Fluxes of dissolved and colloidal organic carbon, along the Purus and Amazonas rivers ( Brazil)

The first study of the molecular size distribution and fluxes of organic ca rbon from the Rio Purus to the Amazon River is reported. Organic carbon mea surements using sequential tangential ultrafiltration (STUF) and discharge measurements give original information for the Purus basin, which has not b een studied in depth until now. For this basin, the total organic carbon (T OC) values and the molecular size distribution are similar to those found f or white waters. This type of water is the most common in the Amazonian bas in and is characterised by a high concentration of mineral material and a l ow content of organic matter. In addition two sampling sites of the Purus R iver show a very large concentration (> 80% of the TOC concentration) in th e dissolved material. This distribution is similar to that of the Rio Madei ra, which is unexpected, since the Rios Purus and Madeira do not drain the same kind of soil. For the Amazon basin, this work has confirmed the carbon size redistribution and flocculation/coagulation phenomena at the confluen ce. This dynamic exchange between the fractions is important for some pheno mena such as metals transported by the organic matter. Indeed the metal tra nsport and the molecular mass distribution of associated metals to organic matter are closely linked to the size of the organic material. (C) 1999 Els evier Science B.V. All rights reserved.