Temporal variability in the quantum yield of sun-induced fluorescence in the Bering Sea: Effects of light and nutrients

This study examines temporal variability in the relationship between sun-induced fluorescence of near-surface phytoplankton (F) and light intensity (E). Parameters derived from the fluorescence-irradiance relationship promise to hold information on the physiological state of phytoplankton, which may be linked to nutrient availability. In particular, the initial slope of the F-E-relationship can be related to the dimensionless quantum yield of fluorescence (&phis;_F), a measure of the efficiency with which the phytoplankton fluoresce the absorbed light and possibly an indicator of physiological performance. The study site is an anticyclonic eddy in the southeastern Bering Sea with surface velocities of 0.2–0.5 m s−1 and a diameter of 120–150 km. An optical drifter was caught in this eddy for more than 100 days, providing a record of both its position and optical properties along its trajectory. Fluorescence parameters showed considerable variability over time with order of magnitude variations in both the initial slope of the fluorescence-irradiance relationship and maximum fluorescence, both normalized to account for absorbed radiation. These two parameters were found to covary, while the light-saturation parameter E_kF stayed relatively constant. There is a lagged correlation between eddy spinning dynamics and phytoplankton parameters, allowing for an interpretation in terms of nutrient availability. Slow rotational velocities of the eddy, which, on a theoretical basis, can be linked with low nutrient input, are associated with high initial slopes of the fluorescence-irradiance relationship. The observed relationship is consistent with the hypothesis that high quantum yields of sun-induced fluorescence are associated with nutrient stress in phytoplankton.