Drivers and variability of CO:O 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 CO to the atmosphere. However, the differences in environmental drivers of CO 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 and CO, as well as the CO:O 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, CO saturation levels varied more (6-1374%) than O saturation levels (85-148%) and hence CO saturation governed the CO:O ratio. The boreal lakes were generally undersaturated with O, while the sub-Arctic and high-Arctic lakes ranged from O saturated to oversaturated. Regardless of location, the majority of the lakes were CO supersaturated. In the boreal lakes the CO:O 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 CO:O ratio in the boreal region together with low alkalinity suggests that in-lake heterotrophic respiration was a major source of lake CO. Contrastingly, the conductivity dependency indicates that CO saturation in the sub-Arctic and high-Arctic lakes was to a large part explained by DIC input from catchment respiration and carbonate weathering.