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 November and December 1996. DOC concentrations were high ( 10 mg C L−1), relatively invariant in time and largely decoupled from discharge. The Surumoni has the highest CDOM levels reported hitherto with slopes of ln-linearized absorption spectra ranging from 0.0085–0.0152 nm−1, a mass specific absorption coefficient at λ=300 nm of 6.45 L mg−1 m−1. Optical signatures indicate that the CDOM is highly aromatic in nature and of terrestrial origin in the Surumoni, whereas autochthonous sources are also likely to contribute significantly to the Orinoco CDOM pool. Hydrologic connectivity of the active channel with fringing floodplains largely determined the spatiotemporal variation 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 substantial amounts of highly degraded, aromatic material from extensive floodplains.