Primary production in the upper sea ice

Abstract

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 cryoand halo-tolerant phytoflagellates accounted for most of the primary production in the upper sea ice interior.