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 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.