Organic matter in Bolivian tributaries of the Amazon River: a comparison to the lower mainstream

We determined the concentrations and compositions of coarse particulate (.63 mm), fine particulate (0.1–63 mm), and dissolved (0.001–0.1 mm) organic matter collected along a river reach extending from a first-order stream in the Bolivian Andes, through the Beni River system, to the lower Madeira and Amazon Rivers. Dissolved organic carbon (DOC) concentrations increased down the total reach from;80 to 350 mM. The percentage of total DOC with a molecular weight greater than;1,000 atomic mass units that could be isolated by ultrafiltration also increased downstream from 40 to 80%. Weight percentages of organic carbon in the ultrafiltered isolates also grew downstream from 5 % at the uppermost station to 37 % in the Amazon mainstem. Organic carbon composed only 0.4–1.2 weight percentage of the total mass of the fine particulate fraction, which accounted for 70–80 % of the total organic carbon (TOC) in transport through the highly turbid (;600–2000 mg L21) Beni sequence. Observed compositional differ-ences were related primarily to the size fractions in which the organic matter occurred. On average, coarse particulate organic material exhibited an atomic C: N of 24, whereas ultrafiltered DOM was nitrogen poor, (C: N)a 5 34, and fine particulate material was nitrogen rich, (C: N)a 5 15. The lignin and stable-carbon isotopic compositions of these fractions indicate tree leaves and other nonwoody tissues from C3 land plants as predominant sources. Three molecular parameters demonstrate that the coarse, fine, and dissolved fractions of individual water samples ar