9315027 Lessard The growing season for phytoplankton in polar oceans is short, but intense. There is an increasing body of evidence that in many Antarctic habitats, the most active period may be very early in the season, a period that has not been emphasized in previous investigations. This project is part of an interdisciplinary program that focuses on the dynamics of the spring phytoplankton bloom in a highly productive subsystem of the Antarctic, the Ross Sea. The overall program will test hypotheses related to the initiation of the phytoplankton bloom shortly after the onset of ice melt, the mechanisms controlling phytoplankton growth and productivity in spring, the implications and short-term fate of high productivity in spring, and the transition from spring to midsummer conditions. The focus of this proposal is the role of microzooplankton in controlling the production and fate of carbon during the two types of blooms. Objectives are: 1) to determine biomass, abundance, size and selected species composition of primary producer assemblages, 2) to determine similar features of nano- and microplanktonic heterotrophic assemblages, 3) to measure total community grazing on heterotrophic bacteria and phytoplankton, 4) to examine which grazers are the major herbivores and bacterivores, and 5) to measure the contribution of microzooplankton and mesozooplankton egesta, sinking of algal cells and colonies, and sinking of protozoan assemblages associated with detritus to the total carbon flux from the euphotic zone through 250 m depth. Water samples for abundance and biomass determinations will be taken and samples will be examined with epifluorescence microscopy. Grazing rates will be measured using the dilution grazing technique and the dual- isotope radiolabeling single cell method. Carbon fluxes will be determined on sinking material collected with particle interceptor traps at the base of the euphotic zone and two deeper depths, using microscopical analys is. An understanding of these processes and other fundamental processes studied by collaborating investigators will contribute to the understanding of the role of the Southern Ocean in present, past and predicted future sequestration of carbon, as well as in other global elemental cycles.
