Decoupling of carbon dioxide and dissolved organic carbon in boreal headwater streams

Abstract

Streams and rivers emit large quantities of carbon dioxide (CO2) to the atmosphere. The sources of this CO2 are in‐stream mineralization of organic carbon (OC) and CO2 input via groundwater inflow, but their relative importance is largely unknown. In this study, we quantified the role of in‐stream OC mineralization as a source of CO2 in a number of nested boreal headwater streams. The results showed that mineralization of stream OC contributed 3% of CO2 supersaturation at time scales comparable to the estimated water travel times in the streams (<24 h). Mass balances showed that downstream losses of OC were ≤3% in low‐order streams, whereas up to 16% of the OC was lost in the largest (fourth order) streams. In contrast, 85% of the CO2 was lost along the stream network (longest total stream length = 17 km). Under the assumption that in‐stream OC mineralization was the main source of stream CO2, higher rates of OC mineralization (6% of OC) than those reported across the literature (≤0.7% of OC) would be required to sustain observed CO2 supersaturation. Further, model results indicated that groundwater inflows were sufficient to sustain observed stream CO2 concentrations. We hence conclude that in‐stream OC mineralization was a minor source of CO2 in these boreal headwater systems and that the main source of stream CO2 was inflowing groundwater transporting CO2 originating from soil respiration.

Hydrological pulses and burning of dissolved organic carbon by stream respiration

“…Much of the net uptake and mineralization observed in this study is likely from labile organic matter (e.g., Fiebig and Lock ; Fischer et al ; Fellman et al ; Fasching et al , ; Drake et al ; Stegen et al ). The DOC at the outlet of the catchment has probably lost most of its rapid biodegradability potential and this may explain in part the extremely slow decomposition rate of recovered natural DOC by reverse osmosis in the Cairn Burn (Stutter et al ) or lack of observed reactivity (e.g., Kothawala et al ; Winterdahl et al ). Further studies combining stream metabolism and DOC quality are required to make stronger inferences (e.g., Fuß et al ; Hutchins et al ), notably at the groundwater (or soil water)—stream water interface coupled with microbial ecology (e.g., Fasching et al ; Fasching et al ; Stegen et al ).…”

Carbon dioxide and methane dynamics in a human-dominated lowland coastal river network (Shanghai, China)

“…The significant correlations between p CO 2 , water temperature, DO, and pH in the rural rivers (Table S8) suggest that mineralization of mobilized terrestrial OC might be an important source contributing to the CO 2 supersaturation in these agricultural rivers [ Butman et al , ]. More importantly, the less constrained hyporheic connectivity of the rural rivers (Table and Figure S2) might favor a more efficient lateral exchange between the river channels and the adjacent terrestrial ecosystems [ Öquist et al , ; Winterdahl et al , ]. Consequently, during high‐flow events, rising of near‐river water tables could intersect more emergent vegetation horizons and flush out more CO 2 ‐rich water previously stored in riparian soils and wetlands [ Johnson et al , ], leading to enhanced longitudinal pathway of CO 2 supply along the river network [ Abril et al , ; Gómez‐Gener et al , ].…”

No long‐term trends in pCO2 despite increasing organic carbon concentrations in boreal lakes, streams, and rivers

“…Streams typically have a strong hydrochemical connectivity with the catchment soils [Hope et al, 2004;Laudon et al, 2011]. Consequently, much of the CO 2 in streams comes from direct inputs of DIC, fixed and mineralized in the catchment and delivered via the groundwater [Leith et al, 2015;Öquist et al, 2009;Winterdahl et al, 2016]. A recent study demonstrated that in situ DOC mineralization was a minor source of CO 2 in small boreal headwater streams and that the main source of stream CO 2 was CO 2 -rich groundwater [Winterdahl et al, 2016].…”

“…Consequently, much of the CO 2 in streams comes from direct inputs of DIC, fixed and mineralized in the catchment and delivered via the groundwater [Leith et al, 2015;Öquist et al, 2009;Winterdahl et al, 2016]. A recent study demonstrated that in situ DOC mineralization was a minor source of CO 2 in small boreal headwater streams and that the main source of stream CO 2 was CO 2 -rich groundwater [Winterdahl et al, 2016]. Likewise, Weyhenmeyer et al [2015b] showed that direct inputs of DIC from the terrestrial surroundings of a lake have a stronger influence on CO 2 concentrations in lake water than do lake internal CO 2 production.…”