Methane fluxes were determined using the static chamber technique, described by Devol et al. (1988, 1990) and Bartlett et al. (1988). The chambers were covered with a thermal and reflective sheet to avoid temperature variations, had an area of 0.066 m2 and a volume of 261. Inside the chamber, a small fan was installed to avoid any air stratification and was turned on at least 30 s before the sampling. The chambers were placed in the sampling site using a boat, with care to avoid perturbations in the water surface and surrounding vegetation. All samples were taken between 11:00 and 16:00 LT (local time), and they were done under conditions of almost no wind. Every 6 min, during 18 min, gas samples were removed through a septum with a 60 ml polyethylene syringe equipped with a 3-way polypropylene stopcock. To verify the linearity and possible perturbation in the chamber deployment, a sample was taken in the first minute after the chamber was placed. Six to eight locations were sampled for each site. Environmental variables that may affect the methane emissions were also measured: water depth, water and air temperatures, and pH.
The methane concentration of all samples was determined with a commercial gas chromatograph (Shimadzu, GC-14A), equipped with a flame ionization detector (FID), a 2.2 ml sample loop and two stainless steel columns that were optimized to perform methane analysis in the Ozone Laboratory at INPE, Sao Jose dos Campos, Brazil. The first column was packed with silica gel (2.5-m long and 1/8" diameter) and it was used to remove the water vapor, CO2 and others heavy organic compounds from the samples, in order to reduce the total retention time. The analysis column (3.0-m long and 1/8" diameter) was packed with a zeolite 5 A molecular sieve. The methane standard (1,749.4 ± 4.5ppbv) used for calibration was acquired from the Climate Monitoring and Diagnostic Laboratory of the National Oceanic and Atmospheric Administration (CMDL/NOAA).
For each syringe, three aliquots were analyzed with a relative precision of 0.7% or better. The minimum detectable methane flux was about 1mg CH4 m-2 d-1. The methane flux was determined from the temporal variation of its mixing ratio inside the chamber during the sampling time, following the method of Schiller and Hastie (1994). The methane fluxes were considered diffusive, if the linear correlation between the mixing ratio change and the elapsed time showed a correlation coefficient (r2) greater than 0.90 (Sass et al. 1992). A second criterion was that the initial concentration obtained by the linear regression (at time t=0) must be close to the measured environmental air concentration. If the flux did not follow the first criterion, and if an abrupt increase of the methane concentration did occur after the first sampling, the flux variation was interpreted as an ebullition. The rate of change of the methane mixing ratio was then determined by subtracting the environmental methane mixing ratio from the mixing ratios obtained at the end of the sampling time, divided by the enclosure time, as proposed by Cicerone et al. (1992) and Keller and Stallard (1994).
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