PHOTOADAPTATION,GROWTH AND PRODUCTION OF BOTTOM ICE ALGAE IN THE ANTARCTIC1

Biomass, chemical composition, growth rates and the photosynthetic response of natural populations of sea ice algae in McMurdo Sound, Antarctica were followed over most of the spring bloom to examine temporal variability under a relatively constant incident irradiance (ca. 1500–1700 μE · m^-2· s^-1 at solar noon). Collection were restricted to bottom 20 cm of the ice sheet in an area with little or no snow (0–5 cm). At low temperature and irradiance these algae normally exhibited low assimilation numbers (ca. 0.1–0.4 mg C · mg Chl^-1· h^-1). Average growth rates (0.02–0.45 d^-1), based on changes in standing stocks, were also low. Biomass, biochemical composition, growth rates, assimilation numbers and photosynthetic efficiencies (mg C · mg Chl^-1· h^-1 (μE · m^-2· s^-1)^-1) displayed large fluctuations over periods of several days during the growth season. On the other hand, I_k which is an index of photoadaptation, and I_m, the optimal irradiance for photosynthesis, were relatively constant with less than twofold variation throughout our study. Substantial nutrient fluxes (3.3–8.0 mmol Si or N · m^-2· d^-1) were necessary to satisfy the minimum nutrient demand for the observed biomass levels and population growth rates; over the 41 days of our study, integrated nutrient demand represented 69–150 mmol N or Si · m^-2, Only 5–25% of this total demand could be met by all of the nutrients in the ice sheet, if they were readily available. However, adequate amounts were present in the top few meters of the water column. With small nutrient gradients in surface waters below the sea ice, vertical eddy diffusivities on the order of 3.8–9.3 cm²· s^- should supply sufficient nutrients to meet algal demand.