PHYTOPLANKTON DYNAMICS IN THE STRATIFIED WATER COLUMN OF LAKE BONNEY,ANTARCTICA .1. BIOMASS AND PRODUCTIVITY DURING THE WINTER-SPRING TRANSITION

Phytoplankton populations in perennially ice-covered Lake Bonney, Anta rctica grow in a unique non-turbulent environment. The absence of turb ulence generated by winds or major streams, combined with strong verti cal gradients in temperature and nutrients, create vertically stratifi ed environmental conditions that support three discrete phytoplankton populations in the east lobe of this lake. Phytoplankton biomass and p hotosynthesis were measured in the east lobe of Lake Bonney during the winter-spring transition (September) to mid-summer (January). During this period, irradiance beneath the ice increased from 0.03 to 1.9 mol quanta m(-2) d(-1). Chlorophyll a concentrations ranged from 0.03 to 3.8 mu g l(-1) within the trophogenic zone (just beneath the permanent ice cover to 20 m) and photosynthesis ranged from below detection to 3.2 mu g C l(-1) d(-1). Our results indicate: (1) phytoplankton photos ynthesis began in late winter (before 9 September, our earliest sampli ng date); (2) maxima for phytoplankton biomass and production develope d sequentially in time from the top to the bottom of the trophogenic z one, following the seasonal increase in irradiance; and (3) the highes t photosynthetic efficiencies occurred in early spring, then decreased over the remainder of the phytoplankton growth season. The spring dec rease in photosynthetic rates for shallower phytoplankton appeared to be related to nutrient availability, while photosynthesis in the deepe r populations was solely light-dependent.