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.

Seasonal Wetlands Make a Relatively Limited Contribution to the Dissolved Carbon Pool of a Lowland Headwater Tropical Stream

“…This value was relatively similar to that obtained using the respiration rates reported by Solano et al. (2023) for the same stream (0.19 g C s −1 during the wet season).…”

“…The highest p CO 2 values at the outlet of Manton Creek occurred during the dry season (Solano et al., 2023), which further suggests that stream CO 2 cannot be sourced mainly from the seasonal wetlands, dry and disconnected during that period. During our sampling campaign, the DIC input from the dolostone groundwater (4.48 g C s −1 ) was much higher than the sum of inputs from the wetland drains (1.24 g C s −1 ), indicating that groundwater inputs were the main source of DIC to the stream.…”

“…Some of the DOC that transits through the seasonal wetlands may be converted to CO 2 through internal respiration before reaching the stream. In‐stream respiration rates were found to be higher in Manton Creek during the wet season when DOC was readily available and water temperature was high (Solano et al., 2023). However, the higher DOC concentrations measured in the wetland drains compared to those measured in the stream (Figure 2d), coupled with short wetland residence times, support the notion that DOC mineralization may not have been an important process within the wetlands.…”

“…We compared this estimate to the mean wet‐season in‐stream respiration rate reported by Solano et al. (2023) for Manton Creek.…”

Amplified Production and Export of Dissolved Inorganic Carbon During Hot and Wet Subtropical Monsoon

“…These rates are usually within the range of 3.2-208.3 mol C/m 2 /yr at subtropical regions, much higher compared to the soil respiration estimated here. In addition, riverine CO 2 evasion and stream respiration are known to affect DIC concentrations and fractionation of δ 13 C (Duvert et al, 2018;Hotchkiss et al, 2015;Marx et al, 2017;Solano et al, 2023). About 11% of dissolved carbon was estimated to evade the stream in karst formations of southwest China (S.-L. Li et al, 2010).…”