Abstract
Streams play an important role in the global carbon (C) cycle, accounting for a large portion of CO2 evaded from inland waters despite their small areal coverage. However, the relative importance of different terrestrial and aquatic processes driving CO2 production and evasion from streams remains poorly understood. In this study, we measured O2 and CO2 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 CO2 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 CO2 concentrations and evasion at multiple timescales. Photosynthesis caused CO2 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 CO2 concentrations in turn influenced evasion, with up to 45% higher rates at night. Throughout the summer, CO2 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 CO2 evasion, suggesting that some stream reaches serve as net sources of CO2, thus creating longitudinal heterogeneity in C production and loss within this stream network. Overall, our results provide the first link between stream metabolism and CO2 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.
Highlights
Streams receive large amounts of carbon (C) from terrestrial ecosystems (Drake, Raymond, & Spencer, 2017) and emit a large fraction of this as CO2 to the atmosphere (Raymond et al, 2013)
We simultaneously assessed continuous O2 and CO2 data to show that aquatic biological processes play an important role in the C cycle of these Arctic streams
In the Swedish northern landscape, the signature of aquatic metabolism was imprinted upon stream CO2 dynamics in two distinct ways: photosynthesis created a clear day– night difference in CO2 evasion and in-stream respiration sustained CO2 evasion from streams throughout the summer
Summary
Streams receive large amounts of carbon (C) from terrestrial ecosystems (Drake, Raymond, & Spencer, 2017) and emit a large fraction of this as CO2 to the atmosphere (Raymond et al, 2013). | 1401 mineralized in soils and subsequently transported to streams in gas form (Öquist et al, 2009), respired within the stream ecosystem (Fisher & Likens, 1973; Hedin, 1990), and/or photo-oxidized in the water column (Cory, Ward, Crump, & Kling, 2014) Resolving these different pathways is necessary to determine the fate of OC at regional scales, including the magnitude of CO2 evasion and water-borne C export to recipient systems (Webb, Santos, Maher, & Finlay, 2018). GPP and ER consume and produce CO2, respectively, and provide estimates of aquatic C processing rates that can be compared to independent measures of CO2 In this way, estimating metabolism modelled from O2 data is a powerful tool to understand CO2 sources to streams (Hotchkiss et al, 2015), yet few studies have coupled high frequency measurements of O2 and CO2 with the goal of resolving these different pathways (but see GómezGener, von Schiller, et al, 2016; Stets et al, 2017). We modelled metabolic rates using the O2 data and estimated CO2 evasion simultaneously
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