Methane emissions from the Orinoco River floodplain, Venezuela

Methane emissions were measured over a 17-month interval at 21 locations on the Orinoco fringing floodplain and upper delta (total area, 14,000 km(2)). Emissions totaled 0.17 Tg yr(-1), or 7.1 mmol d(-1) (114 mg d(-1); standard deviation, +/- 18%) per m(2) of water surface. Ebullition accounted for 65% of emissions. Emission rates were about five times as high for floodplain forest as for open water or macrophyte mats. Emission rates were positively correlated with carbon content of sediment and amount of methane in the water column, and negatively correlated with dissolved oxygen, but the correlations were weak. Emission from floodplain soils occurred only when the water content of soil exceeded 25%, which occurred within 20 m of standing water during floodplain drainage (3 months/yr). Bare soils emitted 60 mmol/day per m of shoreline length; soils covered by stranded macrophyte beds emitted five times this amount. Total emissions were accounted for primarily by flooded forest (94%); macrophyte mats, open water, and exposed soils made only small contributions. The flux-weighted mean delta(1)3C for the floodplain was -62 +/- 8 parts per thousand; for delta D the mean was -271 +/- 27 parts per thousand. The delta(13)C and delta D were negatively correlated. Overall emission rates were notably lower than for the Amazon. The depth and duration of flooding are considerably less for the Orinoco than for the Amazon floodplain; oxygen over sediments is the rule for the Orinoco but not for the Amazon. The Orinoco data illustrate the difficulty of generalizing emission rates. Current information for tropical America, including revised estimates for inundated area along the Amazon, indicate that methane emissions from tropical floodplains have been overestimated.