Primary production in the upper sea ice

Observations and experiments were conducted on fast ice in McMurdo Sound, Antarctica, to investigate seasonal changes in primary production in the upper sea ice interior. 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 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 dinoflagellate 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-December, daily production declined by 77%, but chlorophyll-specific rates of photosynthesis remained high. The decline in production coincided with encystment of P. glacialis and nutrient depletion, the former triggered by the latter. Primary production continued to decrease during December and January. On 9 January 1996, when ice temperatures were similar to-1 degrees C and brine salinity was similar to 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 <1 mg C m(-2). Based on C-14 uptake and brine volume, algal production in the upper 50 cm of sea ice was 181 mg C m(-2) 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 production was greater than or equal to 256 mg C m(-2). Brief early season blooms of cryo- and halo-tolerant phytoflagellates accounted for most of the primary production in the upper sea ice interior.