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 ( p CO 2 ), dissolved methane ( p CH 4 ), specific conductance, and temperature, across 38 points throughout the catchment. Additionally, we collected soil CO 2 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 ( k 600 ) and aquatic CO 2 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 CO 2 where the spring emerged, but rapidly off gassed 99.5% of the p CO 2 in the first kilometre of the stream channel. The calculated aquatic evasion exhibited a similar decreasing pattern, although it was also influenced by k 600 and other variables. Our findings reveal that spatial patterns of aquatic and terrestrial CO 2 fluxes to the atmosphere are decoupled longitudinally and provide insight into ecosystem‐scale dynamics of C across tropical catchments in complex terrain.