NATURAL FLUORESCENCE AND QUANTUM YIELDS IN VERTICALLY STATIONARY PHYTOPLANKTON FROM PERENNIALLY ICE-COVERED LAKES

Phytoplankton in ice-covered lakes near McMurdo Sound, Antarctica, are stratified vertically in distributions similar to deep chlorophyll maxima commonly observed in lakes and seas. We measured natural fluorescence flux rates, chlorophyll concentration (Chl a), phytoplankton absorption spectra (a(ph), photosynthetic efficiency, and spectral irradiance to derive the quantum yields for photosynthesis (PHI(c)) and fluorescence (PHI(f)). Chlorophyll concentrations predicted from natural fluorescence based on mean a(ph) [0.015 m2 (mg Chl a)-1] and mean PHI(f)[0.044 mol photons fluoresced (mol photons absorbed)-1] correlated significantly with measured Chl a (n = 122, r = 0.88). Predictions of primary productivity from natural fluorescence based on mean values for PHI(f) and PHI(c) were poor. Relationships between PHI(c):PHI(f) and temperature and between PHI(c):PHI(f) and irradiance implied that these environmental variables would not provide good bases for correcting predictions of primary production. PHI(c):PHI(f) varied most coherently with distance from the nutricline, due primarily to a large increase in maximum PHI(c) [0.0015-0.051 mol C (mol photons)-1] with proximity to the nutricline. Our results indicate that nutrient supply may be a critical variable to consider when using natural fluorescence methods to estimate primary productivity in vertically stable phytoplankton.