Drivers of Southern Ocean food web structure and impacts of environmental change

The Southern Ocean is experiencing major environmental and ecological changes which could drastically alter communities and impact ecosystem functioning. We still have a poor understanding of the structure of Southern Ocean food webs and their likely responses to ongoing and future changes, which limits our ability to develop and implement effective management and conservation strategies. This thesis employs multiple approaches to investigate several aspects relating to the structure and dynamics of Southern Ocean food webs. First, the links between morphological traits and trophic niches are explored within the demersal fish community of the subantarctic island of South Georgia, highlighting the role of functional traits in driving community structure. Second, functional traits including body size, mobility, foraging habitat and feeding mode are used to identify the drivers of stabilising sub-structures (modularity) across regional food webs. This leads to the hypothesis that habitat heterogeneity is a major determinant of the distribution of modules within networks. Third, an extensive dataset of mesopelagic fish and zooplankton samples from across a latitudinal temperature gradient is used to determine the impact of warming on predator-prey body mass ratios (PPMR). This reveals that ongoing environmental change may reorganise the size-structure of Southern Ocean ecosystems, with implications for their stability. Fourth, the possible consequences of ongoing baleen whale population recovery for competitor biomasses are explored using the Ecopath framework, with the conclusion that strong trade-offs between conservation objectives are likely unless substantial increases in suitable primary production occur. This thesis finishes with a synthesis of these new insights into the structure and dynamics of Southern Ocean food webs and discusses the major future directions for food web research more generally.