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.

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