Primary production in the upper sea ice

Observations and experiments were conducted on fast ice in McMurdo Sound, A ntarctica, to investigate seasonal changes in primary production in the upp er sea ice interior. In November and early December 1995, a dense phytoflag ellate assemblage developed in the brine channels and pockets at a snow-fre e site. Primary production was calculated from C-14 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 dinoflagell ate Polarella glacialis dominated, estimated daily production peaked at 12. 4 mg C m(-2) in the upper 50 cm of ice. On this date, brine temperature was similar to-3 degrees C and brine salinity was similar to 60. By mid-Decemb er, daily production declined by 77%, but chlorophyll-specific rates of pho tosynthesis remained high. The decline in production coincided with encystm ent of P. glacialis and nutrient depletion, the former triggered by the lat ter. Primary production continued to decrease during December and January. On 9 January 1996, when ice temperatures were similar to-1 degrees C and br ine salinity was similar to 20, there was a brief bloom of small pennate di atoms in the upper ice interior, but chlorophyll-specific rates of photosyn thesis were low and estimated daily production was <1 mg C m(-2). Based on C-14 uptake and brine volume, algal production in the upper 50 cm of sea ic e was 181 mg C m(-2) for the season (mid-November through mid-January). Inc reases in phytoflagellate biomass in the upper 90 cm of ice for this same p eriod indicated that production was greater than or equal to 256 mg C m(-2) . Brief early season blooms of cryo- and halo-tolerant phytoflagellates acc ounted for most of the primary production in the upper sea ice interior.