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 Amazonas 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 carried out on 50 1 water samples. The total polysaccharide contents were determined by a colorimetric method, the main classes of organic compounds by pyrolysis and gas chromatography coupled with mass spectrometry (Py-GC-MS), and the lignin or non-lignin phenols by cupric oxide alkaline oxidation and high performance liquid chromatography (HPLC) with photodiode array spectrophotometer detection. Upstream of the confluence of the rio Negro and rio Solimoes, the rivers show characteristic differences. The rio Negro has low discharge, high levels of TOC% for the two fractions, high levels of polysaccharides, low levels of lignin phenols and high levels of hydroxybenzyl non-lignin phenols. The main classes of organic compounds also give a distribution different for the particulate (phenols associated to aromatic compounds) and the colloidal (aromatic compounds and sugars). For the rio Solimoes, the compositional features are generally reversed. Downstream of the confluence, the colloidal fraction showed noteworthy levels of lignin-derived compounds associated with increasing levels of polysaccharides correlated to the distance from the confluence.