Modelling oceanic transport of planktonic species in the Southern Ocean

The Southern Ocean ecosystem is unique on a global scale because of complex ocean dynamics (e.g. fronts, eddies and their interaction with topography), seasonal sea ice advance and retreat, and biochemical cycles relating to all these physical factors. These ecosystems have been a‚Äövú¬µected by climate change over at least the last three decades and the assessment and evaluation of these changes - both at present, and for the future - are urgently required tasks. A key element in understanding and managing Southern Ocean ecosystem change is a better understanding of the physical processes that influence oceanic transport and spatial patterns of recruitment. Simulation models provide a powerful approach to address questions regarding transport processes, including testing alternative theories and predicting conditions under future climate change scenarios. This thesis develops methods for dispersal modelling to investigate transport of larval - juvenile stages of two important species: Patagonian toothfish and Antarctic krill. These approaches treat larval - juvenile stages as passive particles whose transport by ocean currents can be modelled using Lagrangian particle tracking. Key datasets used to inform this work are: i) remotely sensed surface geostrophic velocity data, ii) ocean model output and iii) physical parameters such as oceanic front position, topography and sea ice concentration. This thesis is composed of general introduction (Chapter 1), three main dispersal modelling works (Chapter 2‚Äö-4) and discussion & conclusion (Chapter 5). The second chapter focuses on egg and larval transport of Patagonian toothfish on the Kerguelen Plateau, with results suggesting that successful spawning grounds and transport patterns are coincident with in situ observed data. The second and third chapters focus on Antarctic krill. In particular, Chapter 3 develops a biologically relevant measure of retention time and the results indicate a significant relationship between the length of retention time and observed krill population size. Here retention time is defined as the time that a particle remains in a particular region. Chapter 4 develops a model to test hypotheses for the formation of the observed phenomenon of surface krill patches. Such surface patches are composed mainly of young krill. Results from this chapter support the hypothesis that surfaces patches can form as a result of the release and transport of juvenile krill from the sea ice edge zone during spring and that the distribution of these patches may have shifted to the south due to reductions in sea ice extent over the last 80 years. The tools and methods developed in this thesis have broader applications for other biological particles and will be made available for use by other researchers as an R package.