River Corridor Sources Dominate CO<sub>2</sub> Emissions From a Lowland River Network
Lily Kirk, Matthew J. Cohen
Abstract
Abstract Rivers and streams are control points for CO 2 emission to the air ( f CO 2 ), with emission rates often exceeding internal metabolism (net ecosystem production, NEP). The difference is usually attributed to CO 2 ‐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 f CO 2 . To test this hypothesis, we measured f CO 2 , NEP, and the lateral CO 2 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 f CO 2 , but the remaining CO 2 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 f CO 2 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 CO 2 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 f CO 2 to streams. Our findings imply that true terrestrial CO 2 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.