Stream metabolism controls diel patterns and evasion of CO 2 in Arctic streams

Streams play an important role in the global carbon (C) cycle, accounting for a large portion of CO evaded from inland waters despite their small areal coverage. However, the relative importance of different terrestrial and aquatic processes driving CO production and evasion from streams remains poorly understood. In this study, we measured O and CO continuously in streams draining tundra-dominated catchments in northern Sweden, during the summers of 2015 and 2016. From this, we estimated daily metabolic rates and CO evasion simultaneously and thus provide insight into the role of stream metabolism as a driver of C dynamics in Arctic streams. Our results show that aquatic biological processes regulate CO concentrations and evasion at multiple timescales. Photosynthesis caused CO concentrations to decrease by as much as 900 ppm during the day, with the magnitude of this diel variation being strongest at the low-turbulence streams. Diel patterns in CO concentrations in turn influenced evasion, with up to 45% higher rates at night. Throughout the summer, CO evasion was sustained by aquatic ecosystem respiration, which was one order of magnitude higher than gross primary production. Furthermore, in most cases, the contribution of stream respiration exceeded CO evasion, suggesting that some stream reaches serve as net sources of CO , thus creating longitudinal heterogeneity in C production and loss within this stream network. Overall, our results provide the first link between stream metabolism and CO evasion in the Arctic and demonstrate that stream metabolic processes are key drivers of the transformation and fate of terrestrial organic matter exported from these landscapes.