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We examined the factors controlling the variability in water-column respiration rates in Amazonian rivers. Ourobjectives were to determine the relationship between respiration rates and the in situ concentrations of the sizeclasses of organic carbon (OC), and the biological source (C3 and C4 plants and phytoplankton) of organic matter(OM) supporting respiration. Respiration was measured along with OC size fractions and dissolved oxygen isotopes(d18O-O2) in rivers of the central and southwestern Amazon Basin. Rates ranged from 0.034 mmol O2 L21 h21 to1.78 mmol O2 L21 h21, and were four-fold higher in rivers with evidence of photosynthetic production(demonstrated by d18O-O2,24.2%) as compared to rivers lacking such evidence (d18O-O2.24.2%; 1.3560.22 vs.0.30 6 0.29 mmol L21 h21). Rates were likely elevated in the former rivers, which were all sampled during low water,due to the stimulation of heterotrophic respiration via the supply of a labile, algal-derived substrate and/or theoccurrence of autotrophic respiration. The organic composition of fine particulate OM (FPOM) of these rivers isconsistent with a phytoplankton origin. Multiple linear regression analysis indicates that [FPOC], C:NFPOC ratios,and [O2] account for a high amount of the variability in respiration rates (r2 5 0.80). Accordingly, FPOC derivedfrom algal sources is associated with elevated respiration rates. The d13C of respiration-derived CO2 indicates thatthe role of phytoplankton, C3 plants, and C4 grasses in supporting respiration is temporally and spatially variable.Future scaling work is needed to evaluate the significance of phytoplankton production to basin-wide carboncycling.

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