The composition of sea ice communities in the Ross Sea region was examined during the autumn to winter transition and during the summer. The biomass of autotrophs and heterotrophs in autumn reached maximum values of 709 and 167 mg C m , respectively. During the autumn-winter cruise, most of the biomass was found within ice floes as interior and bottom layer communities. During summer, surface-layer slush communities occurred throughout the ice-covered regions. The biomass was highly variable throughout the study regions during both cruises. Diatoms dominated the autotrophic biomass; however, autotrophic dinoflagellates and autotrophic flagellates contributed significantly to the community make-up. Among heterotrophs, ciliates predominated during both cruises, followed by heterotrophic flagellates and heterotrophic dinoflagellates. Similarity analysis, based on the biomass composition of major groups, showed consistency between and within cruises, with most samples > 70% similar. The autumn to winter samples (all from within floes) showed higher similarity clusters that could be related to changing compositions of diatoms, ciliates, and autotrophic dinoflagellates. Most variable were some summer surface slush samples, where samples dominated by Phaeocystis, Pyramimonas, Gymnodinium, and the ciliate Gymnozoum formed outlying clusters. The dynamics of ice biota may be determined by relatively few taxa that are persistently found within the ice floes. Surface blooms may develop either from a biota within the ice or from opportunistic forms from the water column that are introduced during flooding events. Thus, these assemblages may show considerably more variability in composition than those that develop within the underlying ice. KEY WORDS: Sea ice community · Antarctic · ProtistsResale or republication not permitted without written consent of the publisher

The physical, chemical and biological characteristics of surface and freeboard habitats in the summer pack ice in the eastern Ross Sea, Antarctica, were documented in a continuing effort to determine the factors controlling the distribution, production and succession of sea-ice biota. Three longitudinal transects from approximately 65³ to 74³ S in the western Ross Sea along135³,150³ and 165³ W were visited where samples of slush and slush interstitial water from surface and freeboard habitats as well as sea water were collected at every degree of latitude. Freeboard and surface habitats, found at all stations in the pack ice, contained a large range (five orders of magnitude) of microalgal biomass (measured as chlorophyll a concentrations) and nutrients ranging from below levels of detection to those of the surrounding sea water. The geophysical attributes of the freeboard habitat (i.e. a layer of semi-consolidated ice overlying a layer containing unconsolidated ice crystals and sea water) are consistent with previous descriptions of this environment. However, additional information is presented on the range of biomass concentrations as well as the small-scale distributions of the habitat and biota.

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 µg 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 72°S 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 surface-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 CO 2 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.

Cyanobacteria are important primary producers in many marine ecosystems and their abundances and growth rates depend on their ability to assimilate various nitrogen sources. To examine the diversity of nitrate-utilizing marine cyanobacteria, we developed PCR primers specific for cyanobacterial assimilatory nitrate reductase (narB) genes. We obtained amplification products from diverse strains of cultivated cyanobacteria and from several marine environments. Phylogenetic trees constructed with the narB gene are congruent with those based on ribosomal RNA genes and RNA polymerase genes. Analysis of sequence library data from coastal and oligotrophic marine environments shows distinct groups of Synechococcus sp. in each environment; some of which are represented by sequences from cultivated organisms and others that are unrelated to known sequences and likely represent novel phylogenetic groups. We observed spatial differences in the distribution of sequences between two sites in Monterey Bay and differences in the vertical distribution of sequence types at the Hawai'i Ocean Time-series Station ALOHA, suggesting that nitrogen assimilation in Synechococcus living in different ecological niches can be followed with the nitrate reductase gene.

Abundance of single, non-attached bacteria and viruses <110 nm (likely to have infected bacteria) were determined for surface, interior and bottom ice microhabitats between 66 and 75°S in the Ross Sea during the austral summer of 1999. Emphasis was on sites of ice algal blooms, and bacterial abundance was examined with respect to physical characteristics, chlorophyll a, phaeophytin and, in particular, microbial communities. Bacterial abundance ranged from 1.5 × 10 5 to 6.7 × 10 6 ml -1 melted sample and viral abundance from 6.3 × 10 6 to 1.19 × 10 8 ml -1 melted sample over all the microhabitats. Neither bacterial nor viral abundance differed among microhabitats, and bacterial abundance was not related to physical characteristics of the habitats. Although bacterial abundance was positively correlated with chlorophyll a and phaeophytin concentrations, only chlorophyll a was significant in explaining a small (28%) degree of the variability in bacterial abundance. Abundance of diatoms, heterotrophic dinoflagellates and other flagellates, however, explained 85% of the variability in bacterial abundance; these groups were positively correlated with bacterial abundance. Neither viral lysis nor grazing by bacteriovores appeared to be strong controls of bacterial abundance. Community analysis showed that samples were > 90% similar with respect to abundance of bacteria, viruses, and microeukaryote groups. Distinct clusters could be attributed to different algal bloom stages, with relationships to Phaeocystis spp. blooms being particularly apparent, indicating the strength of algal blooms as processes structuring microbial communities. Microbial communities in Ross Sea summer ice microhabitats could furthermore be viewed within the same general successional sequence characteristic of algal blooms in polar and temperate marine waters.