A PRELIMINARY BALANCED TROPHIC MODEL OF THE ECOSYSTEM OF THE ROSS SEA, ANTARCTICA, WITH EMPHASIS ON APEX PREDATORS

We report on the development of a mass balanced carbon-budget trophic model of the Ross Sea as a step towards investigating ecosystem effects of the fishery for Antarctic toothfish (Dissostichus mawsoni). The model has 30 trophic groups representing all the major biota of the Ross Sea. Many of the lower trophic level species in the model are grouped by functional role because information is not available at greater taxonomic resolution. The model separates the following apex predators by species: Emperor penguin, Adélie penguin, crabeater seal, Weddell seal, orca, sperm whale, Antarctic toothfish. A survey of the available literature and both published and unpublished data provided an initial set of parameters describing the abundance (seasonally and spatially resolved where possible, imports, exports), energetics (growth, reproduction, consumption), and trophic linkages (diets, key predators) for each model group. We also estimated the relative level of uncertainty on these parameters. We describe the method we used to adjust the parameters to give a balanced model taking into account estimates of parameter uncertainty and the large range of magnitude (>6 orders of magnitude) in trophic flows between different groups of organisms. Biomass, production, consumption, export and diet fractions are adjusted simultaneously. We set ecotrophic efficiency to unity for all non-primary producers. Changes to the initial set of parameters needed to obtain balance were significant, especially for bacteria. Excluding bacteria, the adjustments required for balance from the parameters estimated a priori were <46% (biomass), <15% (production, consumption), and <28% (diet fractions). The balanced model presented here has not yet been validated and should be considered a work in progress.