A study of the diatom-dominated microplankton summer assemblages in coastal waters from Terre Adélie to the Mertz Glacier, East Antarctica (139°E–145°E)

In January 2004 the microplankton community from the coastal waters of Terre Adélie and Georges V Land (139°E - 145°E) was studied. Results showed a diatom-dominated bloom with chlorophyll a levels averaging 0.64 µg.l-1 at 5m depth (range 0.21 - 1.57 µg.l-1). Three geographic assemblages of diatoms were identified, based on principal diatom taxa abundances. The stratified waters near the Mertz Glacier presented highest phytoplankton biomasses (0.28 - 1.57 µg Chl a.l-1 at 5m) and diatom abundances (6 507 - 70 274 cells.l-1 at 5m), but low diversity, dominated by Fragilariopsis spp. Lower biomasses (0.38 – 0.94 µg Chl a.l-1 at 5m) and abundances (394 – 9 058 cells.l-1 at 5m) were observed in the mixed waters around the Astrolabe Glacier with a diverse diatom community characterised by larger species Corethron pennatum and Rhizosolenia spp. Finally an intermediate zone between them over the shallower shelf waters of the Adélie Bank represented by Chaetoceros criophilus, where biomasses (0.21 - 0.35 µg Chl a.l-1 at 5m) and abundances (1 190 - 5 431 cells.l-1 at 5m) were lowest, coinciding with the presence of abundant herbivorous zooplankton.Pelagant, contract EV/12/30

Role of sea ice in global biogeochemical cycles: Emerging views and challenges

Observations from the last decade suggest an important role of sea ice in the global biogeochemical cycles, promoted by (i) active biological and chemical processes within the sea ice; (ii) fluid and gas exchanges at the sea ice interface through an often permeable sea ice cover; and (iii) tight physical, biological and chemical interactions between the sea ice, the ocean and the atmosphere. Photosynthetic micro-organisms in sea ice thrive in liquid brine inclusions encased in a pure ice matrix, where they find suitable light and nutrient levels. They extend the production season, provide a winter and early spring food source, and contribute to organic carbon export to depth. Under-ice and ice edge phytoplankton blooms occur when ice retreats, favoured by increasing light, stratification, and by the release of material into the water column. In particular, the release of iron – highly concentrated in sea ice – could have large effects in the iron-limited Southern Ocean. The export of inorganic carbon transport by brine sinking below the mixed layer, calcium carbonate precipitation in sea ice, as well as active iceatmosphere carbon dioxide (CO2) fluxes, could play a central role in the marine carbon cycle. Sea ice processes could also significantly contribute to the sulphur cycle through the large production by ice algae of dimethylsulfoniopropionate (DMSP), the precursor of sulfate aerosols, which as cloud condensation nuclei have a potential cooling effect on the planet. Finally, the sea ice zone supports significant ocean-atmosphere methane (CH4) fluxes, while saline ice surfaces activate springtime atmospheric bromine chemistry, setting ground for tropospheric ozone depletion events observed near both poles. All these mechanisms are generally known, but neither precisely understood nor quantified at large scales. As polar regions are rapidly changing, understanding the large-scale polar marine biogeochemical processes and their future evolution is of high priority. Earth system models should in this context prove essential, but they currently represent sea ice as biologically and chemically inert. Paleoclimatic proxies are also relevant, in particular the sea ice proxies, inferring past sea ice conditions from glacial and marine sediment core records and providing analogs for future changes. Being highly constrained by marine biogeochemistry, sea ice proxies would not only contribute to but also benefit from a better understanding of polar marine biogeochemical cycles