A silver, overcast day in Wilhelmina Bay, Antarctica. Image © owamux, CC BY 2.0 < https://creativecommons.org/licenses/by/2.0 >, via Wikimedia Commons

The story of seasonal sea ice around Antarctica is not as well-known as we might think, and the behaviours and connections of species to the annual cycle less so. But if very recent observations are anything to go by, the alarming changes seen in Antarctic sea ice since 2016 could herald catastrophic days ahead for far-ranging bird species, such as albatrosses and petrels, whose breeding seasons and other behaviours seems to be connected to the sea ice cycle.

Here, Dr Ewan Wakefield of Durham University discusses his recent research, in conjunction with the British Antarctic Survey, that raised as many new questions as answers regarding the interconnectivity between Antarctica’s sea ice and the bird species that seem dependent upon it.

A white-chinned petrel in flight against an icy mountain backdrop. Image © Professor Richard Phillips.

ScienceVega: What birds were the focus of this study?

EW: The study looked at species of birds that breed in the sub-Antarctic. Antarctica is a continent entirely surrounded by sea, by the Southern Ocean. It’s the area below the polar front and within the Southern Ocean, the further south you go, the colder the water gets. But instead of the temperature changing gradually, what actually happens is you reach a point where there’s warm water to the north and cold water to the South. That’s known as the polar front and it’s a line in terms of biology and oceanography where there are key differences on either side of it. The birds we studied breed right on the edge of the polar front in places like South Georgia. That is why they are called sub-Antarctic birds and they tend to go right down to, or near to, Antarctica, but there haven’t been many studies looking at why they do that. We wanted to know whether it had something to do with sea ice and whether there was a connection there.

One reason we wanted to look at this is because the populations are enormous. There are tens of millions of birds and they include what are known as Procellariiformes, often called ‘tube-nosed birds’ because they have nostrils, unlike most birds. Within this group we have the albatrosses and petrels. A wandering albatross is about the size of a human being with a huge wingspan that allows them to fly through a process called ‘dynamic soaring’ which means they barely need to flap their wings at all, essentially gliding almost all of the time enabling them to traverse vast distances. If you measure the heart rate of a soaring albatross to one sitting on its nest, there is very little energetic cost when they are in the air. The most stressful thing that an albatross does, the thing that really gets its heart racing, is walking.

They can cruise around, can take their time, they’re long lived, they typically only have one offspring every one or two years and they can also take a very long time to raise that chick. What all of that means is from these islands where they breed, they can travel a very long way. We wondered whether they travelled down to Antarctica, which they could do at very little cost, and we know that the sea ice region is an absolute bonanza of food in the summer. So it seemed reasonable to think that they would go down there and look for food.

We also knew our colleagues in the British Antarctic Survey had been tracking the birds using GPS trackers for a long time; about 30 years, but that there has been a lot more data collected in the last decade or so. We had access to that tracking data of the birds and we also had anecdotal reports of them going down to where there might be sea ice. This led to our study which was to look at this in more detail and find out if they were going there when the sea ice was there, or whether they were going there when the sea ice has opened up, which it does every spring.

A southern giant petrel. Image © Professor Richard Phillips.

ScienceVega: Why is this seasonal sea ice of importance?

EW: Sea ice is essentially frozen water floating on the sea, but it’s not icebergs that are bits of glaciers that have flowed down into the sea and where they have broken off and floated away. The sea ice we are talking about is formed around Antarctica in the winter, which gets so cold that the sea freezes.

The fresh component of the water freezes, the salt component trickles out and you have sea ice floating on top of the water. That ice can sometimes persist for several years where it gets thicker and thicker and if you look at a satellite picture of Antarctica in late summer through to the winter, the ice almost doubles the surface area of the continent. An obvious result of that is that it covers up the ocean so that birds trying to catch fish or krill struggle.

But the other thing sea ice does is provide something for things to grow on. The freezing process concentrates the nutrients in the ice and during the winter. Dust from South America and Africa settle on the ice and that dust contains nutrients. When we look at the underside of the ice, it is full of little cracks where crystals can grow. It’s really complex, but things grow there such as algae. And where the algae grows, crustaceans come in to feed on it. Then fish feed on the crustaceans and so on. There is a lot of life under the ice during the winter.

As spring progresses, the planet tilts towards the sun, providing longer days and more sunlight. Suddenly there is a cocktail of nutrients, warmer water, sunlight, and the melting water makes a layer of stable, less saline water on the surface. All of that provides the perfect conditions for yet more algae and more plankton to grow. There’s almost an explosion in the amount of algae growing and leading to a bonanza of food and that makes perfect sense why birds that can fly a long way at little cost would come down and feed.

But albatrosses don’t eat plants or algae; they eat squid and fish. So that led to the third part of the research. Having looked at whether they go there at all and then looking at when, we wanted to know if they went there as soon as the ice melted, or whether they waited a few weeks or months until there was a boom in the squid and fish populations so that they could feed on them.

That moved on to look at what was happening on the ice. Around Antarctica in the spring there are a huge amount of seals: crabeater seals, Weddell seals, Ross seals and most of them don’t breed on land; they make holes in the ice and when it is time for them to pup, the pup is born straight onto the ice. It’s quite a messy business, discarded placenta is left behind and some of the pups die. So now there is a lot of food on the ice as well.

Albatrosses do not like walking, they find it very stressful, so this food on the ice is not really for them. But they have relatives called the giant petrels, which look rather like an albatrosses, but are actually more closely related to fulmars, and these birds are happy walking. Giant petrels can do it all; fly really well, walk really well and they are scavengers that like to eat dead seals.

So these were other species we studied. They can also fly very long distances and we know vaguely from other studies that giant petrels do turn up on the sea ice, but it’s never really been known if they’re coming from Antarctica, because they also breed on Antarctica, or whether they’re coming from the sub-Antarctic. Not a lot was known about them, but there has been an idea that maybe they were targeting seals, which would mean they would go onto the ice in the springtime.

So even before we started we had lots of hypotheses we wanted to test. These birds are using the using the sea ice area, so what is the connection? Are they going there after the ice melts? Or are they going there when the sea ice is still there? If the giant petrel does, is it scavenging seals on the ice? For the albatross, do they wait a few weeks or months? They are definitely using the area, but are they waiting until the ice melts?

Close-up of a wandering albatross spreading its wings. Image © Professor Richard Phillips.

ScienceVega: To what extent is sea ice around Antarctica melting?

EW: One of the things that’s very well established now is that the ice in the Arctic is melting. It has been tracked year on year and the amount of ice that forms each winter is less and the amount it melts each summer is greater. We are worried for polar bears which can no longer get around if there is no ice.

But until very recently we had not seen the same effects on sea ice around Antarctica. In fact, satellite data showed it to have increased slightly and that’s been a real conundrum. If we have climate change and the world’s getting warmer, why is there more ice around Antarctica? Why isn’t that declining as well? There have been lots of attempts to explain this, but as the sea ice wasn’t decreasing, it was almost something we didn’t have to worry about.

Until 2016 when, unfortunately, the sea ice around Antarctica melted really quickly in the summer, and it melted far more than it normally does. That was the first time when alarm bells started to ring, and most of the summers since then it’s declined more than it normally does. And over the last three summers and the last winter, it’s been at record low levels and it looks as though there was some kind of delay in the system and it’s taken longer for that heating effect to affect Antarctic sea ice. That effect is now kicking in and it looks like it may go the same way as the Arctic sea ice and we are concerned about how it is going to affect animal populations and in what ways.

Although this is not what we did in this study, we were concerned about climate warming and the possible impact of that on seabird populations. If the ice is melting earlier and therefore the process of creating this food bonanza from the algae upwards is being shortened, perhaps it’s no longer quite the bonanza that it was? We need to know the affect now, because if we wait another 10 or 20 years, the system might be so disrupted that we can never figure that out.

ScienceVega: Have you got any evidence that what has happened since 2016 has had an impact on seabird populations yet or does this require further research?

EW: It does require further research and it’s probably not a change that we would see at the moment; we would probably see that in 10 or 20 years’ time. But my main focus at the moment is on a bird called a snow petrel, a small dove-sized bird that feeds exclusively on the sea ice. They are a relative of the albatross and they breed further south than any other vertebrate, on mountains on the Antarctic continent, and they fly out and feed amongst the sea ice. There has been one long-term study on this bird in the Indian Ocean sector of Antarctica over an almost 50-year period, and it’s very clear that their breeding success is affected by the amount of sea ice. Numerous studies on penguins have shown the same thing. So it is certain that if the amount of sea ice changes, those species are going to be affected.

But what our results suggest is that not only will those species be affected, but these huge populations of albatrosses and petrels that breed on sub-Antarctic islands will also be affected. We don’t know that for sure yet, but that is what we would predict.

An iceberg embedded in sea ice. Image © NASA/ James Yungel, Public domain, via Wikimedia Commons.

ScienceVega: Are there any studies on populations and breeding patterns of albatrosses and petrels in places like South Georgia?

EW: There have been a lot of demographic studies looking at things like the birds’ longevity, their breeding success, and the frequency with which they breed. All of those studies show that they are affected by their environment. There are correlations between, for example, breeding success and the temperature of the ocean.

But what we often don’t know are the kinds of correlations with what are called atmospheric modes, which is a kind of ‘catch all’ number that tries to quantify what state the climate is in.

One mode might be a difference in pressure between the equator and the poles, or the difference between two areas, for instance. When we reduce these major atmospheric pressures down to a single number, we do find correlation between that number and breeding success, for example. What we don’t know is the mechanisms that are in place. What we are saying is one mechanism is probably the variation in sea ice, but that is to be tested.

Dr Ewan Wakefield
Postdoctoral Research Assistant
Department of Geography at Durham University
and
Affiliate researcher at the School of Biodiversity, One Health & Veterinary Medicine at the University of Glasgow.

The study is published in the journal Progress in Oceanography.