Biogeochemical characteristics of organic matter in the particulate and colloidal fractions downstream of the rio Negro and Solimoes rivers confluence

The analysis of organic matter in the rio Negro, rio Solimoes and rio Amazo nas focused on total organic carbon (TOC), as determined at two main sizes, the particulate form (> 0.2 mu m) and the colloidal form (0.2 mu m to 5000 dalton). These fractions were obtained by tangential ultrafiltration carri ed out on 50 1 water samples. The total polysaccharide contents were determ ined by a colorimetric method, the main classes of organic compounds by pyr olysis and gas chromatography coupled with mass spectrometry (Py-GC-MS), an d the lignin or non-lignin phenols by cupric oxide alkaline oxidation and h igh performance liquid chromatography (HPLC) with photodiode array spectrop hotometer detection. Upstream of the confluence of the rio Negro and rio So limoes, the rivers show characteristic differences. The rio Negro has low d ischarge, high levels of TOC% for the two fractions, high levels of polysac charides, low levels of lignin phenols and high levels of hydroxybenzyl non -lignin phenols. The main classes of organic compounds also give a distribu tion different for the particulate (phenols associated to aromatic compound s) and the colloidal (aromatic compounds and sugars). For the rio Solimoes, the compositional features are generally reversed. Downstream of the confl uence, the colloidal fraction showed noteworthy levels of lignin-derived co mpounds associated with increasing levels of polysaccharides correlated to the distance from the confluence.