Tropospheric methane from an Amazonian floodplain lake

Abstract

During July and August 1985, the sources of methane and its flux to the troposphere were measured from a houseboat laboratory anchored in Lago Calado, a stratified, dendritic lake of about 6 km 2 area, located in the central Amazon basin. Methane concentrations in the mixed layer of the lake were varied (0.0001-0.0055 mM) and usually less than 0.004 mM CH,•, with no consistant temporal trend. Methane concentrations increased with depth across the thermocline as oxygen dropped to less than 0.1 mg O 2 L -x. Over 6 weeks, methane increased from less than 0.08 to greater than 0.21 mM in the anoxic hypolimnion below 6 m. Methane in the pore water approached saturation, with a pure methane atmosphere within 5 cm of the sediment/water interface. The gradient-supported flux from the sediments to the overlying water could account for the methane increase in the bottom waters plus the surface flux. The measured methane flux from the surface of the open lake to the atmosphere averaged 27 mg CH,• m-2 d-t. This was consistent with the buildup in ambient methane in the nocturnal surface mixed layer of the troposphere. Ebullition contributed 70% to the average total flux. The diffusive flux measured with a static chamber ranged from 0 to 34 mg CH,• m-2 d-x, with an average of 8.3 mg m-2 d-x. From July 20 to September 2, 1985, average ambient air concentration was 1.89 (+0.16) ppm CH,•, with a diurnal variation of 0.27 ppm. 1. INTRODUCTION Along the middle reaches of the Amazon River near the confluence of the Rio Solimfies and the Rio Negro are large floodplains roughly 150,000 km e in area, dotted with many lakes of various sizes and shapes [Melack, 1984]. The surface areas and depths of these lakes vary as a function of the water level of the Amazon, which changes about 10-12 m at Manacapuru, Brazil, over the course of a year [Meade et al., 1985]. The wetlands associated with the Amazonian floodplain are potentially important regional sources and/or sinks for various forms of carbon [Hedges et al., 1986; Ertel et al., 1986]. Given the high rates of organic production and the common occurrence of anoxia in these environments, one can hypothesize that these regions would be important sources of reduced gases to the atmosphere [Brinkmann and Santos, 1974]. The research described here was undertaken as part of the ABLE 2A/AGE expedition to examine the methane flux through an Amazonian floodplain lake to the troposphere and to determine how the distribution of methane in the water column is related to the dynamics of that flux. A companion paper [Bartlett et al., this issue] describes methane flux from a variety of other environments in the Amazon floodplain. Lago Calado (3ø15'S, 60ø34'W) is a medium-sized dendritic • Now lake about 60 km west of Manaus, Brazil, on the north side of the Rio Solim6es (Figure 1). The lake has been described previously [Melack and Fisher, 1983; Setaro and Melack, 1984; Maclntyre and Melack, 1984]. The surface area of the lake varies from 2 to 8 km 2 and the maximum depth varies from 1 to 12 m, both d...

Comparison of floating chamber and eddy covariance measurements of lake greenhouse gas fluxes

“…The diurnal cycle of FCH 4 is presented in detail by Podgrajsek et al (2014) where it was suggested that the onset of a diurnal cycle of FCH 4 was controlled by water-side convection and formation of methane in the sediment. Such a pattern with convective driven high night-time fluxes was previously observed using flux chambers (Crill et al, 1988;Godwin et al, 2013), while studies from other lakes have found higher daytime CH 4 emissions (e.g. Bastviken et al, 2004Bastviken et al, , 2010Keller and Stallard, 1994).…”

Influence of weather variables on methane and carbon dioxide flux from a shallow pond

“…Another possibility is increased root leakage of substrates for methanogenesis during daytime. The opposite diel patterns has been observed in a deeper wind-sheltered tropical lake which became stratified and accumulated CH4 in anoxic bottom water during daytime, while nighttime convection transported the accumulated CH4 to the surface waters resulting in elevated night-time emissions (Crill et al 1988). It has been suggested that methanotrophs could be inhibited by light (Dumestre et al 1999;Murase and Sugimoto 2005) but presumably this would have a limited effect on methane oxidation in surface sediments.…”

WRF-Chem simulations in the Amazon region during wet and dry season transitions: evaluation of methane models and wetland inundation maps

“…5a3-b3 As WWM is sensitive to different wetland types through the plant-mediated transport mechanism that depends on the vegetation type (Walter et al, 2001a), locations with three Bartlett et al (1988) who estimated the ebullitive flux to account for 48 % in open water and 54 % in flooded forest areas, the ebullitive contribution of WWM to the CH 4 transport to the atmosphere is in the same order of magnitude. However, Crill et al (1988) estimated 70 % contribution of ebullition. Up to this study WWM has only been validated against observations from a swamp region in Panama in the tropics (Walter and Heimann, 2000).…”