Methane emissions from the Orinoco River floodplain, Venezuela

Methane emissions were measured over a 17-monthinterval at 21 locations on the Orinoco fringingfloodplain and upper delta (total area,14,000 km2). Emissions totaled 0.17 Tgyr−1, or 7.1 mmol d−1 (114 mg d−1;standard deviation, ±18%) per m2 of watersurface. Ebullition accounted for 65% of emissions. Emission rates were about five times as high forfloodplain forest as for open water or macrophytemats. Emission rates were positively correlated withcarbon content of sediment and amount of methane inthe water column, and negatively correlated withdissolved oxygen, but the correlations were weak. Emission from floodplain soils occurred only when thewater content of soil exceeded 25%, which occurredwithin 20 m of standing water during floodplaindrainage (3 months/yr). Bare soils emitted 60mmol/day per m of shoreline length; soils covered bystranded macrophyte beds emitted five times thisamount. Total emissions were accounted for primarilyby flooded forest (94%); macrophyte mats, open water,and exposed soils made only small contributions. Theflux-weighted mean δ13C for the floodplainwas −62 ± 8‰; for δD the mean was −271 ± 27‰. The δ13C and δD were negativelycorrelated. Overall emission rates were notably lowerthan for the Amazon. The depth and duration offlooding are considerably less for the Orinoco thanfor the Amazon floodplain; oxygen over sediments isthe rule for the Orinoco but not for the Amazon. TheOrinoco data illustrate the difficulty of generalizingemission rates. Current information for tropicalAmerica, including revised estimates for inundatedarea along the Amazon, indicate that methane emissionsfrom tropical floodplains have been overestimated.

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