Winter ecology of the sea ice biota in Weddell Sea pack ice

During winter 1988, we examined the ice community in the ice edge region of the Weddell and Scotia Seas. We measured chemical and physical characteristics of the ice habitat, chlorophyll a (chl a), particulate organic carbon and nitrogen (POC and PON) and ATP. We also analyzed the composition and biomass of the ice biota by microscopy. Air temperature during the study ranged from above freezing to as low as −18 °C. Large fluctuations over a few days were common. Temperature at the ice surface generally followed air temperature, but with a short lag period. As a result of low temperatures at the ice surface, in situ salinity in the upper layer of ice floes reached > 100 ‰. Samples were taken from newly forming, young, first-year and older sea ice. Ice floes had variable amounts of snow cover. Floes were primarily comprised of congelation ice (56 %) but also contained significant amounts of frazil ice (41 %). Chl a ranged from 29.0 mg chl a m−2. Total integrated chlorophyll as well as chlorophyll concentrations and integrated POC, PON and ATP generally increased with increasing ice age or thickness. High C:chl a, C:N and C:ATP ratios characterized all ice types and suggested substantial detritus in the ice. The ice biota was comprised of bacteria, algae, protozoans and some metazoa. Microscopically estimated biomass in floes ranged from 1000 mg C m−2, with the highest values from older ice floes. Estimates of carbon calculated from ATP showed good agreement with estimates derived from microscopy. The high concentrations of living organisms and detritus in sea ice suggest the potential importance of the ice community to the pelagic system particularly during the winter. The source of unexpectedly high concentrations of detritus, at least in young sea ice, is uncertain. The winter ice assemblage did not differ markedly from the assemblages found during other seasons, and overall the seasonal biomass variation within the pack ice community appears to be low. Resting stages such as archaeomonads and dinoflagellate cysts were common in the ice, and cyst formation for the dinoflagellates appears to take place during the winter as well as in the late summer. Although earlier studies have emphasized the importance of harvesting and concentration of organisms from the water during episodes of frazil ice formation, we did not see evidence for this from our analysis of biomass associated with different structural types of ice. The initial physical con