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The inundation status of the Amazon floodplain affects biogenic gas production andevasion. We analyzed spatial variability of dissolved CO2 concentration and gas evasion in alarge floodplain lake in the lower reach of the Amazon River in four hydrological phases. Wecalculated surficial CO2 concentrations from measurements of pH, dissolved inorganiccarbon, temperature, and conductivity and used meteorological data to calculate gas transfercoefficients to estimate CO2 evasion. Gas transfer coefficients that take into accountboth wind and heating and cooling at the lakefs surface are on the order of 10 cm hr.1,approximately four times higher than values previously used in regional estimates ofgas evasion from lakes on the Amazon floodplain. Supersaturation of CO2 occurredthroughout the lake and was higher in the littoral zone and in regions receiving AmazonRiver inflows. CO2 concentration was reduced in regions with phytoplankton blooms.The range of CO2 concentrations was least at low water, 47 mM to 233 mM, and largestat high water, 1 mM to 656 mM; the average annual value was 125 mM. We estimatemean (±standard deviation) fluxes from open]water in L. Curuai to the atmosphere of44 ± 15, 348 ± 13, 371 ± 23, and 364 ± 20 mmol CO2 m.2 d.1 during receding, low,rising, and high water, respectively. The error associated with these values reflects, foreach hydrological phase, the spatial variation in CO2 concentration in L. Curuai, a likelyrange in atmospheric CO2 levels and temporal variations in gas transfer coefficientwithin 10]day periods.

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