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 Ͻ1% 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 ϽϪ2Њ C and brine salinities elevated.

No abstract

Extreme environmental conditions have been thought to limit algal growth in the upper sea-ice. In McMurdo Sound, Antarctica, chrysophyte statocysts (stomatocysts) and dinoflagellate hypnozygotes (resting cysts) overwinter in $first-and secondyear land-fast sea-ice exposed to tmperatures of -20" C or lower. I n early November, when temperatures in the upper ice are <-8" C and bnne salinities are >126 p s~, dinoflagellate cysts activate and shortly thereafter excyst. During early November, chlysophyte statocysts also begin to excyst. Net daily przmary production occurs in the sea-ice bnne at temperatures as low as -7.1 O C, at brine salinities as high as 129 psu, and at average photon flux densities as low as 5 pmol photons.m-z.s-i. Dinoflagellate densities were >lo6 vegetative cells.L-' of ice while temperatures in the upper ice were between -6.8 and -5.8" C and brine salinities were -100 psu. Chrysophyte densities reached > 106.L-' of ice by ear4 December. High densities of physiologically active clyo-and halotolerant algae can occur in the upper landfast sea-ice under extreme conditions of temperature and salinity.

Observations and expenments were conducted on fast ice in McMurdo Sound, Antarctica, to investigate seasonal changes in primary production in the upper sea ice intenor. In November and early December 1995, a dense phytoflagellate assemblage developed in the brine channels and pockets at a snow-free site. Primary production was calculated from I4C measurements of primary productivity in brine samples combined with estimates of the proportion of the ice volume occupied by brine.On 4 December 1995, when the dinoflagellate PolareUa glacialis dominated, eshmated daily production peaked at 12.4 mg C m-' in the upper 50 cm of ice. On this date, brine temperature was --3°C and brine salinity was -60. By mid-December, daily production d e c h e d by 77 %, but chlorophyll-specific rates of photosynthesis remained high. The decline in production coincided with encystment of P. glacialis and nutnent deplehon, the former triggered by the latter. Pnmary production continued to decrease during December and January. On 9 January 1996, when ice temperatures were --1°C and brine salinlty was -20, there was a brief bloom of small pennate diatoms in the upper ice interior, but chlorophyll-specific rates of photosynthesis were low and estimated daily production was < l mg C m-' Based on I4C uptake and brine volume, algal production in the upper 50 cm of sea ice was 181 mg C m-* for the season (mid-November through mid-January). Increases in phytoflagellate biomass in the upper 90 cm of ice for this same period indicated that produchon was 2 5 6 mg C m-'. Bnef early season blooms of cryo-and halo-tolerant phytoflagellates accounted for most of the primary production in the upper sea ice interior.