At least three floristically distinct communities of algae were distributed down the permanently ice-capped water column of Lake Vanda. Phytoplankton at each depth were highly specialized towards specific conditions of light and temperature. Maximum photosynthesis and algal biomass was at the bottom of the euphotic zone (55–57.5 m) immediately above a region of nutrient-rich anoxic water. This Phormidium-dominated community was adapted towards warm temperatures (18–20 °C) but a dim light regime. Dissolved inorganic N to P ratios in this region were extreme (> 5000:1) and the plankton demonstrated strong cellular P deficiency. A much smaller photosynthetic maximum was recorded in the middle of a large thermohaline convection cell at 25–30 m where P deficiency was less severe, but detectable. A microflagellate community immediately under the ice demonstrated the least response to phosphorus enrichment, but photosynthetic characteristics appeared to be more attuned to the low irradiances which prevail early in the growing season rather than to the relatively bright light regime during midsummer sampling. Algal biomass levels at most depths declined over 4 wk of sampling, further indicating that population maxima are timed early in the season. The low input of phosphorus relative to nitrogen exerts an overall control on phytoplankton biomass in Lake Vanda, but low production rates per unit biomass characterize the shade-adapted populations at all depths.Key words: phytoplankton, photosynthesis, algal biomass, N to P ratios, ice cap

The variability in physical, chemicaland biologicalpropertieswas examinedforanumberof glacier melt streams in south Victoria Land, Antarctica. Streams flowed for between one and two months. Stream water temperatures (range =0-11 "C) varied over short (hr) time scales whilst discharges varied considerably between streams (range 0.001-15 mJ s-1) and over die1 cycles. Solar radiation and air temperature were major determinants of stream discharge. Variability in discharge was reflected in variability in nutrient chemistry and sediment load. Nitrogen and phosphorus varied considerably between streams; the meltwaters early in summer contained 10-20 times higher levels of dissolved X and P than later in the season. Within stream nutrient levels were modified by dense algal growths and penguin rookeries. Epilithic algal communities were made up predominantly of cyanophyceae which formed mats and crusts. Longitudinal and horizontal variability of species in the communities in selected streams is described. Analyses of algal cover and biomass (chlorophyll a ) show that substrate type and flow rates are of greater importance than nutrients in influencing algal abundance and biomass. I n some streams biomass values of over 20 !ig Ch. n cni-2 were recorded. murh of which remains viable but inactive over the antarctic winter.

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