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

Abstract

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 O 2 and CO2 concentrations to estimatethe potential contribution of stream metabolism to the CO2 evasion flux in a tropical lowland headwaterstream. 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 pro-ductivity (GPP) as a negative flux and ecosystem respiration (ER) as a positive flux). Positive NEP values were theresult of a relatively low and stable GPP through the seasons, compared to a higher and more variable ERfavored by the high temperatures and organic matter availability, particularly during the wet season. The CO2evasion flux was relatively low due to low turbulence (median: 1.09 g C m-2 d-1). As a result, daily NEP ratesexceeded 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 trans-ported downstream, where it was ultimately emitted to the atmosphere. Our results highlight the overwhelmingimportance of ER to the C cycle of low-energy, oligotrophic tropical streams.