Hydraulics are a first‐order control on CO₂ efflux from fluvial systems

Abstract

Evasion of carbon dioxide (CO 2 ) from fluvial systems is now recognized as a significant component of the global carbon cycle. However, the magnitude of, and controls on, this flux remains uncertain, and improved understanding of both is required to refine global estimates of fluvial CO 2 efflux. CO 2 efflux data show no pattern with latitude suggesting that catchment biological productivity is not a primary control and that an alternative explanation for intersite variability is required. It has been suggested that increased flow velocity and turbulence enhance CO 2 efflux, but this is not confirmed. Here using contemporaneous measurements of efflux (range: 0.07-107 μmol CO 2 m À2 s À1), flow hydraulics (mean velocity range: 0.03-1.39 m s À1 ), and pCO 2 (range: 174-10712 μatm) at six sites, we find that flow intensity is a primary control on efflux across two climatically different locations (where pH is not a limiting factor) and that the relationship is refined by incorporating the partial pressure of CO 2 (pCO 2 ) of the water. A remaining challenge is how to upscale from point to reach or river basin level. Remote imaging or river surface may be worth exploring if subjectivity in interpreting surface state can be overcome.

Biogeochemical contrast between different latitudes and the effect of human activity on spatio-temporal carbon cycle change in Asian river systems

“…25 Table. 3 shows the comparison of annual runoff and carbon flux in three types of river data in West Siberian Lowland (including Ob River basin) and Mekong River basin. The simulated results are generally in the range of previous data (Schlunz and Schneider, 2000;Coynel et al, 2005;Alin et al, 2011;Dai et al, 2012;Long et al, 2015), as evaluated in the author's previous studies (Fig. A2 in Appendix A) (Nakayama, 2017a(Nakayama, , 2017b.…”

“…(Nakayama, 2017a(Nakayama, , 2017b in order to estimate the carbon cycle both in vertical flux such as CO2 evasion to the atmosphere and sediment storage, and in horizontal transport such as TOC, DOC, POC, DIC-flux to the ocean in previous data of these rivers (Schlunz and Schneider, 2000;Coynel et al, 2005;Rasera et al, 2008;Dubois et al, 2010;Alin et al, 2011;Butman and Raymond, 2011;Dai et al, 2012;10 Striegl et al, 2012;Abril et al, 2015;Lauerwald et al, 2015;Long et al, 2015). There are no data about CO2 evasion but only pCO2 and average efflux data in Mekong River Long et al, 2015), the author estimated CO2 evasion by multiplying the average efflux (gC/m 2 /yr) and water area (km 2 ). It is important that the model captured a variety of carbon budgets in these six rivers of the globe as depicted by Tranvik et al (2009).…”

A review of CO2 and associated carbon dynamics in headwater streams: A global perspective

“…Stream p CO 2 also responds to discharge, which tends to increase the CO 2 gas transfer velocity ( via surface turbulence [ Demars and Manson , ; Long et al , ; Raymond et al , ; Wallin et al , ]. In addition to flow velocity, turbulence depends on stream hydraulics such as stream depth, width, and slope.…”

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

“…In streams, k is highly variable, can have local hot spots, and is dependent on slope, depth, flow velocity, etc. (Long et al, 2015;Raymond et al, 2012;Wallin et al, 2011).…”