Seasonal changes in microbial biomass in the first-year ice of the Terre Ad Adelie area (Antarctica)

The coastal sea ice in the vicinity of Dumont d'Urville station, Antarctica (66degrees 40' S, 140degrees01'E) supports a diverse microbial community. To investigate seasonal changes in bacterial, microalgal and protozoan biomass during the ice-coverage period, a survey was conducted on fast ice in the continental shelf of Terre Adelie. A reference station was sampled twice a month from April to December 1997. In April and May, during sea ice formation, the autotrophic biomass reached relatively high levels ranging from 400 to 800 mgC m(-3) in the subsurface ice. A second increase reaching 1500 mgC m(-3) in mid-December occurred in early summer in the bottom ice layer. Bacterial biomass ranged from < 10 mg C m(-3) in August and September to > 200 Mg C m(-3) in May. Maximal levels of bacterial biomass were detected during ice formation and just before the summer thaw. Total protozoan biomass increased from May to July. Maximum protozoan biomass (31.8 mgC Mm(3)) occurred in the surface ice layer in July. Except for during a short period (late May to early June), ciliates were dominant, accounting for 50 to 77% of the total protozoan biomass. During the ice-covered period, phototrophic biomass was always dominant. Microalgal biomass contributed on average 80.6% of total biomass whereas bacterial and protozoan biomass accounted for only 16.4 and 3%, respectively. Despite their low biomass, protozoa seem to play a major role in bacterial regulation.