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); standa rd deviation, +/- 18%) per m(2) of water surface. Ebullition accounted for 65% of emissions. Emission rates were about five times as high for floodpla in forest as for open water or macrophyte mats. Emission rates were positiv ely correlated with carbon content of sediment and amount of methane in the water column, and negatively correlated with dissolved oxygen, but the cor relations 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 em itted five times this amount. Total emissions were accounted for primarily by flooded forest (94%); macrophyte mats, open water, and exposed soils mad e only small contributions. The flux-weighted mean delta(1)3C for the flood plain was -62 +/- 8 parts per thousand; for delta D the mean was -271 +/- 2 7 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 n ot for the Amazon. The Orinoco data illustrate the difficulty of generalizi ng emission rates. Current information for tropical America, including revi sed estimates for inundated area along the Amazon, indicate that methane em issions from tropical floodplains have been overestimated.