Particulate organic carbon, sterols, fatty acids and pigments in the Amazon River system

Water samples were collected from the Amazon River system during a high flo od period, in June 1989, and lipids associated with particles retained on G F/F filters were examined. Particles showed a highly variable organic carbo n content (1.8-29.0%). Corresponding organic carbon concentrations varied f rom 0.36 to 1.13 mg/l. The flood conditions encountered during the sampling period may feed exceptional inputs of soil organic matter into the tributa ries and the Amazon River. Composition and concentration of sterols, fatty acids and pigments were determined to estimate the relative portion of terr igenous and autochthonous fraction of this complex organic matter. Sterol d istribution patterns were similar to other equatorial rivers, in the Orinoc o (Venezuela) and Solo (Indonesia). In comparison with the dominant profile of the Amazon system, distinct patterns were found in the Trombetas River (29 Delta (5),22 similar to 27 Delta (5) similar to 29 delta (5) > 28 Delta (5), 28 Delta (5,22)) and in the Tapajos River (27 Delta (5),22 > 27 Delta (5) > 29 Delta (5), 28 Delta (5), 28 Delta (5),22). These fingerprints ref lect different vegetation types of drainage basins and distinct planktonic pools. The distribution of even-carbon numbered saturated fatty acids in th e carbon range of 24-36 revealed low inputs of constituents associated with cuticular waxes of vascular plants in Black waters and in the Tapajos Rive r (similar to 200 ng/l), higher in White waters (328-483 ng/l) and highest in the Trombetas River (704 ng/l) and in stagnant waters of a small lake cl ose to the Amazon (1088 ng/l). Pigment concentrations showed that in the ma in river and most tributaries vegetal carbon did not represent more than 2% , except for the Tapajos River (6.2%) and in relatively stagnant waters sam pled along the main river (9.2%). Based on relative abundance of Chl b, Chl c, fucoxanthin, peridinin, alloxanthin, and zeaxanthin, various phytoplank tonic assemblages were evidenced in the Amazon River system. Branched fatty acids in the carbon range of 15-17 gave insight into bacterial signatures. They showed low microbial contribution to the fatty acid pool, with slight est higher contribution in a swamp of stagnant waters and in the White wate rs of the Solimoes River.