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

The population dynamics of interior ice microalgae were investigated a t a snow-free site on annual land-fast sea ice in McMurdo Sound, Antar ctica, during the austral spring and summer of 1995-96. A dynamic succ essional sequence was observed with life history transformations playi ng an important role. During late November and early December (austral spring), cryo- and halotolerant dinoflagellates and chrysophytes bloo med in brine channels within the upper ice. At this time, competition and grazing pressure are low because of the inability of most marine s pecies to grow under the extreme environmental conditions found in the upper ice during the austral spring. In November and December, dinofl agellates, chrysophytes, and prasinophytes contributed an average 66%, 44%, and <1% of the of the phytoflagellate biomass, respectively. Bot h the dinoflagellates and the chrysophytes encysted in December, and f lushing 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 (astral summer), when ice temperatures were s imilar to those in the water column, pennate diatoms replaced flagella tes as the photosynthetic dominants in the upper sea ice. The upper la nd-fast sea ice undergoes dramatic seasonal changes in light availabil ity, temperature, brine salinity, and inorganic nutrient availability. Ephemeral blooms of cyst-forming phytoflagellates exploit this habita t in the austral spring, when both inorganic nutrients and light are a vailable but temperatures <-2 degrees C and brine salinities elevated.