Seasonality

The seasonality of the emission rates during the wet and dry periods may be observed in Fig. 7.4, which presents the box plot diagrams for both the periods. For the average diffusive fluxes, a statistically difference (a=0.05) was observed between the wet (18.5±26.1 mg CH4 m-2 d-1) and the dry (9.5±15.1 mg CH4 m-2 d-1) periods, with greater fluxes during the wet periods, following increases in the water temperature and depth.

Although the correlation between the fluxes and the environmental parameters was low, the statistical difference observed between both periods shows some influence on the fluxes measured. The separation of the sites between floodplains and lakes showed that the variability with the depths was experienced mainly in the lakes. In general, the floodplains present a small slope, so that in the wet period, the depth increased about 0.5 m, while in the lakes, the depths increased more than one meter. In all floodplains, the influence of seasonality was observed mainly in their area, with a large decrease during the dry period, such that the Sao Joao site was completely dry in 2005.

Depth and temperature were altered from the wet to dry season in the lakes, but their flooded area had a small increase in the wet season. The results of this work, in some aspects, are different from those obtained in Amazonia by Bartlett et al. (1990) and Devol et al. (1990), since in the Pantanal, the diffusive fluxes, although lower in both periods, showed statistic differences between the wet and the dry periods, mostly in the floodplains. Following Keller and Stallard (1994), higher temperatures and input of substrates due to flooding in the wet period, could have influenced positively the diffusive fluxes, including some effects of depth.

The ebullitive fluxes did not show a statistically significant variation from wet (average: 281.6 ± 376.3 mg CH4 m-2 d-1) to dry (average: 277.2 ± 405.3 mg CH4 m-2 d-1) season; there was only an increase in the data dispersion in the dry season. During the dry period, were observed some higher fluxes, including the two highest ones. The separation of the ebullitive fluxes in the lakes and the floodplains components shows that the average fluxes were higher in the floodplains than in the lakes, with similar values in both periods, although the observed temperature and depth were statistically different for the wet and the dry seasons. This result may be associated with the buoyancy of the bubbles in the sediments, which is related to the depth, even if a correlation was not detected statistically. One indication of this is

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