Planktonic interactions and particulate flux in Ellis Fjord, East Antarctica

The Southern Ocean is one of the largest marine ecosystems in the world, and is a major sink for atmospheric carbon dioxide. Thus, it plays a considerable role in the mitigation of the greenhouse effect and resultant global warming. Marine micro-organisms dominate the plankton biomass in Antarctic waters, consume much of the primary production, and are principal determinants of the transfer and vertical flux of photosyntheticallyfixed carbon. This thesis examines the composition and trophodynamics of a plankton community dominated by microzooplankton grazers, and their role in vertical carbon flux in an east Antarctic fjord. Ellis Fjord is a semiisolated marine inlet that is usually ice-covered throughout the year, and supports a zooplankton community that has low species richness and is dominated by microzooplankton. As such, it is akin to a macrocosm in which the role of microzooplankton in carbon dynamics can be studied in detail, in the relative absence of strong hydrodynamic forcing and the influence of higher trophic levels. The seasonal succession of the plankton community in Ellis Fjord was similar to that commonly observed in the wider Southern Ocean; changing from dominance by microplanktonic diatoms and small herbivorous copepods during early summer to nanoflagellates and protozoa during late summer. MicroplanIctonic diatom blooms and herbivorous grazers are commonly regarded as contributing to carbon export in the Southern Ocean, while communities dominated by auto- and heterotrophic nanoplankton favour the retention and respiration of carbon in pelagic waters. In Ellis Fjord, the physiological state of the cells appeared to determine their buoyancy, as microplanktonic diatoms did not directly sediment until the bloom declined. While there was evidence of near-surface export of microplanktonic diatoms, heterotrophic nanoflagellates, and microzooplankton faecal pellets, these contributed little to vertical flux to depth. Grazing by microzooplanIcton retarded the flux of phytoplankton by reducing the number of cells available to sediment directly, by producing faecal pellets of a morphology and ultrastructure that inhibited sinking, and by coprophagous degradation and recycling of pellets. Most pellets at depth were minipellets that contained little carbon, many of which appeared to be 'false' minipellets caused by coprophagy and degradation. Surprisingly, protozoan pellets that contained only empty diatom frustules contained more carbon per pellet than small oval copepod pellets. Differences in the ecology of the dominant small copepods, Oithona similis and Oncaea curvata, affected the morphology, persistence, and carbon content of their pellets, and the distribution of their biomass in the water column. Despite differences within and between small copepod and protozoan taxa, models of carbon flux in Ellis Fjord indicate that these microzooplankton contribute to the retention of both new and regenerated production. This reduces the draw-down of atmospheric carbon in Antarctic waters and the capacity of these waters to ameliorate global climate change.