The role of streams and rivers in the global carbon (C) cycle remains unconstrained, especially in headwater streams where CO₂ 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 O₂ and CO₂ 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 CO₂ inputs (GW_CO2) from riparian well CO₂ measurements. Paired O₂–CO₂ data reveal stream CO₂ supersaturation driven by groundwater CO₂ inputs and large in-stream production of CO₂. At short time scales, CO₂ 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.

溪流和河流在全球碳 (C) 循环中的作用仍然不受限制，特别是在向大气排放 CO ₂ (F _CO2 ) 高的源头溪流中。与温带溪流相比，热带地区的溪流 C 循环研究不足，热带溪流可能具有最高的 F _CO2，这是由于温度较高、持续的有机物质输入以及溪流和周围土壤中的高呼吸速率。在本文中，我们展示了来自哥斯达黎加低地雨林源头流的成对的流内 O ₂和 CO _{2传感器数据，以探索气体浓度和生态系统过程的时间变化。}此外，我们估计地下水 CO ₂输入（GW_CO2 ) 来自河岸井 CO ₂测量值。配对的 O ₂ -CO ₂数据揭示了由地下水 CO _{2输入和大量 CO 2}流内生产驱动的河流 CO ₂过饱和。在短时间尺度上，CO ₂在风暴事件期间被稀释，但在较长的季节尺度上增加。我们研究范围内的面积通量表明，与 GW _{CO2相比，F CO2}受到更大的流内代谢支持。我们的研究结果强调了热带源头河流作为大气中二氧化碳的主要贡献者的重要性，并表明逃逸的 C 可以来自河流和陆地来源。