Production Strategies in Antarctic Inland Waters: Phytoplankton Eco-Physiology in a Permanently Ice-Covered Lake.

Three distinct population strategies were observed within the summer algal plankton of Lake Fryxell (Taylor Valley, South Victoria Land, 77°35'S, 163°15'E). Phytoplankton immediately under the ice (Ochromonas and Chlamydomonas) were adapted to relatively bright light but were limited by nitrogen availability. A deep maximum of Chroomonas and Pyramimonas was recorded at the bottom of the euphotic zone. This community did not respond to nitrogen or phosphorus enrichment. It was highly shade adapted but at this depth ambient irradiance was below that required to saturate photosynthesis. Net population increases in both the upper and lower euphotic communities occurred very early in the season. Flagellated algae in the middle of the oxygenated water column swam up to depths of greater light during the day and returned to lower depths of greater nutrient supply at night. These mid-euphotic populations continued to grow throughout midsummer. Comparisons with other Dry Valley lakes suggest that nutrient supply, rather than in situ light or temperature, determines the large lake-to-lake and depth variations in primary productivity. Nutrient availability appears to control algal biomass, but in contrast to arctic ecosystems, low light rather than low temperature dampens algal photosynthesis to cellular rates that are well below those recorded at lower latitudes. The complexity of production strategies in Lake Fryxell, and the occurrence of population maxima early in the season when sampling is logistically difficult, challenge the view that the desert lakes of Antarctica offer ideal and simple systems for limnological study. [ABSTRACT FROM AUTHOR]