Carbon Evasion Dynamics in a Tropical, High-Elevation, Peatland Catchment are Mediated by A Threshold in Watershed Area

Inland waters emit large amounts of carbon and are key players in the global carbon budget. Particularly high rates of carbon emissions have been reported in streams draining mountains, tropical regions, and peatlands. However, few studies have examined the spatial variability of CO2 concentrations and fluxes occurring within these systems, particularly as a function of watershed morphology. Here we evaluated spatial patterns of CO2 in three tropical headwater catchments in relation to the river network and upland landscape characteristics. We measured dissolved carbon dioxide (pCO2), aquatic CO2 evasion, and stream surface area at high spatial resolutions along multiple stream reaches. Confirming previous studies, we found that tropical headwater streams are an important source of CO2 to the atmosphere. More notably, we found marked, predictable spatial organization in aquatic carbon fluxes as a function of landscape position. For example, pCO2 was consistently high (>10,000 ppm) at locations close to groundwater sources and just downstream of hydrologically connected wetlands, but consistently low (<1,000 ppm) in locations of high turbulence or with large drainage areas. Our results suggest that the primary driver of CO2 evasion is stream-size dependent. For these tropical páramo catchments, a drainage area threshold of around 12.5-20 ha determines the partitioning between supply- and transport-limited CO2 evasion from streams. Further, we observed the highest rates of evasion in both the smallest and largest stream reaches. We suggest that accurate estimation of CO2 emissions requires understanding of dynamics across the entire stream network, from the smallest seeps to larger streams.