Stream respiration exceeds CO₂ evasion in a low-energy, oligotrophic tropical stream

Carbon dioxide (CO₂) can be either imported to streams through groundwater and subsurface inputs of soil-respired CO₂ or produced internally through stream metabolism. The contribution of each source to the CO₂ evasion flux from streams is not well quantified, especially in the tropics, an underrepresented region in carbon (C) cycling studies. We used high-frequency measurements of dissolved O₂ and CO₂ concentrations to estimate the potential contribution of stream metabolism to the CO₂ evasion flux in a tropical lowland headwater stream. We found that the stream was heterotrophic all year round, with net ecosystem productivity (NEP) values ranging from 0.84 to 4.06 g C m⁻² d⁻¹ (median 1.29 g C m⁻² d⁻¹; here we expressed gross primary productivity (GPP) as a negative flux and ecosystem respiration (ER) as a positive flux). Positive NEP values were the result of a relatively low and stable GPP through the seasons, compared to a higher and more variable ER favored by the high temperatures and organic matter availability, particularly during the wet season. The CO₂ evasion flux was relatively low due to low turbulence (median: 1.09 g C m⁻² d⁻¹). As a result, daily NEP rates exceeded the CO₂ evasion flux with a potential contribution of 129% (median; 120–175% interquartile range), despite the strong seasonal changes in flow regime and landscape connectivity. The CO₂ excess was likely transported downstream, where it was ultimately emitted to the atmosphere. Our results highlight the overwhelming importance of ER to the C cycle of low-energy, oligotrophic tropical streams.