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.

M3STRACT Nitrapynn, an lnh~bitor of NH; oxldlz~ng bactena was used to estlmate the a c t~v~t y of NH: oxldlzing bacteria in the bottom 1 to 15 cm of annual sea Ice and in the water column at vanous locations In McMurdo Sound and along the Ross Ice Shelf (RIS) N~t r a p y r~n s~gn~flcantly ~n h~b i t e d dark ' 4 C -H C 0 3 uptake in virtually all sea-ice samples, indlcahng the presence of NH: oxld~zlng bacteria Inorganic carbon futahon by sea ice NH: o x~d~z e r s was only a small fract~on of that futed by sea-lce photoautotrophs, both on an hourly and annual b a s~s Despite their relative lack of ~mportance to lnorganlc carbon fixation NH: oxidizing bactena may have an important role in the N d y n a m~c s w~t h l n the b~o g e n i c layer of annual sea Ice both In terms of N H~ utlllzatlon and eventual NO; product~on Inorgamc carbon futatlon in the water column beneath sea ice was generally not i n h~b~t e d s~gnif~cantly by nitrapynn NH: oxldizer actlvlty was also not detectable in deep water flow~ng beneath (southward) or from under (northward) the RIS N 2 0 (a by-product of NH: oxidation) levels in p e l a g~c samples were always near 100 % of saturation w t h respect to the alr above the sea surface corroborating the low levels of NH:ox~dizer activity found In the water column

The response of photosynthesis, and of the uptake of NO3-. NH4+ and serine, to irradiance was measured in diatom-dominated sea ice microbial assemblages from bottom ice and surface ice of McMurdo Sound, Antarctica. Uptake responses for dissolved inorganic nitrogen (DIN; N o 3 -and NH4+) could be fitted to a standard model used for photosynthesis after the addtion of a dark uptake parameter; serine uptake showed no dependence on irradiance. The derived uptake models were used to predict the patterns of photosynthesis and DIN uptake over die1 irradiance cycles. According to model predictions, uptake rates in the bottom ice assemblage were always limited by irradiance; neither light saturation nor photoinhibition regulated photosynthesis or DIN utilization in this assemblage. Conversely, photosynthesis in the surface ice assemblage was nearly always light-saturated, whereas DIN uptake was photo-inhibited near midday and saturated at the minimum irradance. Integrated daily C : DIN uptake ratios (g:g) in the bottom ice and surface ice assemblages were 8.6 and 9.7, respectively, corresponding to particulate C:N ratios (g:g) of 8.1 and 5.8 for these respective diatom-dominated communities. Our results indicate that information on d e l patterns of photosynthesis and N uptake is required to evaluate accurately the stoichiometric balance of essential elements in sea ice microalgae.