Carbon dioxide evasion from headwater systems strongly contributes to the total export of carbon from a small boreal lake catchment

Abstract

Inland waters are hotspots for carbon (C) cycling and therefore important for landscape C budgets. Small streams and lakes are particularly important; however, quantifying C fluxes is difficult and has rarely been done for the entire aquatic continuum, composed of connected streams and lakes within the same catchment. We investigated carbon dioxide (CO 2 ) evasion and fluvial fluxes of dissolved inorganic carbon and dissolved organic carbon (DIC and DOC) in stream and lake systems within the 2.3 km 2 catchment of a small boreal lake. Our results show pronounced spatial and temporal variability in C fluxes even at a small spatial scale. C loss from the catchment through CO 2 evasion from headwaters for the total open water-sampling period was 9.7 g C m À2 catchment, dominating the total catchment C loss (including CO 2 evasion, DIC, and DOC export from the lake, which were 2.7, 0.2, and 5.2 g C m À2 catchment, respectively).Aquatic CO 2 evasion was dominated by headwater streams that occupy~0.1% of the catchment but contributed 65% to the total aquatic CO 2 evasion from the catchment. The importance of streams was mainly an effect of the higher gas transfer velocities than compared to lakes (median, 67 and 2.2 cm h À1 , respectively). Accurately estimating the contribution of C fluxes from headwater streams, particularly the temporal and spatial dynamics in their gas transfer velocity, is key to landscape-scale C budgets. This study demonstrates that CO 2 evasion from headwaters can be the major pathway of C loss from boreal catchments, even at a small spatial scale.

High terrestrial carbon load via groundwater to a boreal lake dominated by surface water inflow

“…Therefore, it is important to consider that 2013 was a comparatively dry and warm year, with 30% lower precipitation and 2°C higher temperature than the long‐term annual means of 1961–1990 (SMHI) and thus very low discharge. Terrestrial C load is consequently likely to be considerably higher in a wet year [ Kokic et al ., ] (see supporting information). It seems plausible to assume that the interannual variation in groundwater C load to boreal lakes is proportional to the interannual variation in stream C load, since also the streams receive terrestrial C load via groundwater inflow through riparian soils.…”

“…Samples for DOC analyses were filtered in the laboratory through precombusted glass fiber filter (25 mm Whatman GF/F) and analyzed with a Shimadzu TC analyzer within 48 h from sampling. Particulate organic carbon (POC) has been earlier found to be a quantatively insignificant contribution to the terrestrial C load to Lake Gäddtjärn [ Kokic et al ., ] and was therefore not measured in this study.…”

Freshwater methane and carbon dioxide fluxes : Spatio-temporal variability and an integrated assessment of lake and stream emissions in a catchment

“…On the other hand, streams could be more important than lakes with regard to the release of CO2, according to some studies (Kokic et al, 2015;Lundin et al, 2013), and the marked dominance of streams and rivers for global CO2 emissions has been estimated by Raymond et al (2013). There are also great differences with regard to the amount of turbulence generated in lakes and rivers/streams due to morphological variation.…”

Carbon dioxide degassing at the groundwater-stream-atmosphere interface: isotopic equilibration and hydrological mass balance in a sandy watershed

“…Hence, in larger rivers, internal processes become a more significant source of CO 2 degassing (Hotchkiss et al, 2015), but still based on terrestrial organic carbon losses (Cole and Caraco, 2001). Moreover, several studies on headwaters have been conducted in temperate (Butman and Raymond, 2011;Polsenaere and Abril 2012), boreal (Wallin et al, 2013;Kokic et al, 2015) and tropical (Johnson et al, 2008;Davidson et al, 2010) ecosystems at different spatial scales. These works came to the same conclusion that headwaters are hotspots of CO 2 degassing, i.e., as regions that exhibit disproportionately high reaction rates, relative to the surrounding area (Vidon et al, 2010).…”