Abstract
Contemporary environmental change is reshaping species’ distribution ranges at an unprecedented rate and scale, yet responses in the Southern Hemisphere, and in the ocean, remain far less understood than in the Northern Hemisphere and on land. With the High Seas Treaty coming into force in January 2026, there is an urgent need for us to address such knowledge gaps. Antarctica, in particular, plays a major role in shaping the spatial and temporal dynamics of the Southern Ocean. With global warming, research indicates that there could be a tipping point past which drastic changes will occur in Antarctica and the Southern Ocean.
The giant kelp, Macrocystis (in te reo Māori: rimurimu), is a habitat-forming macroalgal taxon widely distributed in temperate waters of the Americas, southern Africa, Australia, New Zealand and most of the sub-Antarctic islands. Although not (yet) established in Antarctica, it has been found cast up on Antarctic beaches carrying diverse non-native holdfast invertebrates. As warming drives poleward range shifts, there is a pressing need to understand the origins, dispersal pathways and Antarctic viability of Macrocystis, not only to understand the risk of its future establishment in Antarctic ecosystems, but also to evaluate the resilience and adaptive capacity of its native populations.
In this thesis, I combined genomic analyses, physiological experiments, and dispersal and species distribution modelling, in order to address these issues. I used genotyping-by-sequencing (GBS) to study the population connectivity and structure of Macrocystis populations around the Southern Hemisphere, which revealed high-resolution population structure that informed inferences on evolutionary history and dispersal patterns while also enabling future fine-scale source tracking of beached Macrocystis specimens. I explored the physiological tolerance (buoyancy, photosynthesis, reproduction) of Macrocystis to cold and dark Antarctic conditions in experimental studies, comparing it with Durvillaea, one of the other buoyant temperate kelp genera that can raft to Antarctic beaches. I used Lagrangian oceanographic modelling to investigate potential dispersal pathways of Macrocystis across the Southern Ocean and how they could arrive in Antarctica. Lastly, a species distribution model (SDM) was used to examine current and future suitability of the Antarctic environment to Macrocystis establishment.
Overall, my results suggested that Macrocystis is likely to establish in Antarctica in the future. Macrocystis populations are connected across the Southern Hemisphere by the Antarctic Circumpolar Current (ACC), however, such connectivity has not maintained strong gene flow, with regions exhibiting local diversification likely due to density-dependent processes. The results revealed fine-scale (tens of kilometres in some regions) population structure and genetic diversity not detected by previous research using lower-resolution, traditional genetic methods. Populations were more closely related to those of nearby regions than to those further way, consistent with an Isolation-by-Distance hypothesis. I also hypothesised, and demonstrated, that regions more connected by the ACC were more genetically related than to regions less affected by the ACC (‘Connectivity-by-ACC’).
The physiological health (buoyancy, photosynthesis, reproduction) of both Macrocystis and Durvillaea declined somewhat with cold and freezing temperature exposures. Some Durvillaea samples lost buoyancy completely, while all Macrocystis samples remained afloat throughout the experiments. Both taxa were, however, still able to produce spores or gametes after freezing. These results suggest temperate kelps could cope with cold temperature exposure and potentially withstand harsh Antarctic winter conditions.
The Lagrangian dispersal simulation model supported empirical genetic inferences that Southern Ocean coasts are connected. While some simulated particles reached Antarctic beaches, many more crossed the Southern Ocean fronts into Antarctic waters, assisted by highly variable meso-scale eddies and Stokes drift. The SDM showed no suitable habitats for Macrocystis in Antarctica under current conditions, whereas under moderate and severe warming scenarios there are several regions that may become suitable. In the meantime, multiple populations in its current range, especially in temperate regions, would experience range contractions and, in some cases, local extinctions. Together, both the oceanographic and the SDM suggested that the “point-of-entry” for Antarctic Macrocystis population establishment could be the Antarctic Peninsula and the Ross Sea region under future environmental conditions.
These results provide insights into the future trajectory of Southern Ocean and Antarctic coastal ecosystems through the lens of the foundational kelp species Macrocystis. By highlighting potential ecosystem shifts, my results help to identify conservation priorities and support effective and holistic conservation planning.