ABSTRACT: Measurements of spectral scalar irradiance were made in a shallow estuarine system during a phytoplankton bloom in summer 1994. High concentrations of dissolved organic matter (DOM) and pigments resulted in strong attenuation of light within the upper 1 m of the water column. Spectral analysis showed an increase in the ratio of far-red (>700 nm) irradiance to total photosynthetically available radiation (PAR) as depth increased. Under these conditions it is expected that state transition may occur, as far-red light is preferentially absorbed by Photosystem 1 (PS1). Measurements of fluorescence emission at 77 from natural phytoplankton samples as well as from laboratory-grown unialgal cultures showed that redistribution of excitation energy between the photosystems occurred in response to changes in the spectral light environment. Exposure to the far-red-enriched lower euphotic zone induced State 1 (whereby more light energy was directed towards Photosystem 2, PS2), whereas algae reverted to State 2 (redirection of light energy away from PS2) at the surface or after exposure to near-dark conditions at the bottom of the euphotic zone. Differences in the kinetics of state transition between green algae and cyanobacteria together with estimation of the wind-induced mixing time of the water column are discussed with respect to photoacclimation. For both natural phytoplankton and laboratory cultures, state transition occurred on a time scale equal to or faster than the time required for 1 vertical mixing cycle. The faster response of cyanobacteria to changes in spectral irradiance in the water column may be an important factor contributing to cyanobacterial dominance in hypertrophic waters under light-limited conditions.

KEY WORDS: Algal blooms . Cyanobacteria . State transition . Photosynthesis . Vertical mixing . Underwater light . Acclimation . Light-harvesting pigments . Low temperature fluorescence emission

Published in MEPS Vol. 128 (1995) on November 23
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1995