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 O-2 and CO2, as well as the CO2:O-2 ratio in three groups of lakes separated by steps of approximately 10 degrees latitude in South-Eastern Norway (near 60 degrees N), sub-Arctic lakes in the northernmost part of the Norwegian mainland (near 70 degrees N) and high-Arctic lakes on Svalbard (near 80 degrees N). Across all regions, CO2 saturation levels varied more (6-1374%) than O-2 saturation levels (85-148%) and hence CO2 saturation governed the CO2:O-2 ratio. The boreal lakes were generally undersaturated with O-2, while the sub-Arctic and high-Arctic lakes ranged from O-2 saturated to oversaturated. Regardless of location, the majority of the lakes were CO2 supersaturated. In the boreal lakes the CO2:O-2 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:O-2 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.