Export of organic carbon in run-off from an Amazonian rainforest blackwater catchment



Rainfall, run-off and dissolved and particulate organic carbon concentrations were measured to investigate the hydrological export of organic carbon out of the blackwater Igarapé Asu rainforest catchment over a two-year period. Annual rainfall was above average (2442 mm) at 2976 mm in 2002 and below average at 2054 mm in 2003. Surface run-off dominated the flow out of the catchment, with groundwater outflow being negligible. Streamflow totals amounted to 1362 mm in 2002 and 780 mm in 2003. Dissolved organic carbon (DOC) concentrations in rainfall were similar to those measured in rainfall elsewhere in the Amazon Basin at 1·2 mg l−1, leading to atmospheric DOC deposition estimates of 3·5 g m−2 in 2002 and 2·4 g m−2 in 2003. Daily average DOC concentrations in run-off ranged from 8 mg l−1 under low flow conditions to 27 mg l−1 during large quickflow events. Suspended sediment (10–2000-µm size fraction) consisted for 28% of carbon and had a median concentration of 4·1 mg l−1. Daily run-off varied between 1·2–2·5 mm day−1 during dry periods with corresponding organic carbon exports of 0·009–0·031 g m−2 day−1. Exports associated with large storms were much higher, reaching a daily maximum of 1·02 g m−2 day−1 for a discharge event of 38·4 mm. Export of carbon during the wet seasons amounted to 70% of the total. Annual exports in river water were different between the years because of differences in run-off, varying between 26·2 g C m−2 in 2002 and 11·7 g C m−2 in 2003. Organic carbon exports were dominated by DOC, with exports in sediment constituting 6–8% of the total. Net carbon export, corrected for rainfall inputs, amounted to 22·7 and 9·3 g m−2 in 2002 and 2003, respectively. The present study indicates that accounting for exports of organic carbon in stream run-off (averaging 19·0 g m−2 year−1 over the 2 years) would reduce the rainforest sink strength, estimated at 300–400 g m−2 year−1 from eddy covariance measurements, by 5–6% for this blackwater catchment. Copyright © 2006 John Wiley & Sons, Ltd.