Spatial and seasonal variation of methane emission from an Amazon floodplain lake

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Abstract
Methane (CH4) emission from a floodplain lake in central Amazonia was measured during a complete annual cycle using a static chamber method. The results show significant differences in the CH4 flux rates from different habitats in the lake: open water, floating vegetation (mainly the aquatic grass Paspalum repens) and floodplain forest. The highest CH4 emission rates (10–198 mg CH4 m−2 d−1) were recorded during inundation in the seasonally flooded forest. Lower emission rates occurred in the floating vegetation (3–79 mg CH4 m−2 d−1), which is found in areas with a short subaerial phase and in the permanently aquatic part of the lake (4–65 mg CH4 m−2 d−1) free of macrophytes and with the lowest productivity. Seasonal fluctuations in water level resulted in distinct seasonal patterns in CH4 release at different sites in the lake. The permanently aquatic part of the lake had lowest emission rates during periods of water inflow and subsurface oxygen replenishment into the lake. At the periodic aquatic sites the release of CH4 started immediately after flooding and showed a tendency of increased fluxes with length of period of inundation. This tendency was interrupted when the water level was at its maximum. Results from this study indicate that the spatial and seasonal variations in CH4 emission can be related to the supply of organic material to the different habitats and a shift from aerobic to anaerobic conditions favorable for methanogenesis.

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