Comparison of radiocarbon and fluorescence based (pump and probe) measurements of phytoplankton photosynthetic characteristics in the Northeast Atlantic Ocean

Signif~cant advances in rapid non-destructive means of measuring the photosynthetic energy conversion of phytoplankton in the ocean have been made in recent years The new techniques offer the potential to obtain detailed spatial and temporal information about photosynthetic rates 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 Probe Fluororneter [PPF) and 'v photosynthesis vs irradiance [Pvs IJ incubations over a wide vanety of biological, optical and physical conditions. A comparison of the photosynthetic characteristics obtained from the instantaneous PPF and time-integrated I4C approaches demonstrated correlations between parameters, although the absolute values were significantly different. Significant correlations were observed for both 1, and P,. while the light-dependent rate constant a was not significant despite a strong correlat~on between '"C-derived a and PPF measurements of the efficiency of photoconversion, A@,. The primary reason for the observed discrepancies in the absolute values derived from the 2 appi-oaches were the spectral differences between the artificial light source used for the I4C incubations and the natural underwater light in which the PPF measurements were made. Future studies must consider and correct, via normalization, for such spectral differences. Although the dataset was limited, there was some indication that photoaccl~mation of the phytoplankton assemblage may have occurred during the 2 h 'T incubations that took place late in the day. The use of the PPF approach is promising with regard to extending the measurement of photosynthetic character~stics on to the temporal and spatlal 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. KEY WORDSPhotosynthetic characteristics . lntercompanson Active fluorescence . Radiotracer methods Photoacclimation