Abstract
Macroalgae are fundamental components of most marine ecosystems, creating habitat and food sources for a wide range of other organisms. Macroalgal distributions are strongly linked to water temperature, and global environmental change is therefore likely to drive major shifts both in the distributions and compositions of marine communities. Phylogeographic research on macroalgae and associated organisms can reveal ecological changes that have occurred with global warming since the Last Glacial Maximum (LGM), which can help us to predict what might happen under future climate change scenarios. Such research shows that many macroalgae have changed their distributions, broadly shifting poleward or into deeper waters. Importantly, for organisms to change their distributions in response to climate change, they must be able to disperse, sometimes long distances. Some buoyant, robust macroalgae are extremely good long-distance travellers, and others have apparently been able to disperse across oceans indirectly, such as via rafting. However, not all macroalgal species are capable of long-distance dispersal, and with global warming, ecosystems thus do not simply slide poleward in their entireties, but both move and change. Studies are already showing that contemporary climate change is affecting the distributions of macroalgal-dominated ecosystems. This chapter summarizes some of the ways in which Southern Hemisphere macroalgal distributions are inferred to have shifted with past climate change, and speculates on how they might change in the future. Processes underpinning these changes, such as climate drivers and dispersal capacity, are also discussed.