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

We determined the concentrations and compositions of coarse particulate (>6 3 mum). fine particulate (0.1-63 mum), and dissolved (0.001-0.1 mum) organi c 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 similar to 80 to 350 muM. The percentage of total DOC with a molecular weight greater than similar to1,000 atomic mass units that could be isolated by ultrafiltration also increased downstream from 4 0 to 80%. Weight percentages of organic carbon in the ultrafiltered isolate s also grew downstream from 5% at the uppermost station to 37% in the Amazo n mainstem. Organic carbon composed only 0.4-1.2 weight percentage of the t otal mass of the fine particulate fraction, which accounted for 70-80% of t he total organic carbon (TOC) in transport through the highly turbid (simil ar to 600-2000 mg L-l) Beni sequence. Observed compositional differences we re related primarily to the size fractions in which the organic matter occu rred. On average, coarse particulate organic material exhibited an atomic C :N of 24, whereas ultrafiltered DOM was nitrogen poor, (C:N)a = 34, and fin e particulate material was nitrogen rich, (C:N)a = 15. The lignin and stabl e-carbon isotopic compositions of these fractions indicate tree leaves and other nonwoody tissues from C3 land plants as predominant sources. Three mo lecular parameters demonstrate that the coarse, fine, and dissolved fractio ns of individual water samples are increasingly degraded downstream. Elemen tal nitrogen, amino acids, and basic amino acids are all preferentially ass ociated with fine minerals. Observed geographical patterns included more po sitive delta C-13 values in particulate organic matter from high altitude s ites and an increase in the abundance and degradation of ultrafiltered diss olved organic matter down the drainage system. Many of these compositional patterns are imprinted within materials carried by low-order, high-altitude tributaries and appear to reflect processes occurring on the landscape.