Methane flux from the Amazon River floodplain: Emissions during rising water

Abstract

During April and May of 1987, an extensive methane flux data set from Amazonian wetland habitats was collected during the wet season as river water levels were high and rising. This work extends measurements made in the dry season of 1985, when water levels were falling. Comparison of these two data sets allows estimation of the amount of seasonal variability in this globally significant source of tropospheric methane. A total of 284 flux measurements were made in the three primary floodplain environments of open‐water lakes and channels, floating grass mats, and flooded forests, along approximately 1500 km of the central floodplain. Emissions (means and standard errors) were 74±14 mg CH 4 /m 2 /d (open water), 201±35 mg CH 4 /m 2 /d (grass mats), and 126±20 mg CH 4 /m 2 /d (flooded forests). These values were not significantly different from the majority of those from 1985, in part due to the high variability in flux seen at both times. Although ebullition was a significant component of methane emissions at both periods, the frequency of bubbling and its contribution to total flux was lower during the period of rising water than during falling water. A prominent diurnal pattern in atmospheric methane concentrations was observed, with minimum levels of about 1.75 ppm at midday and a maximum of 2.12 ppm at about midnight. Given the relatively small seasonal changes observed in flux at the two stages of the river's hydrographic curve, our earlier estimates of regional methane flux remain largely unchanged. Revision of global estimates of wetland methane sources based on these tropical data and recently published figures for northern peatlands indicated that tropical wetlands may be more important than previously suggested, but that wetland sources overall remain at approximately 110 Tg/yr.