Discharges of old groundwater can greatly increase carbon dioxide emissions from streams and other surface waters.
Streams and rivers are increasingly recognized as an important component in the carbon cycle, from local to global scales. A recent study measured carbon dioxide (CO2) and methane (CH4) emissions from tropical rainforest streams in both the wet and dry seasons. Measurements were made in a stream receiving inputs of very old (about 3,000 years) groundwater and in other streams without such inputs.
Measuring elevated stream CO2 degassing rates might suggest that an ecosystem has elevated respiration and is a net source (rather than sink) with respect to atmospheric CO2. In ecosystems with inputs of old, high-carbon groundwater, however, knowing that elevated stream CO2 degassing is supported and driven by a large input of nonbiogenic CO2 from old groundwater helps to avoid an overestimation of ecosystem respiration and provides a more accurate picture of the ecosystem’s carbon source and sink status.
CO2 and CH4 degassing was measured in two rainforest streams at La Selva, Costa Rica: one stream fed only by young (<10 years old) local groundwater recharged within the watershed, and another fed by about two-thirds young groundwater and one-third older groundwater (about 3,000 years old) from a large regional aquifer system. Regional groundwater inputs had no measurable effect on stream gas exchange velocity, stream water CH4 concentration, or stream CH4 emissions, but it significantly increased stream water CO2 concentration and degassing. CO2 emissions from the stream receiving regional groundwater averaged 5.5 moles of carbon per m2 of stream surface per day, about 7.5 times higher than the average from the stream with no regional groundwater input. Carbon emissions from both streams were dominated by CO2, with CH4 accounting for only 0.06 percent to 1.70 percent of the total (average CH4 degassing rate from both streams was 0.005 moles of carbon per m2 of stream surface per day). Annual stream degassing fluxes normalized by watershed area were 299 and 48 moles of carbon per m2 of watershed surface in the watersheds with and without inputs of old regional groundwater, respectively. Stream degassing of CO2 is a major carbon flux in the watershed receiving inputs of old regional groundwater, and is similar in magnitude to the average net ecosystem exchange estimated by eddy covariance. Examining the effects of watershed connections to underlying hydrogeological systems can help avoid overestimation of ecosystem respiration and advance understanding of the carbon source and sink status and overall carbon budgets of terrestrial ecosystems.
Daniel Stover, SC-23.1, email@example.com, 301-903-0289; and Jared DeForest, SC-23.1, firstname.lastname@example.org, 301-903-1678
North Carolina State University
This work was funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, Terrestrial Ecosystem Science program (award DE-SC0006703).
Oviedo-Vargas, D., D. P. Genereux, D. Dierick, and S. F. Oberbauer. 2015. “The Effect of Regional Groundwater on Carbon Dioxide and Methane Emissions from a Lowland Rainforest Stream in Costa Rica,” Journal of Geophysical Research Biogeosciences 120(12), 2579–95. DOI: 10.1002/2015JG003009. (Reference link)
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