Stream metabolism controls diel patterns and evasion of CO2 in Arctic streams

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.

Keywords: Arctic; CO2 evasion; carbon cycle; carbon processing; stream metabolism.

Figure 1

Map of the Miellajokka catchment with the coloration indicating changes in elevation. The black dots represent the location of the measuring sites in this study. The inset shows the location of the Miellajokka catchment within Scandinavia, and the dashed line represents the Arctic circle

Figure 2

The diel change in CO₂ concentration (ΔCO₂) for each site in relationship to K ₆₀₀. Each point represents the average of each site, while the bars denote the 0.05–0.95 quantiles for both ΔCO₂ and K ₆₀₀

Figure 3

Daily variations in pCO₂ (panels a, c and e) and the relationship between the diel change in CO₂ evasion and gross primary production (GPP; panels b, d and f), in the three streams with low K ₆₀₀ (Figure 2). ΔCO₂ is the daily change in pCO₂ from midnight, where each solid line represents 1 day, and the dashed lines denote the average for years 2015 and 2016. The solid lines in panels (b), (d) and (f) are the linear regressions between the diel change in CO₂ evasion and GPP

Figure 4

Daily variations in pCO₂ (panels a, c and e) and the relationship between the diel change in CO₂ evasion and gross primary production (GPP; panels b, d and f), in the three streams with high K ₆₀₀ (Figure 2). The ΔCO₂ is the daily change in pCO₂ from midnight, where each solid line represents 1 day, and the dashed lines denote the average for years 2015 and 2016. The solid lines in panels (b), (d) and (f) are the linear regressions between the diel change in CO₂ evasion and GPP

Figure 5

The coupling between O₂ and CO₂ in Arctic streams, and the contribution of net ecosystem production (NEP) to CO₂ evasion. (a) Departure from atmospheric equilibrium of CO₂ and O₂, where each point is an individual hourly observation and the ellipse for each site represents the 0.95 confidence level. (b) CO₂ evasion and NEP (with inverted sign) values for each day and all sites. Both parameters have the same units, and the dashed line is the 1:1 line where CO₂ evasion is equal to NEP. Therefore, points below the line have higher NEP than evasion. (c) Boxplots of the proportion of CO₂ evasion corresponding to stream NEP for each site, sorted from smallest (M17) to largest (M1) catchment area

Figure 6

Patterns of CO₂ concentrations and fluxes in a losing water stream. (a) Downstream change in discharge along the 2 km stream reach. The stream reach loses water as it passes through an alluvial deposit (see Figure S13 for a spatial version of this figure). We therefore expect that the contribution of terrestrially respired CO₂ is negligible as there are no groundwater inputs. (b) How the pCO₂ increases a twofold along this reach. By assuming that lateral inputs are negligible, we can do a mass balance to quantify the CO₂ produced within the stream. (c) Calculated inputs and export of CO₂ for five stream segments of the 2 km stream reach. The CO₂ was produced at a rate of 2.6 g C m⁻² day⁻¹ in this reach, and the net ecosystem production the same day measured at the site M1 (~800 m downstream) was 2.8 g C m⁻² day⁻¹