Résumé
Although winter conditions play a major role in determining the productivity of the western Antarctic Peninsula (WAP) waters for the following spring and summer, a few studies have dealt with the seasonal variability of microorganisms in the WAP in winter. Moreover, because of regional warming, sea-ice retreat is happening earlier in spring, at the onset of the production season. In this context, this study describes the dynamics of the marine microbial community in the Melchior Archipelago (WAP) from fall to spring 2006. Samples were collected monthly to biweekly at four depths from the surface to the aphotic layer. The abundance and carbon content of bacteria, phytoplankton and microzooplankton were analyzed using flow cytometry and inverted microscopy, and bacterial richness was examined by PCR-DGGE. As expected, due to the extreme environmental conditions, the microbial community abundance and biomass were low in fall and winter. Bacterial abundance ranged from 1.2 to 2.8 x 10(5) cells ml(-1) showing a slight increase in spring. Phytoplankton biomass was low and dominated by small cells (< 2 mu m) in fall and winter (average chlorophyll a concentration, Chl-a, of, respectively, 0.3 and 0.13 mu g l(-1)). Phytoplankton biomass increased in spring (Chl-a up to 1.13 mu g l(-1)), and, despite potentially adequate growth conditions, this rise was small and phytoplankton was still dominated by small cells (2-20 mu m). In addition, the early disappearing of sea-ice in spring 2006 let the surface water exposed to ultraviolet B radiations (UVBR, 280-320 nm), which seemed to have a negative impact on the microbial community in surface waters.