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1.5 2.0 log (Flux)

The Pantanal region was characterized by a drought during the years of the cam-paings, mainly in 2005, when the water maximum peak level in the Paraguai River was 3.29 m, while for the normal flooding condition, it ranges from 5.0 to 6.0 m (Embrapa 2006). In 2004, the maximum water level was 4.26 m, what characterizes a small flooding condition. This strong drought also contributed for lowering the depth of the lakes to 1.4 m in 2004, and to 0.9 m in 2005.These drought levels possibly influenced the organic matter distribution in the water.

The pH measured at 10 cm below the surface ranged from 6.4 to 9.8, and 90% of them fell in the range 6.5-7.7, which is the optimum range for the methane production by the methanogenic bacteria (Yang and Chang 1998). However, for this short range, relationships between fluxes and the pH are unlikely.

Marani and Alvala (2007) observed differences of occurrence in the diffusive and in the ebullitive (bubbles) fluxes while considering lakes or floodplains, with a tendency of the higher occurrence of bubbles in the floodplains. While the diffusive fluxes were concentrated in the lower portion of the emission range (average: 13.1±20.7mg CH4 m-2 d-1, median: 5.0 mg CH4 m-2 d-1), the ebullitive fluxes had higher values, with a greater dispersion (280.7±390.0mg CH4 m-2 d-1, median: 128.5 mg CH4 m-2 d-1). This later result reflects the more intense nature of the emission, which occurred only in 39% of the measured fluxes. These higher bubble fluxes observed in the Pantanal floodplains reveal the importance of ebul-litive fluxes in the emission of methane in the region when considering regional emission. Bubbling events are episodic, but when they occur, they dominate the methane release, thus resulting high average fluxes (Devol et al. 1990).

The fraction of occurrence of bubble events at different depth ranges for flood-plains and for lakes is presented in Fig. 7.3. In the first ones, the bubble fluxes represented 51% of the occurrences, with a depth range from 0.1 to 1.4 m, while in the lakes they represented 27%, with a depth range from 0.1 to 3.5 m; also, in

Depth (m)

Fig. 7.3 Ratio of ebullition (i.e., fraction of fluxes measurements with detectable bubbles) at different depths to lakes and floodplains

Fig. 7.3 Ratio of ebullition (i.e., fraction of fluxes measurements with detectable bubbles) at different depths to lakes and floodplains the same range of depth (0.1-1.4 m), the relative number of ebullitive fluxes in the floodplains was higher than the one observed in the lakes.

In contrast, in Amazonia, the bubbling emission corresponded to up to 73% of the overall fluxes, with each one showing different contribution (Bartlett et al. 1990; Devol et al. 1990). Bastviken et al. (2004) observed that, in temperate lakes, there is no linear correlation between the frequency of occurrence of ebullitive fluxes and the depth; their frequency ranged from 25 to 80% of the measurements, depending on the water depth, with the higher values occurring in deep waters. Smith et al. (2002) verified that although the correlations of methane fluxes with a number of environmental variables are statistically significant, they are too weak to serve as a basis for either the prediction, or the analysis of emission mechanisms.

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