Alternative energy pathways in Southern Ocean food webs : combining theoretical, empirical and modelling approaches

The flow of biomass and energy in Southern Ocean food webs is not well characterised, but is crucial in determining how Antarctic marine ecosystems may respond to environmental change and harvesting, and for formulating appropriate management and conservation strategies. Energy flow in Southern Ocean ecosystems is often described in terms of the short, highly efficient food chain, with Antarctic krill (Euphausia superba) dominating energy transfer from primary producers to higher trophic levels. However, there is increasing evidence that alternative energy pathways through other types of zooplankton and mesopelagic fish and squid are equally, if not more important in some regions or years. The question of the importance of these alternative pathways has not yet been examined at a circumpolar scale. The thesis draws together diverse approaches from theoretical ecology, empirical research and quantitative ecosystem modelling to generate novel and important insights into Southern Ocean food web structures and the pathways for energy flow through mid-trophic levels at varying spatial scales ranging from regional to circumpolar. Chapter 1 reviews the ecosystem modelling process and how data signatures, in particular stable isotopes, may be more powerfully used in the development and evaluation of size- and species- based food web models to address questions of ecosystem structure and energy flow. Chapter 2 used network analyses informed by dietary data available from the SCAR Southern Ocean Diet and Energetics Database (SODD) to show that food web structures are fundamentally different between the major oceanic sectors of the Southern Ocean; the mid-trophic groups responsible for maintaining marine mammal and bird populations vary between each sector. Chapter 3 develops a decision framework for species aggregation in Antarctic food web models utilising information from the SODD to justify the functional group structure of an Ecopath model for Prydz Bay, East Antarctica. Chapter 4 presents the development and implementation of a Prydz Bay Ecopath model and provides the first quantitative analysis of the importance of different mid-trophic-level groups for directing energy flow in East Antarctic marine food webs. Finally, Chapter 5 synthesises these new insights into alternative energy pathways in Southern Ocean food webs and considers future research directions for identifying key environmental drivers of these differences.