COMPARISON OF RADIOCARBON AND FLUORESCENCE-BASED (PUMP AND PROBE) MEASUREMENTS OF PHYTOPLANKTON PHOTOSYNTHETIC CHARACTERISTICS IN THE NORTHEAST ATLANTIC-OCEAN

Significant advances in rapid non-destructive means of measuring the p hotosynthetic energy conversion of phytoplankton in the ocean have bee n made in recent years. The new techniques offer the potential to obta in detailed spatial and temporal information about photosynthetic rate s that cannot be achieved using more traditional methods. A study of a mesoscale coccolithophore bloom in the Northeast Atlantic enabled the comparison of photosynthetic parameters derived from the Pump and Pro be Fluorometer (PPF) and C-14 photosynthesis vs irradiance (P vs I) in cubations over a wide variety of biological, optical and physical cond itions. A comparison of the photosynthetic characteristics obtained fr om the instantaneous PPF and time-integrated C-14 approaches demonstra ted correlations between parameters, although the absolute values were significantly different. Significant correlations were observed for b oth I-k and P-m, while the light-dependent rate constant a was not sig nificant despite a strong correlation between C-14-derived alpha and P PF measurements of the efficiency of photoconversion, Delta phi(m). Th e primary reason for the observed discrepancies in the absolute values derived from the 2 approaches were the spectral differences between t he artificial light source used for the C-14 incubations and the natur al underwater light in which the PPF measurements were made. Future st udies must consider and correct, via normalization, for such spectral differences. Although the dataset was limited, there was some indicati on that photoacclimation of the phytoplankton assemblage may have occu rred during the 2 h C-14 incubations that took place late in the day. The use of the PPF approach is promising with regard to extending the measurement of photosynthetic characteristics on to the temporal and s patial scales required for the development of more robust bio-optical models and to complement currently available in situ measurement rates of physical and chemical parameters.