Spatial Decoupling of Aquatic and Terrestrial Carbon Fluxes in a Tropical Andean Headwater Corridor

Abstract

The global carbon (C) cycle increasingly recognizes the importance of inland waters, and in particular, headwater streams. These streams transport and transform C that has been produced on land and will ultimately be delivered to the ocean, stored in the ground or released to the atmosphere. Studies that quantify C fluxes in inland waters traditionally focus on one type of aquatic system (i.e., wetland, stream, lake), leaving a gap in our understanding of how hydrologic processes affect whole ecosystem C fluxes across the continuum from terrestrial to aquatic systems. In this study, we characterized C flux dynamics across ~4 km of a first‐order, spring‐fed stream within a stratovolcano catchment of the Andes Mountains. We collected a suite of environmental variables, including dissolved carbon dioxide (pCO2), dissolved methane (pCH4), specific conductance, and temperature, across 38 points throughout the catchment. Additionally, we collected soil CO2 flux in areas adjacent to the stream as well as morphological data on stream channel including width, depth, slope, and water velocity to calculate gas transfer velocity (k600) and aquatic CO2 evasion. Our analysis revealed pronounced longitudinal gradients and spatial heterogeneity in aquatic C concentrations and fluxes, which contrasted drastically with the lack of such spatial heterogeneity in terrestrial C fluxes. The stream was highly enriched in dissolved CO2 where the spring emerged, but rapidly off gassed 99.5% of the pCO2 in the first kilometre of the stream channel. The calculated aquatic evasion exhibited a similar decreasing pattern, although it was also influenced by k600 and other variables. Our findings reveal that spatial patterns of aquatic and terrestrial CO2 fluxes to the atmosphere are decoupled longitudinally and provide insight into ecosystem‐scale dynamics of C across tropical catchments in complex terrain.