Biophysical modelling of Antarctic krill in a changing climate : key habitats and transport

The Southern Ocean hosts globally unique and important ecosystems characterized by extreme environmental variability. Superimposed upon this, these systems are now experiencing climate change impacts at rates greater than the global average. Antarctic krill – an ecologically and commercially important species – are highly adapted to their variable polar environment. At annual scales, krill have evolved a life cycle synchronized to seasonal oscillations between productive open-ocean habitats in the summer, and sea ice-dominated habitats in the winter. At interannual scales, krill recruitment is episodic and highly dependent upon high-quality habitat, created by optimal environmental conditions occurring every 4-5 years. This project therefore aims to improve understanding of the environmental drivers of krill habitat quality and population success in both open-ocean and sea-ice habitats, in order to better assess their current status and continued survival in a changing Southern Ocean.
In the open ocean, krill habitat quality can be influenced by a number of environmental drivers, including temperature and food availability. However, published projections of open-ocean habitat quality have been limited by biases in Earth System Model (ESM) representations of primary production. This thesis minimizes biases in an ESM ensemble using a novel model selection and weighting approach, allowing for projected changes in temperature and primary production to be accounted for. By providing the first circumpolar projections for krill growth potential, this project finds a seasonal shift in habitat quality during the main reproductive season which may have implications for krill population dynamics and interactions with the krill fishery.
The environmental drivers of sea-ice habitat quality remain somewhat speculative. Current understanding is limited by challenges in observing the sea-ice environment and so is primarily informed by correlative relationships with relatively simple indicators of sea-ice habitat availability, such as extent. Robust future projections require an improved understanding of the mechanisms underpinning observed correlations. This thesis employs a high-resolution sea ice-ocean model characterise the sea-ice environment. This new information is used to identify relationships between krill recruitment dynamics and the sea-ice environment that reflects a more mechanistic understanding of the underlying processes of krill habitat use. By identifying more mechanistically informed environmental drivers of sea-ice habitat quality, and integrating them across overwinter transport routes, this thesis significantly advances understanding of regional connectivity as well as regional variability in sea-ice habitat drivers.
Using these innovative quantitative approaches, this thesis addresses significant knowledge gaps in our understanding of the environmental drivers of current and future krill population success. Together, this work represents an important advance in improving our knowledge of how the biophysical environment influences krill biology, and towards using these biophysical relationships to support robust projections of Southern Ocean systems into the future.