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Sediment greenhouse gases (methane and carbon dioxide) in the Lobo‐Broa Reservoir, São Paulo State, Brazil: Concentrations and diffuse emission fluxes for carbon budget considerations

2005; Wiley; Volume: 10; Issue: 4 Linguagem: Inglês

10.1111/j.1440-1770.2005.00277.x

1440-1770

Donato Seiji Abe, Donald D. Adams, Corina V. Sidagis Galli, Elizabeth Sikar, José Galízia Tundisi,

Groundwater flow and contamination studies

Abstract Carbon gases (methane, CH 4 , and carbon dioxide, CO 2 ) were measured for the first time in sediments of the Lobo‐Broa Reservoir, near São Carlos in São Paulo State, Brazil. It is believed these are the first measurements of this kind in any of the many reservoirs located in Brazil. Even though the Lobo‐Broa Reservoir is classified as oligotrophic, the sediment gas concentrations were exceedingly high, ranging from 0.4–3 mmol L −1 for CH 4 and 1–9 mmol L −1 for CO 2 . Both gases exceeded their in situ gas saturation values at these shallow water depths (7 m in central basin; 11 m at dam), resulting in numerous sediment bubbles. Organic matter was highly concentrated in the reservoir sediments, averaging 25.5% loss on ignition (LOI) (dam) to 26.9% LOI (central basin) for the 0–12 cm depth interval, with values as high as 29–30% LOI (12% organic carbon) in the surface 0–5 mm layer. The theoretical flux of dissolved pore water carbon gases to the sediment–water interface (SWI) averaged 3.4 mmol L −1 m −2 day −1 CH 4 and 7.3 mmol L −1 m −2 day −1 CO 2 for the surface 0–10 mm. From gas emission measurements at the water surface, it was calculated that 90% of CH 4 is consumed either at the SWI or in the water column, resulting in a loss of 0.31 mmol L −1 m −2 day −1 of CH 4 to the atmosphere. However, only 20% of the total CO 2 gas transported across the water–atmosphere interface (36.3 mmol L −1 m −2 day −1 , or 1600 mg CO 2 m −2 day −1 ) was produced in the sediments. The remaining 80% of CO 2 probably comes from other carbon sources. With CH 4 oxidation in the aerobic water column, close to 30% of the carbon gas flux to the atmosphere could be accounted for by gas production of CO 2 and CH 4 in the sediments and their diffuse transport to the water column.