Fast repetition rate fluorometry (FRRF) measuring phytoplankton productivity: A case study at the entrance to the Gulf of Finland, Baltic Sea

The fast repetition rate (FRR) fluorometry, based on variable fluorescence of chlorophyll a, has been introduced as a potential tool to evaluate the primary productivity (PP) in aquatic systems. I measured PP in situ with both the 14C and the FRR techniques at the entrance to the Gulf of Finland, Baltic sea, and noticed that the relation between the 14C-based (Pb) and the FRR-based (Pf) estimates of PP was non-linear. This discrepancy between Pf measuring the rate of linear photosynthetic electron flow, and Pb measuring the rate of C fixation should be resolved before the PP can be reasonably estimated with the FRR technique. The deviation from linearity took place at irradiances above 200 µmol quanta m–2 s–1, since Pb reached light-saturation at lower irradiances than did Pf. Within the regression fits, the daily variability in Pb explained at least 85% of the daily variability in Pf (a total of four days, n = 11 on each day). The period of time over which PP was determined was substantially different for the two approaches; over 2 h for Pb, and approximately 10 s for Pf. Consequently, most of the deviation around the resulting fits was caused by the sensitivity of the FRR technique to the momentary incident irradiance at the time of the measurement. The FRR-based photosynthetic efficiency of phytoplankton in the upper parts of the water column could be divided into three layers: reduced levels in the near-surface layer due to inhibiting effect of supraoptimal irradiance, the layer beneath this having the highest levels, and the deepest layer inhabited by a senescent algal population characterized by lowered levels.