Subantarctic bull kelp suggests ice age was icier
Genetic patterns in subantarctic southern bull kelp (Durvillaea antarctica) suggest that Antarctic sea ice extent during the last ice age may have been much greater than currently estimated.
Dr Ceridwen Fraser, formerly of the Allan Wilson Centre at the University of Otago, New Zealand, said the genetic analysis of bull kelp from many subantarctic islands, including South Georgia, Crozet, Kerguelen, Gough, Marion and Macquarie islands, showed the kelp had very little genetic diversity. This suggests it has only recently colonised the islands from a common origin.
In contrast, bull kelp collected from the New Zealand subantarctic region (Auckland, Campbell, Antipodes and Snares islands), and along the coasts of mainland New Zealand and central Chile, was comparatively genetically diverse.
Speaking at the Third International Forum on the Sub-Antarctic, Dr Fraser said that bull kelp was a species that could not survive ice scouring caused by glaciers or sea ice.
This means that during the last ice age, which peaked about 20 000 years ago, the coastal-growing kelp could not have survived in coastal areas affected by sea ice. But when the ice age ended and sea ice and glaciers began to retreat, new habitat would have opened up to early colonising species.
‘In the northern hemisphere there are many examples of early colonising species that were able to rapidly multiply in newly ice-free areas,’ Dr Fraser said.
‘This led to a distinct loss of genetic diversity in the recolonised regions, as the early colonisers quickly expanded into the new habitat.’
Dr Fraser’s genetic analysis of the bull kelp suggests that such a ‘post-glacial recolonisation’ of the subantarctic has also occurred, most likely from glacial refugia around New Zealand’s subantarctic islands, via the Antarctic Circumpolar Current. Similar genetic patterns have also been observed in some of the small crustaceans living in the holdfasts of the kelp, supporting this hypothesis.
‘Post-glacial recolonisation of the subantarctic by bull kelp is a logical hypothesis from this research,’ Dr Fraser said.
‘But the problem is that current estimates of where sea ice extended during the last glacial maximum don’t cover many of the islands where we found evidence of recolonisation.’
Dr Fraser and her colleagues examined the data used to compile estimates of sea ice extent and a recent paper which re-analysed this data and made new estimates of ice cover. This data consisted of sediment cores containing fossil diatoms (microscopic algae), some species of which are common in sea ice.
‘We found there was uncertainty about sea ice extent, due to a lack of sediment core data, in a large area that includes several of the subantarctic islands where we found evidence of recent recolonisation,’ she said.
‘Based on our kelp evidence, we think sea ice may have extended as far as Marion and Crozet islands and even past Macquarie Island, in roughly the 50° latitude zone.’
On this map of Antarctica and the subantarctic, the blue area shows the current estimated extent of winter sea ice at the Last Glacial Maximum (LGM), while the red dotted line shows the equivalent of the Antarctic Polar Front at the LGM. This red line is where Dr Fraser and her colleagues think sea ice actually extended during the LGM. The pie charts show the distributions and proportions of the genetic variants (haplotypes) of southern bull-kelp found around the Southern Ocean. Only one haplotype (red circles) was observed on the subantarctic islands that fall within the proposed new sea ice extent, and in the part of southern Chile that was covered by glacial ice during the last ice age. This haplotype most likely originated from the New Zealand subantarctic, after bull kelp from the region hitched a ride on the Antarctic Circumpolar Current.
Further support for the recolonisation hypothesis comes from observations of bull kelp off South America.
‘You see a lot of genetic diversity along the Chilean coast but in the fiordland of the south there’s only a single genotype across more than 1000 kilometres. This area corresponds to the region thought to be covered by the Patagonian ice sheet during the Last Glacial Maximum, so we think it’s a clear example of post-glacial recolonisation.’
Dr Fraser has conducted genetic analyses on other subantarctic ice-affected and ice-resilient seaweed species, to see if this hypothesis is supported by more biological data. Further studies using other scientific approaches are also needed to resolve the issue.
If the estimates of sea ice extent are found to be wrong, current climate models may need to be adjusted to more accurately predict what will happen in the future.
This research, conducted by Dr Fraser, Prof Jon Waters, Prof Hamish Spencer and Dr Raisa Nikula, was published in the Proceedings of the National Academy of Science, USA 106: 3249-3253 in 2009, and Marine Ecology Progress Series 405: 221–230 in 2010.
Corporate Communications, Australian Antarctic Division
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