MEPS 149:215-226 (1997)  -  doi:10.3354/meps149215

Significant 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 Fluorometer (PPF) and ¹⁴C photosynthesis vs irradiance (P vs I) incubations over a wide variety of biological, optical and physical conditions. A comparison of the photosynthetic characteristics obtained from the instantaneous PPF and time-integrated ¹⁴C approaches demonstrated correlations between parameters, although the absolute values were significantly different. Significant correlations were observed for both I_k and P_m, while the light-dependent rate constant alpha was not significant despite a strong correlation between ¹⁴C-derived alpha and PPF measurements of the efficiency of photoconversion, Delta phi_m. The primary reason for the observed discrepancies in the absolute values derived from the 2 approaches were the spectral differences between the artificial light source used for the ¹⁴C 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 photoacclimation of the phytoplankton assemblage may have occurred during the 2 h ¹⁴C 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 spatial 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.

Photosynthetic characteristics · Intercomparison · Active fluorescence · Radiotracer methods · Photoacclimation