Partitioning inorganic carbon fluxes from paired O 2 –CO 2 gas measurements in a Neotropical headwater stream, Costa Rica

The role of streams and rivers in the global carbon (C) cycle remains unconstrained, especially in headwater streams where CO 2 evasion (F CO2 ) to the atmosphere is high. Stream C cycling is understudied in the tropics compared to temperate streams, and tropical streams may have among the highest F CO2 due to higher temperatures, continuous organic matter inputs, and high respiration rates both in-stream and in surrounding soils. In this paper, we present paired in-stream O 2 and CO 2 sensor data from a headwater stream in a lowland rainforest in Costa Rica to explore temporal variability in gas concentrations and ecosystem processes. Further, we estimate groundwater CO 2 inputs (GW CO2 ) from riparian well CO 2 measurements. Paired O 2 –CO 2 data reveal stream CO 2 supersaturation driven by groundwater CO 2 inputs and large in-stream production of CO 2 . At short time scales, CO 2 was diluted during storm events, but increased at longer seasonal scales. Areal fluxes in our study reach show that F CO2 is supported by greater in-stream metabolism compared to GW CO2 . Our results underscore the importance of tropical headwater streams as large contributors of carbon dioxide to the atmosphere and show evaded C can be derived from both in-stream and terrestrial sources.