An end-to-end food web model was developed to investigate potential effets of changes in the structure of the marine food web in the Marguerite Bay region of the western Antarctic Peninsula (WAP). Data on abundance, diet composition and consumption of food web components, which range from microbes to whales, were used to construct a trophic framework that represents the current state of the WAP system. This end-to-end model was then used in a top-down framework to estimate the biomass of the mid and lower trophic levels necessary, on an annual basis, to sustain the observed biomass of the top predators. The top-down information was then inverted to get a bottom-up depiction of the food web, where the production of prey determines the intake by predators, in accordance with the expected direction of the energy flux in the ecosystem. The bottom-up framework allows testing of the effect of modifications in the energy fluxes in the lower and mid food web that may arise from changes in ocean circulation and water mass properties on annual production (and on the long term in the biomass) of the top predators. The food web model allows investigation of complex linkages between the different ecosystem components and potentially reveals indirect and nonlinear feedbacks that may result from modifications of diet and biomass of one of more trophic groups. For example, diverting 50% of the primary production (PP) grazed by Antarctic krill to salps (usually considered a trophic dead-end) results in reduction of the annual krill production (15-35%) and a consequent decrease in the annual production of the krill-dependent top-predators. Adélie penguins (7-17%), crabeater seals (14-32%), minke whales (14-32%) and humpback whales (9-20%). A similar decrease in the amount of PP that goes to herbivorous copepods results in a decrease in the annual production of ctenophores (5-12%), cephalopods (7-9%), pelagic and benthic fish (5-23%) and Weddell seals (6-11%). A small amount of predation on salps by benthic fish results in an (indirect) increase in the annual production of Weddell seals, for which the benthic fish represent an important percentage (40%) of the diet.

Tosca Ballerini received a M.S. degree in Natural Sciences from the University of Florence in 2001 and a Ph.D. in Ecology and Biology of Polar Habitats from the University of Siena in Italy in 2007. She is currently a postdoctoral researcher at the Center for Coastal Physical Oceanography at Old Dominion University. Her research is focused on development of end-to-end food web models for the Southern Ocean, with emphasis on the western Antarctic Peninsula region, and on comparison of the trophic structure of this region to similar studies ongoing for the northwest Atlantic, northeast Pacific, and Gulf of Alaska. This study is part of the final synthesis and integration phase of the U.S. Global Ocean Ecosystem Dynamics (GLOBEC) program.

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