Summary
Plankton and chemical studies conducted during a synoptic cruise of the Amazon in May-June 1977 indicate that inorganic and organic substances of terrestrial origin could influence the microbial activity in the mainstem, tributaries, and varzea (floodplain) lakes. The major limiting factors to plankton primary production (PPR) were light attenuation by high levels of suspended particulate matter (SPM) in the mainstem and tributaries (average 69 mg SPM liter-1), and low nutrient levels in river mouthbays and várzea lakes (average 0.12 μmole PO4-P liter-1 and 1.6 μmole NO3-N liter-1). In riverine systems, PPR averaged 4.04 mg C m-3 hr-1, and electron transport system measures of respiration (Re) averaged 0.67 mg C m-3 hr-1; and in the more productive mouthbays and várzea lakes, PPR averaged 26.37 mg C m-3 hr-1, and Re, averaged 2.30 mg C m-3 hr-1. :Re ratios of 8 for the riverine systems and 17 for the mouthbays and varzea lakes indicated that Re was not as limited as PPR in the rivers. Bacterial densities, 14C-acetate rate constants for uptake, and particulate organic C:N of 20, implied that microflora was influenced by terrestrial carbon. Shifts of ΣCO2 components due to more free CO2 and H2CO3 in surface waters of some lacustrine environments and tributaries were suggested by high partial pressures of carbon dioxide (~ 5,000 to ~ 15,000 x 10-6 atm). It is suggested that carbon dioxide oversaturation of some of the water with respect to the atmosphere was partly due to decomposing allochthonous matter. Calculations imply that respiration balanced by evasion is sufficient to explain the high carbon dioxide vapor pressures.