Assessment of photosynthesis in a spring cyanobacterial bloom by use of a fast repetition rate fluorometer

Estimates of gross primary production (GPP) based on fast repetition rate fluorometer (FRRF) measurements were compared with independent C-14 and O-2 at three stations during a spring bloom in the North Atlantic. A photosynthesis versus irradiance (P-E) curve was constructed for each station from the observations of in situ photon efficiency of photosynthesis. This composite P-E curve was compared with P-E curves determined for discrete samples from C-14 assimilation. Estimates of alpha (Chl) and P-m(Chl) from the C-14-uptake method were 1.5-2.5-fold lower than those estimated from the FRRF data. Much of this discrepancy can be accounted for if C-14 assimilation approximates net phytoplankton photosynthesis with use of a photosynthetic quotient of 1.4 mol O-2 (mol CO2)(-1). Photosynthetic oxygen consumption may have also contributed to the difference. In situ GPP was calculated from incident irradiance, light attenuation, light absorption by phytoplankton, and the light dependence of the in situ photon efficiency. This estimate of GPP was two times greater than net community photosynthesis determined from diel changes of in situ oxygen concentration. Thus, the in situ net O-2 and in vitro C-14 techniques yielded similar estimates of phytoplankton photosynthesis that were about twofold lower than the estimates of GPP provided by FRRF Uncertainties in the FRRF technique associated with choosing an appropriate value of photosynthetic unit size and fitting a P-E curve to the in situ measurements are discussed. Despite these uncertainties, the FRRF results were consistent with the independent estimates of phytoplankton productivity.