A COMPARISON OF NET AND GROSS RATES OF OXYGEN PRODUCTION AS A FUNCTION OF LIGHT-INTENSITY IN SOME NATURAL PLANKTON POPULATIONS AND IN A SYNECHOCOCCUS CULTURE

In vitro rates of gross and net oxygen production were measured as a function of light intensity in some plankton communities collected from Bedford Basin, Nova Scotia, and in a monoclonal culture of Synechococcus. The rate of gross oxygen production was measured by a technique in which the stable oxygen isotope, O-18, serves as a photosynthetic tracer. Net oxygen production was measured by automated Winkler technique. The rate of community respiration in the light was then determined by the difference between gross and net rates of oxygen production. In the natural populations examined, neither gross nor net oxygen production rates were significantly inhibited at the highest light intensity measured (500-800-mu-E m-2 s-1). In a sample in which the dark respiration rate was small relative to the maximal rate of production [P(max; sensu Platt et al. (1980) J. Mar. Res., 38, 687-701] the rates of 'light' respiration were approximately 3 times greater. In two other communities, with high rates of dark respiration relative to P(max), the rates of 'light' respiration were closer to rates of dark respiration. In the Synechococcus clone, both gross and net oxygen production rates were inhibited at high light intensities. Rates of 'light' respiration were found to vary as a function of light intensity. The greatest rates of respiration were measured in samples incubated at light intensities that were just saturating (approximately 100-mu-E m-2 s-1). The rates of C-14 production were also measured as a function of light intensity. The photosynthetic quotients, based on C-14 production rates and gross oxygen production rates, average approximately 1.9.