The Pantanal is a hydrological complex plain constituting a large sedimentary basin, which is periodically flooded by the Paraguay River and its tributaries. Its altitude varies between 80 and 120m, with a total area estimated 138,183km2, located mostly within Brazil, but with small areas in Paraguay and Bolivia. The Paraguay River and its tributaries carry continuously organic material and during the flood period, the water overflow spreads the sediments over the entire region. This organic material that is deposited in the lakes and in the floodplain is the most important source of nutrients for the methanogenic bacteria. The total area under flood
conditions, including the floodplain, lakes, rivers, and channels that link lakes and rivers, was estimated by Hamilton et al. (2002) as 130,920 km2, thus constituting the largest floodplain in South America. Using a 100-year data set of the Paraguay River level (from 1900 to 1999), Hamilton et al. (2002) also estimated an average flooded area of 34,880 km2. The flooding of the region occurs after December, during the Southern Hemisphere summer, when the rain water that is falling on the riverhead of the basin since October reaches the region. The maximum flooded area occurs at the end of March. After April, up to the end of September, there are only a few rain episodes and the river level thus decreases to its lowest depth (Alvala and Kirchhoff 2000). Normally, the period in which half of the maximum-flooded area stays flooded is of 172 days per year, and there is a delay of months between the summer rains and the flooding period, due to the slow passage of flood waters through the Pantanal plain.
The flux measurements were performed over water environments, consisting of open water and emergent macrophytes, at five sites near the Miranda River as shown in Fig. 7.1. Among these sites, three are considered floodplains: Sao Joao (19°24'S, 57°03'W), Bati (19°19'S, 57°03'W) and Arara Azul (19°19'S, 57°03'W); and two lakes: Mirante (19°23'S, 57°03'W) and Medalha (19°34'S, 57°00'W) where the water body is permanent, with less influence of the seasonal flooding. During the experiments, the Base de Estudos do Pantanal - BEP (19°34'S, 57°01'W), which belongs to the Universidade Federal do Mato Grosso do Sul, was used as the local logistic support. Hamilton et al. (2002) evaluated that permanent lakes and rivers in the Pantanal cover about 3,120 km2, with small variation during the year. The remaining flooding areas of the Pantanal are characterized by vast plains subjected to seasonal flooding, mainly due to water overflow from the rivers, but also caused by local rain (Marani and Alvala 2007). In general, these plains are shallow in comparison with the lakes and many of them dry completely during the dry season.
Eight methane campaigns were performed during the years 2004 and 2005, thus resulting in the determination of 560 methane fluxes in two lakes and three flood-plains of the Southern Pantanal Region. The overall average methane flux was 116.8±276.0mg CH4 m-2 d-1, with the individual measurements varying from 1 to 2,187.0mg CH4 m-2 d-1. Figure 7.2 presents the frequency distribution of all fluxes on a logarithmic scale. Although the fluxes to the atmosphere show skewness toward the smaller fluxes in the logarithmic scale, a Gaussian distribution follows at the 1% level. This skewness resulted from the relative higher number of diffusive fluxes («=342, but with lower flux values), in contrast with the ebullitive fluxes («=218, with higher flux values).
Fig. 7.2 Frequency distribution of all fluxes on a logarithmic scale
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