Hydrologic controls on pCO₂ and CO₂ efflux in US streams and rivers

Abstract

Streams and rivers emit petagrams of CO2, yet there is little known about how discharge variability impacts stream CO2 at broad scales. This article analyzed changes of pCO2 and CO2 efflux with discharge in 813 conterminous US streams and rivers. Half of the streams and rivers (49%) showed positive pCO2–discharge responses, while significantly more (80%) showed positive CO2 efflux–discharge responses. More CO2 efflux therefore occurred at higher discharge. While CO2 efflux tended to increase with discharge at all stream orders, pCO2 tended to decrease with discharge in small rivers (stream order 1–6) and increase in large rivers (stream order 7–10). Small streams and rivers tended to release more CO2 at higher discharge due to highly responsive gas transfer velocity. While high CO2 efflux in small streams (stream order 1–3) pointed to strong terrestrial input, high CO2 responsiveness in large rivers was likely due to increased riverine metabolism and river–corridor exchanges.

Dynamics and potential drivers of CO2 concentration and evasion across temporal scales in high-alpine streams

“…Overall our b C values are lower than those reported for 1st-to 6th-order streams (b C ranging from −0.05 to 0) and larger rivers (b C ranging from 0.02 to 0.24) throughout the USA (Liu and Raymond 2018). Low b C values are indicative of dilution as a response to increasing q, which in mountain streams is certainly Figure 2.…”

“…As shown above, q drives solute dilution and concentration behavior in streams and at the same time it influences the gas exchange velocity through the water surface (Raymond et al 2012. A recent survey by Liu and Raymond (2018) shows that roughly 50% of the streams and rivers throughout the USA had positive responses of CO 2 concentration to q, which would suggest increased CO 2 deliveries from the catchment outbalancing F CO2 from these systems. This is in line with observations that hydrological connectivity, as encapsulated by changes in q and its relationship with groundwater, can affect the transportation of CO 2 from various sources (e.g.…”

“…We collapsed the 10 min interval data from the time series to daily median values of CO 2 concentration, q, gas transfer velocity of CO 2 (k CO2 ) and F CO2 . We followed the same approach as Liu and Raymond (2018) to identify the responses of CO 2 concentration, k CO2 , and F CO2 to q, respectively. We used power law functions with transformed data (log (x+50)) to analyze whether the responses were chemostatic or chemodynamic, as well as the magnitude of the response (Godsey et al 2009, Meybeck andMoatar 2012).…”

“…Godsey et al 2009, Meybeck andMoatar 2012). Numerous studies have focused on the behavior of conservative solutes, including dissolved ions, but few have adopted the C-q scaling approach to gases, such as CO 2 (Liu and Raymond 2018), to understand their temporal and spatial dynamics. Unlike non-gaseous solutes that are transported downstream through advective flow, CO 2 may not only be converted to other carbonate species such as HCO 3 − , but may also be outgassed vertically from the streamwater into the atmosphere (figure 1).…”

“…On the other hand, high responsiveness of both CO 2 concentration and F CO2 to q also enhances F CO2 but because of elevated CO 2 concentrations in the streamwater. Our conceptual framework serves as guidance to understand the balance between CO 2 supply to the stream and its outgassing from the stream, which ultimately affects the role of streams for large-scale CO 2 fluxes (Liu andRaymond 2018, Rocher-Ros et al 2019).…”