High‐Resolution Spatial Sampling Identifies Groundwater as Driver of CO2 Dynamics in an Alpine Stream Network. Issue 7 (10th July 2019)

• Record Type: Journal Article
• Authors: Horgby, Åsa; Boix Canadell, Marta; Ulseth, Amber J.; Vennemann, Torsten W.; Battin, Tom J.
• Is Part Of: Journal of geophysical research. Volume 124:Issue 7(2019)
• Page Start: 1961
• Page End: 1976
• Publication Date: 2019-07-10
• Subjects: CO2 sources -- headwater stream -- stable isotopes -- hydrological connectivity -- control point -- CO2 evasion; Geobiology -- Periodicals; Biogeochemistry -- Periodicals; Biotic communities -- Periodicals; Geophysics -- Periodicals; 577.14
• DOI: 10.1029/2019JG005047
• Languages: English
• ISSNs: 2169-8953

Abstract

Abstract: Inland waters are major sources of CO2 to the atmosphere. The origin of this CO2 is often elusive, especially in high‐altitude streams that remain poorly studied at present. Here we study the spatial and seasonal variations in streamwater CO2, its potential sources and drivers in an Alpine stream network (Switzerland). High‐resolution sampling combined with stable isotope analysis and mixing models enabled us to capture the fine‐scale spatial heterogeneity in streamwater p CO2 as the stream network expanded and contracted during seasons. We identified soil respiration as a major source of CO2 to the stream. We also identified a major groundwater upwelling zone as an ecosystem "control point" that disproportionately influenced stream biogeochemistry. This was particularly pronounced when the stream network expanded during snowmelt, when it covered a five times larger area compared to winter (35, 300 m 2 compared to 7, 100 m 2 ). Downstream from this control point, CO2 evaded rapidly owing to high gas transfer velocity. The stream network was a net source of CO2 to the atmosphere with an average areal evasion flux of 30.1 (18.0–43.1) μmol · m ‐2 · s ‐1 and a total flux at network scale ranging from 237 (141–339) kg C/day in winter to 1793 (1069–2565) kg C/day during spring snowmelt. Our study highlights the role of stream network dynamics and control points for the CO2 dynamics in high‐altitude streams. Plain Language Summary: Streams are significant sources of CO2 to the atmosphere. However, the origin of this CO2 and its spatial and temporal dynamics are often not clear. Here we study the small‐scale spatial variation of streamwater CO2 and its evasion from an Alpine stream network in Switzerland. We found that groundwater upwelling, as constrained by local geomorphology, delivered large quantities of respiratory CO2 from adjacent soils to the stream. The relevance of these groundwater deliveries changes seasonally as the stream network expanded and contracted. Our spatially resolved sampling design allowed us to identify hot spots of CO2 evasion, based on which we were able to assess CO2 evasion at the scale of the entire stream network. Our findings emphasize the relevance of such approaches to properly constrain CO2 evasion fluxes beyond the reach scale. Key Points: We identified a geomorphological "control point" to deliver respiratory CO2 from soils to an Alpine stream via groundwater We show how streamwater CO2 dynamics and evasion varies with stream network expansion and contraction CO2 evasion from mountain streams might be underestimated due to high spatiotemporal variability in p CO2