Differential Spatial Patterns in Aquatic and Terrestrial Carbon Fluxes in a Headwater Tropical Catchment

The global carbon cycle increasingly recognizes the importance of inland waters, and in particular, headwater streams. They transport and transform carbon, which has been produced on land and will ultimately be delivered to the ocean, stored in the ground, or released to the atmosphere. In this study, we characterized carbon 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 morphological data on stream width, depth, slope, and water velocity to calculate gas transfer velocity (k₆₀₀) and aquatic CO₂ evasion and assess spatial trends. Our analysis revealed pronounced longitudinal gradients and spatial heterogeneity in aquatic carbon concentrations and fluxes, which contrasted drastically with the lack of such spatial heterogeneity in terrestrial carbon fluxes. The stream was highly enriched in dissolved CO₂ where the groundwater surfaced, but rapidly off gassed 99.5% of the pCO₂ in the first kilometer, indicating a geologic control. The calculated aquatic flux values exhibit a similar pattern, although they are also influenced by variable k₆₀₀ and other geomorphological drivers. My findings reveal that aquatic and terrestrial CO₂ fluxes to the atmosphere are decoupled longitudinally and provide insight into large-scale carbon dynamics across tropical catchments in complex terrain.