Stream respiration exceeds <scp>CO<sub>2</sub></scp> evasion in a low‐energy, oligotrophic tropical stream
Vanessa Solano, Clément Duvert, Christian Birkel, Damien T. Maher, Erica A. García, Lindsay B. Hutley
Abstract
Abstract Carbon dioxide (CO 2 ) can be either imported to streams through groundwater and subsurface inputs of soil‐respired CO 2 or produced internally through stream metabolism. The contribution of each source to the CO 2 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 CO 2 concentrations to estimate the potential contribution of stream metabolism to the CO 2 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 CO 2 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 CO 2 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 CO 2 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.