How are Antarctic planktonic microbial food webs and algal blooms affected by melting of sea ice? Microcosm simulations

The influence of the melting of Antarctic sea ice on the planktonic microbial food web and the formation of microalgal blooms was investigated under controlled laboratory conditions in 20 1 microcosms at the onset of austral spring in the southern Atlantic. Experiments were performed with first-year ice and water from the Antarctic Circumpolar Current (ACC) and the ACC-Weddell Gyre Boundary (AWB) at 6 degrees W, a region usually lacking ice-edge blooms in austral spring. The experiments comprised the following simulations: (1) stratification of the water column, (2) seeding of sympagic organisms, (3) inoculation of dissolved organic matter from sea ice and (4) grazing impact of mesozooplankton (Calanoides acutus). Microcosms were sampled almost daily over a 2 wk period. The samples were analyzed for inorganic nutrient concentrations, primary and bacterial production, abundances of algae (in the size classes <20 and >20 mu m), bacteria and heterotrophic flagellates (in the size classes <2, 2 to 10 and 10 to 20 mu m). Additionally, maximum uptake rates of glucose and aminopeptidase activity were measured. Stratification alone did not induce bloom formation within the time scale of the experiments. Simulated seeding of sympagic organisms provoked a strong increase in abundances and activities of auto- and heterotrophic microbes and the development of microalgal blooms (cells >20 mu m). Primary and bacterial production was enhanced up to 28- and 24-fold, respectively, compared to the control, while abundances increased by a factor of about 20 for microalgae and 12 for bacteria. Initial high activities of auto- and heterotrophs were restricted to sympagic organisms, which maintained their high metabolic rates for several days. Addition of dissolved organic substances mainly stimulated the pelagic bacterial populations. Their production was enhanced up to 12-fold compared to the control, while autotrophs were virtually unaffected. In the presence of mesozooplankton, bacterial blooms occurred but no accumulation of nano- and microalgae. Thus, this region of the Atlantic sector of the Southern Ocean does have a strong potential for the formation of ice-edge blooms, but phytoplankton accumulation is efficiently controlled by grazing activity. Bacterial blooms may be more common during ice melt. These findings agree with remote-sensing data, which show no bloom formation during ice melt in this area.