DISSOLVED ORGANIC-MATTER AND ITS OPTICAL-PROPERTIES IN A BLACKWATER TRIBUTARY OF THE UPPER ORINOCO RIVER, VENEZUELA

Colored dissolved organic matter (CDOM) and dissolved organic carbon ( DOC) concentrations were investigated in a blackwater tributary (river Surumoni) of the upper river Orinoco (South Venezuela) during Novembe r and December 1996. DOC concentrations were high (similar to 10 mg C L-1), relatively invariant in time and largely-decoupled from discharg e. The Surumoni has the highest CDOM levels reported hitherto-with slo pes of In-linearized absorption spectra ranging from 0.0085-0.0152 nm( -1), a mass specific absorption coefficient at lambda = 300 nm of 6.45 L mg(-1) m(-1). Optical signatures indicate that the CDOM is highly a romatic in nature and of terrestrial origin in the Surumoni, whereas a utochthonous sources are also likely to contribute significantly to th e Orinoco CDOM pool. Hydrologic connectivity of the active channel wit h fringing floodplains largely determined the spatio-temporal variatio n of CDOM in the Surumoni. The upriver channel is straight, braided in some reaches, and CDOM optical properties remained largely invariant along its course. Downriver, the meandering channel receives substanti al amounts of highly degraded, aromatic material from extensive floodp lains. (C) 1998 Elsevier Science Ltd. All rights reserved.