The springtime stratospheric ozone (O ₃ ) layer over the Antarctic is thinning by as much as 50 percent, resulting in increased midultraviolet (UVB) radiation reaching the surface of the Southern Ocean. There is concern that phytoplankton communities confined to near-surface waters of the marginal ice zone will be harmed by increased UVB irradiance penetrating the ocean surface, thereby altering the dynamics of Antarctic marine ecosystems. Results from a 6-week cruise (Icecolors) in the marginal ice zone of the Bellingshausen Sea in austral spring of 1990 indicated that as the O ₃ layer thinned: (i) sea surface- and depth-dependent ratios of UVB irradiance (280 to 320 nanometers) to total irradiance (280 to 700 nanometers) increased and (ii) UVB inhibition of photosynthesis increased. These and other Icecolors findings suggest that O ₃ -dependent shifts of in-water spectral irradiances alter the balance of spectrally dependent phytoplankton processes, including photoinhibition, photoreactivation, photoprotection, and photosynthesis. A minimum 6 to 12 percent reduction in primary production associated with O ₃ depletion was estimated for the duration of the cruise.

Algal chlorophyll and carotenoid distributions were measured periodically in the euphotic zone of Sta. ALOHA (22°45'N, 158°00'W) between February 1989 and October 1991 to document the variability in phytoplankton abundance and composition. The annual mean depth‐integrated (0–200 db) concentration of Chl a displayed significant interannual variability. Seasonal patterns in Chl a concentration were found to be depth‐dependent. Elevated Chl a in the mixed layer is the result of photoadaptation as the mixed layer deepens in winter. Increases in Chl a at the deep chlorophyll maximum layer (DCML) in spring are explained by increased nutrient availability caused by a deepening of the DCML relative to the σ_θ = 24.25 density surface.An algorithm based on the ratios of Chl a to diagnostic pigments present in specific algal taxa was used to estimate the contribution to total Chl a by the major algal groups represented within the DCML. Results indicate the presence of a phytoplankton community at the DCML with the following mean composition: Prochlorococcus spp. (39%), cyanobacteria (24%), prymnesiophytes (22%),and chrysophytes (13%). No single taxon is responsible for the springtime increase in Chl a observed in this habitat. Results from size fractionation and normal‐phase high performance liquid chromatography confirm that Prochlorococcus spp. are the principal contributors of Chl a to the DCML.