Plankton Metabolism and Carbon Processes in the Amazon River, Its Tributaries, and Floodplain Waters, Peru‐Brazil, May‐June 1977

Abstract

A synoptic study of photosynthetic and respiratory activity of plankton communities in different Amazon surface waters indicates that large—scale events such as flooding can have a major impact upon the cycling of carbon and nutrients in these aquatic ecosystems. During high water, the major factors influencing primary production appeared to be nutrient concentrations in the mouthbays and varzea (floodplain) lakes and high levels of suspended matter in the Amazon mainstem. In riverine systems, plankton primary production (PPP) averaged 4.04 mg C°m—3°h—1, and measures of respiration (Re) averaged 0.67 mg C°m—3°h—1. In the more productive varzea lakes and mouthbays, PPP averaged 26.37 mg C°m—3°h—1 and Re averaged 2.30 mg C°n—3°h—1. Bacterial densities, 14 C—acetate rate constants for uptake, and plankton carbon: ATP ratios implied that heterotrophic microbiota were important components of the plankton communities in riverine waters. The importance of terrestrial organics to metabolic activity in all waters was implied by high particulate carbon: nitrogen ratios (20:1). These features were especially evident in riverine surface waters where integrated respiration rates exceeded those of plankton primary production. Riverine respiratory levels may be attributed to several factors: adequate supplies of terrestrial organic carbon, sufficient dissolved nutrient concentrations, increased surface area of suspended matter for microbial attachment and growth, and shading of phytoplankton by suspended matter which reduces photosynthetic activity. Observed supersaturation of Amazon waters by carbon dioxide was similar to observations for other rivers of the world. Shifts of CO2 solute components to CO2 in surface waters of varzea lakes and mouthbays and of some tributaries implied high partial pressures of carbon dioxide (°500—1500 Pa). The primary source of CO2 is most likely decomposing organic matter in planktonic and benthic environments of the rivers, lakes, and flooded terrestrial lowlands. The hypothesis that respiratory input of CO2 balanced by evasion (gas lost to atmosphere) is sufficient to explain high CO2 vapor pressures in the Amazon River appeared true from our calculations but needs further examination. Particular attention should be given to periods of rising water, when planktonic respiration appears to be two orders of magnitude greater than at periods of high water. Subsequent seasonal studies of the Amazon and other large rivers are needed to determine how the plankton community, the chemistry of terrestrially derived organics and their biological oxidation in water, and inorganic factors control CO2 supersaturation and exchange with the atmosphere.