Development of sea ice microbial communities during autumn ice formation in the Ross Sea

Sea ice communities were sampled across the Ross Sea in the austral autumn. The biota in first-year pack ice was assessed by measuring chlorophyll a (chl a), phaeopigments, total particulate carbon and nitrogen (POC and PON, respectively) and collecting samples for identification by microscopy. Physical and chemical parameters were also measured to characterize the environment. Chl a concentrations in ice ranged from 0 to 96.9 mug l(-1) in discrete samples and from 0.02 to 20.9 mg m(-2) for values integrated throughout floes. Maximum values were similar to those observed in first-year pack ice at other Antarctic locations. Chl a concentrations varied with ice structure and with latitude. POC:chl a and C:N ratios (molar) were high, possibly indicating detritus accumulations. The higher chl a levels north of approximately 72degreesS were apparently a result of ice forming in the south early in the season with subsequent advection to the north. These dynamics would result in older ice in the mid- or northern pack ice zone that was maintained in a favorable light and temperature regime during the seasonal progression of formation and drift. Chlorophyll levels were low in surf ace-layer communities. High chlorophyll concentrations were associated with internal communities. Bottom-layer algal populations, while present, did not reach the levels of high biomass reported for autumn blooms in some land-fast ice regions. Apparent nutrient and CO2 depletion were correlated with biomass parameters but accounted qualitatively for only a fraction of the biomass accumulation measured. Overall, autumn ice-associated production in the Ross Sea may be lower than expected because of the ice drift dynamics, apparently low production in the near-surface layers of first year ice flows, and the absence of rich bottom-layer assemblages.