Abstract
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.