Giant kelp looks like it ought to be one big, well-mixed family. It’s a single species, Macrocystis pyrifera, growing from California to Chile and around the cold coasts of the far south, and its gas-filled bladders keep torn-off fronds afloat, so they can drift a long way. But a new genetic study suggests that, in the Southern Hemisphere at least, giant kelp populations are far more separate from one another than that picture implies, and nowhere more so than around New Zealand.
The paper was published online on 18 August in the Journal of Phycology. Its authors are Xiaoyue Pluto Liu, Alana Alexander, Narissa Bax, Donna Bond and Ceridwen Fraser. Fraser is at the University of Otago and is the same researcher whose earlier genomic work showed southern bull kelp rafting between distant coastlines. This time the team sampled giant kelp from three widely separated regions: New Zealand, Tasmania and the Falkland Islands (Malvinas). They used genotyping-by-sequencing, which reads genetic variation at many points across the genome rather than the small handful of markers older studies relied on.
The biggest surprise was how separate the three regions turned out to be. Giant kelp fronds can float for months, and the Antarctic Circumpolar Current links these coastlines, yet the genetic differences between New Zealand, Tasmania and the Falklands were strong. That contradicts earlier studies using traditional genetic markers, which had concluded that southern giant kelp was highly connected. New Zealand and Tasmania were more closely related to each other than either was to the Falklands, a pattern known as isolation by distance, where populations further apart are more genetically different. Within regions, the picture varied: Tasmania and the Falklands showed only weak structure, but New Zealand stood out. There, populations could be told apart over distances as short as 9 to 13 kilometres, and New Zealand also had the highest genetic diversity of the three regions.
Why does this matter? Giant kelp has had a hard few decades in parts of its southern range. Tasmania had the lowest genetic diversity of the three regions, and a separate genome study published this year notes that giant kelp cover along Tasmania’s east coast is reported to have fallen by 95 to 98 percent since 1940. When forests are being protected, mapped or rebuilt, it helps to know whether a coastline holds one population or several distinct ones, and how far apart they are. The authors frame their results as groundwork for future conservation decisions and for predicting how the species’ range may shift.
There are limits to what one study can show. It covers three regions, not the whole Southern Hemisphere range: mainland South America, for example, isn’t part of this dataset. And a genetic map shows where populations differ, not what those differences mean for how each copes with warmer water. The team looked for genes under natural selection and found none. The authors suggest the patterns come from currents, distance, and the difficulty incoming kelp has establishing where kelp already grows. They see the next steps as tracing drifting kelp back to its source populations and building local differences into models of how the species’ range will shift. Still, the result is a useful correction: even a floating, far-ranging seaweed can be surprisingly local.
Further reading: Liu, X.P., Alexander, A., Bax, N., Bond, D. & Fraser, C.I., “Genomic analyses of giant kelp reveal restricted connectivity across the Southern Hemisphere and striking diversity in Aotearoa (New Zealand),” Journal of Phycology (2026); Scharfenstein, H.J. et al., “Two Australian genome assemblies expand the genomic blueprint of giant kelp,” BMC Genomics (2026).