USE OF ACTIVE FLUORESCENCE TO ESTIMATE PHYTOPLANKTON PHOTOSYNTHESIS IN-SITU

We describe the theory and practice of estimating photosynthetic rates from light-stimulated changes in the quantum yield of chlorophyll flu orescence. By means of a pump-and-probe fluorescence technique, where weak probe flashes are used to measure the change in the quantum yield of fluorescence induced by the strong pump flash, it is possible to d erive the absolute absorption cross sections for photosystem 2, the qu antum yield for photochemistry, and the maximum rate of photosynthetic electron transport at light saturation. In conjunction with a semiemp irical biophysical model of photosynthesis, these parameters can be us ed to calculate the instantaneous rate of gross photosynthesis in situ under ambient irradiance. A profiling pump-and-probe fluorometer was constructed and interfaced with a CTD, and vertical profiles of variab le fluorescence were obtained on four cruises in the northwest Atlanti c Ocean. The derived photosynthetic rates were compared with concurren t estimates of production based on radiocarbon uptake. The correlation coefficient between the two estimates of primary production, normaliz ed to Chl a, was 0.86; linear regression analysis yielded a slope of 1 .06. There is a 3-4 fold range in the maximum change in the quantum yi elds of photochemistry and absorption cross-sections in natural phytop lankton communities. Uncertainties in the pump-and-probe-derived estim ates of photosynthesis are primarily due to temporal mismatches betwee n instantaneous and time-integrated measures of production and in biol ogical variability in the ratio of the number of PS2 reaction centers to total Chl a.