Early last year, a Chilean colleague, Dr Erasmo Macaya, was walking along a beach in Antarctica when he stumbled on something incredible. Cast up by the ocean, lying dying on the sand, was part of an organism that did not belong in Antarctica, and that could neither swim nor fly. How on Earth could it have reached the icy southern land?
We used high-resolution genomic analyses and found that the organism found on the beach that day was a genetic match to northern populations tens of thousands of kilometres away, and could only have floated to Antarctica ... yet such a voyage ought to have been impossible! The Southern Ocean, surrounding Antarctica, is the windiest place on Earth, including the region commonly known as the ‘roaring forties’. The world’s strongest ocean current, the eastward-flowing Antarctic Circumpolar Current, encircles Antarctica, flowing from west to east. The ocean currents also drift northward at the surface, because the Coriolis force – which results from the rotation of the Earth – deflects them to the left. This means that floating objects at the surface of the Southern Ocean should drift away from Antarctica, to the east and to the north. Modelling suggests that even features such as eddies, which move parcels of water away from the main ocean currents, are not able to overcome the strong north-east drift.
Waves at the surface can, however, push floating objects – such as plastics, aeroplane crash debris, pumice from volcanoes, driftwood, seaweeds, and messages in bottles – in unusual directions. This effect is known as Stokes drift. Each time a wave passes, the kelp moves a short distance forward with the wave. You can think of this as surfing, except the waves move faster than the floating object. In the Southern Ocean, the waves arising during standard storms are 10-15 m high (in fact, the Southern Ocean recently saw the Southern Hemisphere’s largest recorded wave, at >23 m!), so the Stokes drift is large. When we included this wave-driven surface motion in our modelling of floating particles in the Southern Ocean, many were able to overcome the northward drift of the ocean currents, and turn southward to reach Antarctica.
A video showing the modelling done by Adele Morrison and Andy Hogg to reveal how kelp was able to raft to Antarctica. In this video, 140 virtual, drift particles were released from South Georgia - the orange ones will reach Antarctica, and the blue won't.
The effect of waves on the dispersal of floating material has not previously been part of Southern Ocean models, but it’s clear that waves can drastically alter dispersal pathways in stormy seas. These findings could therefore change the way we model drift pathways of floating objects throughout the world’s oceans, because wave-driven surface motion is also likely to be important in other stormy regions.
So what was on the beach, and why does it matter?
The dying organism (well, organisms – there were two, and DNA analysis shows that each one came from a different source – one from Kerguelen, around 20,000 km away, and one from South Georgia, even further upstream) that reached Antarctica was southern bull-kelp (Durvillaea antarctica). Southern bull-kelp is ecologically important species that grows only in cold-temperate southern regions including New Zealand, Chile, and the sub-Antarctic. We know that this species can carry many other species of plants and animals with it when it detaches and floats at sea, so the discovery that this kelp can raft to Antarctica means we could see major ecological changes in Antarctic marine ecosystems as the climate warms.
Southern bull-kelp is a keystone species in cold-temperate regions in the Southern Hemisphere, blanketing rocky shores and providing habitat for many other seaweeds and animals.
Antarctica is not biologically isolated
Up until now, Antarctica has been thought to be biologically isolated from the rest of the world. There is almost no evidence of colonisations of Antarctica from northern regions in human history, and many Antarctic plants and animals are distinct from those found on other continents and sub-Antarctic islands.
The kelp specimens found in Antarctica are the first foreign organisms known to have drifted across the Southern Ocean. One piece started its journey from the Kerguelen Islands and the other from South Georgia. Their journeys are the longest ever recorded biological rafting events.
We modelled millions of virtual kelp floating across the ocean, and found that – with surfing – many are able to reach Antarctica. This means that Antarctica’s ecological differences are not really due to ‘isolation’, but instead are probably because the harsh Antarctic climate prevents new plants and animals from establishing there.
Parts of Antarctica are among the fastest warming regions on Earth. As Antarctica and the ocean around it warms, the kelp rafts – and other floating organims including invertebrates hanging onto the kelp, seeds, driftwood that could harbour insects, and larvae – may one day be able to colonise. By the end of this century, when parts of Antarctica are expected to be similar to current sub-Antarctic environments, we might see many new species establishing in Antarctica, and dramatic ecosystem change.
The fact that Antarctica is not physically isolated might have implications for floating plastic debris too. While there are few sources of plastic litter on Antarctica itself, there are garbage patches in the South Atlantic and South Pacific, just north of the Southern Ocean. The storms and Stokes drift that transport kelp southward could also transport plastic debris to Antarctica, putting extra pressure on vulnerable ecosystems.
You can watch a video summary of the findings, here: https://www.youtube.com/watch?v=6Yqy9DpraHA&feature=youtu.be
This blog post includes sections from an early draft of an article, written by several of the research team members, for The Conversation.
The results described here were published in Nature Climate Change:
Fraser CI, Morrison AK, Hogg AM, Macaya EC, van Sebille E, Ryan PG, Padovan A, Jack C, Valdivia N & Waters JM (2018). Antarctica’s ecological isolation will be broken by storm-driven dispersal and warming. Nature Climate Change, 8: 704–708 . [doi: 10.1038/s41558-018-0209-7] Free-view link: https://t.co/hE8BWHsV2C