Late winter primary production and bacterial production in sea ice and seawater west of the Antarctic Peninsula

Abstract

The Southern Ocean is believed to be unproductive during winter due pnncipally to low irradiance. On the 1985 Wintercruise of the R N Polar Duke, considerable microbial blomass and rates of primary produchon and bactenal production were found in sea ice up to 1.79 m thick. Microbial activity associated with sea ice was equal to that found in several meters of underlying seawater. Downwelling irradiance was adequate for net production near the surface of ice-free water and in sea ice. Approximately 40 % of the newly fixed carbon incorporated by ice microalgae was assimilated into protein, suggesting that net growth was taking place without nutrient limitation. We propose that annual estimates of primary production should be revised upward by as much as 25 % to account for this unexpected productivity during late winter in the Southern Ocean. In addition, sea ice should be viewed as a concentrated source of microalgal carbon for grazers such as krill during late winter when phytoplankton in the water column are scarce. In situ observations by divers suggest that sea ice may also serve as an important nursery ground for larval knll during this time of year. We conclude that both the quantity of sea ice associated production and seasonal timing of this production are important factors in Antarctic trophodynamics.