Patterns and controls of carbon dioxide concentration and fluxes at the air–water interface in South American lowland streams

Abstract

Carbon dioxide (CO2) emission from fluvial systems represents a substantial flux in the global carbon cycle. However, variation in fluvial CO2 fluxes at the air–water interface as well as its drivers are poorly understood, especially in non-forested headwaters. Here, we measured CO2 concentration and fluxes in 14 lowland open-canopy streams (Pampean region, Argentina) that cover a wide range of land use and water quality. We also analyzed drivers of CO2 concentration and fluxes, including factors related to metabolism, gas solubility, alkalinity, and gas transfer. Metabolic rates varied considerably among the study sites, but most streams (i.e., 8 out of the 11 where we were able to estimate ecosystem metabolism) were net heterotrophic. Metabolic differences among sites were mostly driven by the aromatic carbon content and the percent of the stream reach covered by primary producers. All streams, even those that were not net heterotrophic were CO2 supersaturated. Alkalinity combined with in-stream primary production explained 52.3% of the variance in the partial pressure of CO2 (pCO2), but our observations suggest that pCO2 might be controlled by external groundwater inputs of dissolved inorganic carbon rather than by internal metabolism. All streams were net emitters of CO2 to the atmosphere. Significantly more variance in CO2 flux was explained by gas transfer velocity (63.7%) than by pCO2 (21.9%). We also observed high spatial heterogeneity in CO2 fluxes within each stream, which was associated with flow variation and the presence of different macrophyte and algae patches. Overall, our results indicate that CO2 emission in these extremely low turbulence streams is controlled by a combination of external and internal biogeochemical processes and limited atmospheric exchange.