River Corridor Sources Dominate CO2 Emissions From a Lowland River Network

Rivers and streams are control points for CO2 emission to the air (fCO(2)), 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 fCO(2). To test this hypothesis, we measured fCO(2), 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 fCO(2), 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 fCO(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 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 fCO(2) 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. Plain Language Summary Globally, rivers and streams emit a similar amount of carbon dioxide (CO2) to the atmosphere as the land retains. To help curb CO2 emissions that cause climate change, we need to understand what is generating these high rates of stream CO2 emissions. While biological processes in the stream can generate CO2, the bulk of stream CO2 is thought to come from upland soils, carried by run-off from the land to the streams where it off-gases. In this study of seven streams of varying sizes in the same river network, we measured CO2 fluxes from both the stream and land to better understand where the CO2 emissions come from. We observed that a large fraction-sometimes all-of the CO2 delivered to the stream channel from groundwater comes from the wetlands next to the stream. Indeed, about half of CO2 emissions came from stream organisms consuming organic carbon, and one-third from the thin band of riparian wetlands bordering the stream. Thus, we conclude that the narrow river corridor (the stream plus adjacent wetlands) is the source of 87% of the CO2 emitted by streams, and that the uplands that comprise most of the land area are a much smaller source than expected.