Seasonal dynamics and localscale emissions

In general, northern wetlands show a distinct seasonal pattern with a growing season peak dominating annual emissions (for example Crill et al, 1988; Whalen and Reeburgh, 1992; Rinne et al, 2007). When emissions are at the growing season peak in high-emitting northern wetlands they average 5-10mg CH4 m-2 hr-1. Recently, high northern permafrost wetlands have been shown to have some interesting additional peak emissions associated with the freeze-in period (Mastepanov et al, 2008). The generality of this feature and frequency of occurrence remains to be documented in more detail.

Tropical emissions are closely related to the seasonal flooding appearing in major parts of the wetland areas. The levels of emission are generally higher than for northern wetlands during the flooded seasons but they drop dramatically during the non-flooded part of the year. Average peak season emission may exceed 15mg CH4 m-2 hr-1. Figure 3.4 shows a generalized diagram comparing seasonal variation in emissions from the high northern tundra over northern wetlands in general to the tropical regions.

Figure 3.4 A general schematic illustration of differences in seasonal variations of methane emissions in tropical and northern wetland settings

Note: Tropical emissions are primarily influenced by the spatial scale of flooding while northern wetlands has a distinct temperature response. Line 'a' represents a constantly flooded tropical high-emitting wetland. Line 'b' a very common seasonally flooded tropical wetland. Line 'c' is a general representation of a northern seasonally temperature-dependent emission pattern and line 'd' represents the special dynamics associated with permafrost environments and recently discovered freeze-in bursts associated with those (stippled line 'e'). Source: The generalized schematic is based on flux measurements presented in Bartlett et al (1988), Melack et al (2004), Mastepanov et al (2008) and Jackowicz-Korczyriski et al (in press)

The emission pathways may also vary seasonally. During the growing season, where there are vegetated surfaces, the percentage of the flux arising from ebullition may be below 50 per cent as the vascular transport mechanisms by the plants dominate. Purely diffusive flux is normally a very minor part of the overall flux (Bartlett et al, 1988; Christensen et al, 2003b). During the non-growing season in northern wetland, ebullition flux may be relatively more dominant as an emission pathway, and physical processes influencing emissions of stored gas are important (Mastepanov et al, 2008).

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