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Estimating the primary productivity of phytoplankton as they are mixed through the surface layer is often hampered by methodological or conceptual constraints. Fast Repetition Rate Fluorometry (FRRF) allows some of these constraints to be overcome by providing measurements of the instantaneous, depth-dependent rates of primary productivity of phytoplankton  in situ . Data acquired by FRR fluorescence is used in this paper to determine the parameters of the photosynthesis–irradiance curve and the instantaneous photosynthetic rates for phytoplankton from the mixed layer during a cruise in the Celtic Sea in May 2000. FRR fluorescence-based estimates of the initial slope of the photosynthesis–light curve (α B ) ranged from 7.5 in well-mixed conditions to 12.7 g C (mol photons) −1  m 2  (g Chl  a ) −1  under stratified conditions. FRR fluorescence-based estimates of the light saturation parameter,  E  k , were strongly correlated with the logarithm of the surface PAR. FRR fluorescence-based estimates of the Chl  a -specific, light saturated photosynthesis rate ( P  m  B ) ranged from 1.4 to 2.8 g C (g Chl  a ) −1  h −1 . The FRRF determined values of  P  m  B  were consistent with those measured using radiocarbon techniques, however, there were systematic differences when the estimates of α B  and  E  k  were compared. It was found that 86% of the variance in the instantaneous, integral water column primary productivity determined from FRR fluorescence measurements could be accounted for through a logarithmic relationship to surface PAR indicating that light, rather than nutrient limitation, was the dominant factor influencing photosynthesis.

A methodology to determine primary production and phytoplankton photosynthetic parameters from Fast Repetition Rate Fluorometry