the boreal lakes the CO₂:O₂ ratio was mainly related to DOC concentration, in contrast to the sub-Arctic and high-Arctic localities, where conductivity was the major statistical determinant. While the southern part is dominated by granitic and metamorphic bedrock, the sub-Arctic sites are scattered across a range of granitic to sedimentary bed rocks, and the majority of the high-Arctic lakes are situated on limestone, resulting in contrasting lake alkalinities between the regions. DOC dependency of the CO₂:O₂ ratio in the boreal region together with low alkalinity suggests that in-lake heterotrophic respiration was a major source of lake CO₂. Contrastingly, the conductivity dependency indicates that CO₂ saturation in the sub-Arctic and high-Arctic lakes was to a large part explained by DIC input from catchment respiration and carbonate weathering.