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 ma xima commonly observed in lakes and seas. We measured natural fluoresc ence flux rates, chlorophyll concentration (Chl a), phytoplankton abso rption spectra (a(ph), photosynthetic efficiency, and spectral irradia nce to derive the quantum yields for photosynthesis (PHI(c)) and fluor escence (PHI(f)). Chlorophyll concentrations predicted from natural fl uorescence 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 sig nificantly with measured Chl a (n = 122, r = 0.88). Predictions of pri mary productivity from natural fluorescence based on mean values for P HI(f) and PHI(c) were poor. Relationships between PHI(c):PHI(f) and te mperature and between PHI(c):PHI(f) and irradiance implied that these environmental variables would not provide good bases for correcting pr edictions of primary production. PHI(c):PHI(f) varied most coherently with distance from the nutricline, due primarily to a large increase i n maximum PHI(c) [0.0015-0.051 mol C (mol photons)-1] with proximity t o the nutricline. Our results indicate that nutrient supply may be a c ritical variable to consider when using natural fluorescence methods t o estimate primary productivity in vertically stable phytoplankton.