Distinct concentration-discharge dynamics: CO2 exhibits chemostasis, while CH4 exhibits dilution
Abstract
Inland waters, aside from exporting terrestrial carbon to the ocean, process and release carbon as carbon dioxide (CO2) and methane (CH4). Studies have shown that greenhouse gas fluxes from inland waters are significant to the global carbon cycle. However, regional and global estimates of fluxes from streams and rivers are still uncertain. These flux estimates are, in part, reliant on models of pCO2 and pCH4 oversaturation. Beyond broad-stroke spatial variability, variations in pCO2 and pCH4 are not included in upscaling efforts.
We sampled pCO2 and pCH4 over three years in the Connecticut River mainstem and at seven sites on streams and rivers in a sub-basin of the Connecticut River Watershed. Fluxes of CO2 and CH4 were calculated from the measured air-water concentration gradient and the gas transfer velocity, which was derived from water velocity and stream slope. Concentration-discharge relationships were used to explore CO2 and CH4 variability. Contingency tables of outliers were used to identify controls on extreme CO2 and CH4 fluxes. Overall, pCO2 has lower coefficients of variation and spans fewer orders of magnitude than pCH4 at all sites. In fact, CO2 ranges from undersaturated to eight times oversaturated, while CH4 ranges from undersaturated to more than 500 times oversaturated. Further, pCO2 remains relatively stable across flow conditions, whereas pCH4 decreases with increasing discharge. This chemostatic behavior of CO2 likely results from other controls (e.g., flow path activation, carbonate equilibria) acting in tandem with discharge. Moreover, extreme CO2 fluxes are associated with extreme flows, while extreme CH4 fluxes are associated with extreme pCH4 values. These findings elucidate controls on CO2 and CH4 in streams and rivers, and in particular, clarify how discharge governs concentrations and fluxes. For instance, discharge is a more important factor for upscaling pCH4 than for upscaling pCO2. This study also sheds light on storm dynamics; turbulent flows act on a relatively constant CO2 stock but on a diminishing CH4 stock. As more frequent and intense droughts and storms are predicted with climate change, discharge-driven variability should be included in CO2 and CH4 evasion estimates, and, in turn, more accurate carbon cycle models.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B32B..08A
- Keywords:
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- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 0458 Limnology;
- BIOGEOSCIENCES;
- 0495 Water/energy interactions;
- BIOGEOSCIENCES;
- 1818 Evapotranspiration;
- HYDROLOGY