POPULATION DYNAMICS OF MICROALGAE IN THE UPPER LAND-FAST SEA ICE AT A SNOW-FREE LOCATION

The population dynamics of interior ice microalgae were investigated at a snow-free site on annual land-fast sea ice in McMurdo Sound, Antarctica, during the austral spring and summer of 1995 – 96. A dynamic successional sequence was observed with life history transformations playing an important role. During late November and early December (austral spring), cryo- and halotolerant dinoflagellates and chrysophytes bloomed in brine channels within the upper ice. At this time, competition and grazing pressure are low because of the inability of most marine species to grow under the extreme environmental conditions found in the upper ice during the austral spring. In November and December, dinoflagellates, chrysophytes, and prasinophytes contributed an average of 66%, 44%, and < 1% of the phytoflagellate biomass, respectively. Both the dinoflagellates and the chrysophytes encysted in December, with cyst formation most intense just prior to surface melt and flushing of the ice. The cysts appear to be an adaptation for survival and dispersal in the plankton during ice decay and/or overwintering in the sea ice. In January (austral summer), when ice temperatures were similar to those in the water column, pennate diatoms replaced flagellates as the photosynthetic dominants in the upper sea ice. The upper land-fast sea ice undergoes dramatic seasonal changes in light availability, temperature, brine salinity, and inorganic nutrient availability. Ephemeral blooms of cyst-forming phytoflagellates exploit this habitat in the austral spring, when both inorganic nutrients and light are available but temperatures