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 and are principal
determinants of the transfer and vertical flux of
photosynthetically-fixed 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 semi-isolated
marine inlet that is usually ice-covered
throughout the year, and supports a
zooplankton community that has low species
richness. 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. Microplanktonic 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 microzooplankton retarded
the flux of phytoplankton by reducing their
direct sedimentation, 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. 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.