Ab STRACT: Spectral irradiance, in vivo absorption spectra and quantum yield of photosynthesis (Q) were measured in December 1984 for both congelation ice and platelet ice microalgae and for a bloom of the planktonic prymnesiophyte Phaeocystis pouchetii from McMurdo Sound. Antarctica. Profiles of spectral irradiance through the ice column demonstrated that the irradiance environment of sea ice was vertically and horizontally heterogeneous, changing from blue-dominated to green-dominated with depth in the column, and varying from site to site depending on snow cover and ice algal patchiness. In response to reductions in irradiance, platelet Ice microalgae consistently showed enhanced absorption of blue-green light relative to congelation ice microalgae. Samples of the planktonic alga P. pouchetii from under the seasonal fast ice of McMurdo Sound also exhibited enhanced blue-green absorption relative to samples from open waters of the Ross Sea. The mean specific absorption coefficient, "a, for sea ice microalgae ranged between 0.0058 and 0.0097 m2 (mg chl a)-', values characteristic of microalgae in green productive waters. For P. pouchetii, "a was greatest for samples taken from open water at the ice edge (0.015 to 0.029 m2 [mg chl a]-'), and decreased to about 0.008 m2 (mg chl a)-' for samples taken from under the seasonal ice of McMurdo Sound. The quantum yield of photosynthesis for these microalgae ranged from 0.013 to 0.055 m01 C (Ein absorbed)-', and no significant differences in cP were found between congelation ice and platelet ice algae.

Changes in spectral irrad~ance beneath annual sea ice were measured during the development of sea ice microalgal communities in McMurdo Sound, Antarctica. Five different light regimes were initially estabhshed by varying surface snow cover on 10 m X 10 m sea ice quadrats. The presence of ice algae in quadrats w t h 6 5 cm snow cover was indicated by a spectral shift with increased attenuation between 400 and 550 nm and at 671 nm, wavelengths absorbed by diatom pigments. Snow cover had a profound effect on both the rate of community development and community loss by ice ablation. A simple model of factors determining changes in ice algal biomass is described.