Two small pieces of seaweed found by a Chilean scientist on an Antarctic beach set in train research that may transform our understanding of ocean drift and reveal what the future holds for Antarctic ecosystems affected by climate change.
It all started in January 2017, when sharp-eyed marine biologist Erasmo Macaya spotted two clumps of southern bull kelp washed up on the tide line of an Antarctic beach.
Most of us would have walked right on by, but it stopped Macaya in his tracks. To him it was as if an alien had just landed – and in many ways that was exactly what had happened.
Every piece of science he knew said that this species of kelp should never have ended up in Antarctica. Its home was the regions around New Zealand, Chile and the sub-Antarctic islands. Indeed, a genetic test later confirmed that the pieces he found had travelled tens of thousands of kilometres from the Kerguelen and South Georgia islands.
So how did the kelp get to Antarctica?
The ocean barrier
Many scientists considered such a journey impossible, because of the fierce barrier of winds and currents that encircle Antarctica. These winds – known to sailors as the Roaring Forties – combine with the world’s strongest ocean current, the Antarctic Circumpolar Current, and the Coriolis force generated by Earth’s rotation.
Together, these forces push floating objects east and north, away from Antarctica. Before Macaya’s discovery, this barrier was thought to be impenetrable to floating debris.
But if kelp and other organisms could make it to Antarctica, this would have profound consequences for Antarctic ecosystems. So was there a way for the kelp to drift through that barrier?
We took up the challenge, using our ocean models. The mystery deepened when our first modelling attempts suggested that the Southern Ocean was indeed uncrossable by floating kelp. Even ocean eddies – the “weather” of the ocean – were not able to push floating objects southward away from the main ocean currents.
Yet the kelp had undeniably made the crossing. This led us to think about other influences on ocean drift that could play a role. We decided to add a very small effect known as Stokes drift to our models.
You can think of Stokes drift as deep ocean surfing. Waves can push floating objects in unusual directions. In the kelp’s case, each time a wave passes, the kelp will move a short distance with the wave. This drift is slow when waves are small, but in regions with large waves (such as the Southern Ocean) it can be much faster.
During storms around Antarctica, waves are typically 10-15m high. The largest wave ever recorded in the Southern Hemisphere, more than 23m, was in the Southern Ocean off New Zealand. Stokes drift must be large here.
When we added this factor to our ocean models, the change was instant. The massive waves generated by Antarctic storms pushed a small proportion of floating objects southwards. As we report in Nature Climate Change today, this conceivably explains the kelp’s voyage to Antarctica.
We calculated that the kelp specimens must have drifted at least 20,000km to reach Antarctica – the longest biological rafting events ever recorded.
Our results will also change the way that drift pathways for floating objects – such as plastics, aeroplane crash debris, pumice from volcanoes, driftwood, seaweeds, and messages in bottles – will be calculated, particularly in stormy oceans.
What this means for Antarctica
The implications don’t stop there. Until now, Antarctica was thought to be an isolated ecosystem, largely insulated from environmental change. This is not in fact true.
Southern bull kelp can carry many other species of plants and animals when it detaches and floats out to sea. The discovery that this kelp can raft to Antarctica means we could see major ecological changes in Antarctic marine ecosystems as the climate warms.
So far there is almost no evidence of natural colonisations of Antarctica from northern regions in the past few tens of thousands of years. Many Antarctic plants and animals are distinct from those found on other continents and sub-Antarctic islands.
In fact, the kelp strands Macaya found are the first recorded foreign organisms to have drifted across the Southern Ocean. But our models suggest these are unlikely to be the only ones to have made the trip.
This means that Antarctica’s ecological differences are not really due to physical isolation. It is more likely that the harsh Antarctic climate prevents new plants and animals from establishing themselves.
But Antarctica is changing. Parts of the frozen continent are among the fastest-warming regions on Earth. As Antarctica and the ocean around it warms, the kelp rafts – and other floating organisms, 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 colonising Antarctica, bringing dramatic ecosystem change.
Other human-caused influences may also be felt. If kelp can break through the barrier, then floating plastic debris from the large garbage patches in the South Atlantic and South Pacific, just north of the Southern Ocean, could conceivably make a similar journey.
Plastic litter is still very rare in the waters around Antarctica. But with ever-growing amounts of plastic entering our oceans and the new drift pathways we have discovered, more plastic will likely find its way south to pollute one of our last near-pristine environments.
And all of this has been revealed through the discovery of two small pieces of kelp on a distant beach, and the application of a relatively insignificant piece of ocean physics. From these small beginnings we now know that one of the world’s last great wildernesses might not escape our influence.
Adele Morrison, Research Fellow, Australian National University; Andy Hogg, Associate Professor, Australian National University; Ceridwen Fraser, Senior lecturer, Australian National University, and Erik van Sebille, Associate Professor in Oceanography and Climate Change, Utrecht University
Antarctica’s ice sheets could totally collapse if the world’s fossil fuels are burnt off, according to a recent climate change simulation. While we are unlikely to see such a dramatic event any time soon, we are already observing big changes and it’s worth considering what the worst case scenario might look like for the continent’s ecosystems. How long before Antarctica turns into grassy tundra?
For now, life thrives mostly at the very edge of the continent – it’s driven by the plankton-rich Southern Ocean and clustered around seasonally ice-free areas of coastal land. The interior might be sparsely inhabited, but the continent is not as barren as many think. There are around 110 native species of moss and two flowering plants, the Antarctic hairgrass and pearlwort. These plants have flourished along the relatively mild Antarctic Peninsula in recent decades. However they can’t go much further – they already occur at almost the most southern suitable ice-free ground.
With ice-caps and glaciers receding already in the Peninsula region, native land plants and animals are benefiting from more easily available liquid water. Already we are starting to see increased populations, greater areas occupied and faster growth rates, consequences only expected to increase – everything is currently limited by the extreme physical environment.
It may eventually prove too warm for some native species, but the bigger issue in upcoming decades and centuries will be whether new and currently “non-native” species will arrive that are stronger competitors than the native organisms.
Native polar species are inherently weak competitors, as they have evolved in an environment where surviving the cold, dry conditions is the overriding selective pressure rather than competition from other biological sources. If humans (or other wildlife expanding their range southwards) bring new competitors and diseases to Antarctica, that may pose a very grave risk to the existing biodiversity. Some native species would likely be pushed into the remaining more extreme regions where they can avoid competition and continue to rely on their inherent stress tolerance abilities.
We usually split the process of natural colonisation – which applies even today in Antarctica – and that of movement of “alien” species by human agency. The best available data for the Antarctic region come from some sub-Antarctic islands, where it appears humans have been responsible for many more successful colonisations than nature. In fact, over the recent centuries of human contact with the region we have introduced 200-300 species compared to just two or three known natural colonisations.
Penguins, seals and flying seabirds move between islands and the Antarctic Peninsula, so there is potential for some natural colonisation. Vagrant birds are regularly observed across the sub-Antarctic and even along the Peninsula, some of which have colonised successfully (such as the starlings, redpolls and mallard ducks on Macquarie Island).
Migrants such as skuas and gulls, which spend time on land at both ends of their migration, could be important natural vectors of transfer for invertebrates, plant seeds and spores, and microbes into an ice-free Antarctica. Importantly, bird colonies also fertilise surrounding rock and soil with faeces, eggshells and carcasses. Plant and animal life flourishes near seabird colonies, encouraged by this enrichment.
However it can be tough to predict what Antarctic melt would mean for individual species, never mind entire ecosystems. Take penguins, for instance – they have already survived previous inter-glacial retreats, but at reduced population sizes. This time round it is likely that Adélie and emperor penguins who are more dependent upon sea ice would decline, while less ice-dependent species such as gentoos and chinstraps might benefit. Indeed, there is already some evidence that emperors are struggling (although also that they may be adapting and learning to emigrate).
However the fact fish-eating gentoo penguins are increasing on the Peninsula while Adélies and chinstraps (both krill eaters) aren’t doing so well suggests prey availability can be more to blame than ice cover. Figuring out the impact of large-scale environmental change at ecosystem or food-web level is hard – it’s a complex process that will no doubt throw up some unexpected results.
The sub-Antarctic islands are full of examples of such unexpected impacts. Pigs, dogs, cats, sheep, reindeer and rabbits have all been intentionally introduced in the past, with often devastating effects. Rats and mice were introduced to South Georgia and other islands accidentally by sealers and whalers, for instance, and have decimated seabird populations. A recent eradication campaign appears to have been successful and pipits, ducks and small seabirds are showing some immediate signs of recovery.
The removal of non-native cats from Macquarie and Marion Islands has similarly helped native burrowing seabirds, although responses in such ecosystems can be far more complex and unpredictable – the removal of cats from Macquarie also led to increase in the introduced rabbit population, and considerably increased damage to sensitive native vegetation.
Antarctic biodiversity is far more complex than widely assumed, with up to 15 distinct biogeographic regions that have been evolutionarily isolated for many millions of years. Humans present the greatest threat, not only of introducing new species, but also of moving “native” species between regions within Antarctica. This could be even more damaging, as these native species would already be pre-adapted to polar life.
Visitors to Antarctica are subject to increasingly strict biosecurity measures but accidental introductions continue to occur, often through food shipments for scientists. Changes in sea and land ice affect access to new areas, so we can only expect plant and invertebrate invasions to increase unless biosecurity becomes more effective.
While cost issues may be raised, it is worth remembering that prevention will always be better – and cheaper – than subsequent control and eradication, even if such action is possible.
The link below is to an article warning of a Fire Ant invasion of Sydney – this is a very important problem and warning for Sydney.
The article below reports on the threat to Antarctica posed by weeds brought in by human visitors. This is a threat that will continue to grow with climate change.
The link below is to an article reporting on the invasion of weeds and pests in Antarctica. The threat is growing with climate change and human visitation.
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