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Microzooplankton Community Structure and Grazing Impact along the Western Antarctic Peninsula

Microzooplankton (zooplankton 20 200 pm) are an integral part o f aquatic food webs as they can be significant grazers o f phytoplankton and bacteria, remineralizers o f nutrients, and prey for higher trophic levels. The importance o f microzooplankton in many pelagic ecosystems has been established, yet compared to larger zooplankton, microzooplankton are understudied in the Southern Ocean. I quantified microzooplankton community composition and grazing rates along the Western Antarctic Peninsula (WAP) as part o f the Palmer Antarctica Long-Term Ecological Research project (PAL LTER). This region is experiencing rapid warming, causing declines in sea ice and changes in the marine food web. I determined distributions o f major microzooplankton taxa in January 2010 and 2011 within the top 100 m of the water column, along both north-south and coastal-offshore gradients o f the WAP, using microscopy. I found that microzooplankton are potentially adjusting to climate changes along with other trophic levels, as there was generally higher microzooplankton biomass in the south compared to the north. Biomass was higher in surface waters compared to 100 m, and variability in microzooplankton biomass between years and with distance from shore appeared to be influenced by sea ice dynamics. M icrozooplankton biomass was also positively correlated with chlorophyll-# and particulate organic carbon (POC), and biomass o f several microzooplankton taxonomic groups peaked near Marguerite Bay, historically a productivity hot spot. I also calculated phytoplankton and bacterial growth and grazing mortality rates using the dilution method at select stations along the WAP in January 2009 2011 and in the near shore waters near Palmer Station in February March 2011. Microzooplankton exerted higher grazing pressure on bacteria, relative to grazing on phytoplankton. Microzooplankton also exhibited selective grazing on smaller phytoplankton (picoautotrophs and nanophytoplankton), and on the more actively growing bacterial cells, thus shaping phytoplankton and bacterial assemblages and effectively cropping production. There was a significant (albeit weak) positive correlation between temperature and phytoplankton grazing mortality. My research is the first to describe both the microzooplankton structure and grazing impact in the PAL LTER study region. This study contributes valuable information to studies modeling the flow o f carbon through the WAP food web and provides a reference point for studying how future changes will affect microzooplankton community structure and food web dynamics in this region. Lori Michelle Price SCHOOL OF MARINE SCIENCE THE COLLEGE OF WILLIAM AND MARY IN VIRGINIA