River Corridor Sources Dominate CO₂ Emissions From a Lowland River Network

Abstract

Rivers and streams are control points for CO2 emission to the air (fCO2), with emission rates often exceeding internal metabolism (net ecosystem production, NEP). The difference is usually attributed to CO2‐supersaturated groundwater inputs from upland soil respiration and rock weathering, but this implies a terrestrial‐to‐aquatic C transfer greater than estimated by terrestrial mass balance. One explanation is that riparian zones—rich in organic and inorganic C but mostly neglected in terrestrial mass balances—contribute disproportionately to fCO2. To test this hypothesis, we measured fCO2, NEP, and the lateral CO2 contributions from both terrestrial uplands (TER) and riparian wetlands (RIP) for seven reaches in a lowland river network in Florida, USA. NEP contributed about half of fCO2, but the remaining CO2 emission was generally much larger than measured TER. The relative importance of RIP versus TER varied markedly between contrasting hydrogeologic settings: RIP contributed 49% of fCO2 where geologic confinement forced lateral drainage through riparian soils, but only 12% where unconfined karst allowed deeper groundwater flowpaths that bypassed riparian zones. On a land area basis, the narrow riparian corridor yielded far more CO2 than the terrestrial uplands (33.1 vs. 1.4 g‐C m−2 yr−1), resulting in river corridors (i.e., stream channel plus adjacent wetlands, NEP + RIP) sourcing 87% of fCO2 to streams. Our findings imply that true terrestrial CO2 subsidies to streams may be smaller than previously estimated by aquatic mass balance and highlight the importance of explicitly integrating riparian zones into the conceptual model for terrestrial‐to‐aquatic C transfer.

Lotic‐SIPCO2: Adaptation of an open‐source CO2 sensor system and examination of associated emission uncertainties across a range of stream sizes and land uses

“…The relationships between land cover and median CO 2 concentration and emission rates also follow previous studies (Borges et al 2018), where the importance of certain sources of CO 2 to streams and the production of CO 2 within the stream are influenced by land cover (Marescaux et al 2018). For example, wetlands can provide a large supply of CO 2 to lotic ecosystems (Abril et al 2014; Kirk and Cohen 2023), where CO 2 produced in wetlands is transported to streams and emitted there. Similarly, urbanization has been shown to increase riverine CO 2 emissions (Gu et al 2021; Zhang et al 2021), where increased inputs of nutrients and organic matter fuel in‐stream production of CO 2 .…”

Seasonal Wetlands Make a Relatively Limited Contribution to the Dissolved Carbon Pool of a Lowland Headwater Tropical Stream

“…It is reasonable to expect that these findings from northern regions would also apply to tropical streams. Recently Kirk and Cohen (2023) found that despite their modest spatial extent, riparian corridors supplied up to 49% of the CO 2 outgassing from a subtropical stream in Florida. While our study did not provide a conclusive answer on the role of the riparian corridor in fueling the stream C pool, indirect evidence suggests that it acted as a major source of C to Manton Creek.…”

Impacts of Carbonate Buffering on Atmospheric Equilibration of CO2, δ13CDIC, and Δ14CDIC in Rivers and Streams

“…Despite their importance in the terrestrial carbon cycle, the relative balance of CO 2 sources that support carbon fluxes to the atmosphere, which include groundwater sources such as soil respiration and subsurface chemical weathering reactions, or stream corridor sources including respiration of organic matter within the hyporheic zone and water‐column as balanced by photosynthesis, remains uncertain. More recently, studies have shown the potential role of carbonate buffering reactions to contribute to evasing fluxes and spatial patterns of stream CO 2 partial pressure ( p CO 2 ) (Duvert et al., 2019; Kirk & Cohen, 2023; Stets et al., 2017; Wang et al., 2021). Crucially, these buffering dynamics are not typically included in freshwater system carbon budgets and may represent an overlooked aspect of terrestrial‐aquatic carbon cycling.…”

“…Further, Stets et al (2017) suggest that this effect is responsible for the observations of high downstream pCO 2 in high alkalinity streams despite significant degassing during downstream transport. These buffering reactions may also contribute significantly to total stream emissions (e.g., Kirk & Cohen, 2023); for example, Duvert et al (2019) found that up to 60% of emissions were supported by carbonate buffering at some locations within a high alkalinity, tropical Australian river system.…”