Antarctic Species Under Climate Change

Due to its high salinity, the Southern Ocean’s water doesn’t freeze at zero degrees Celsius, and the water temperature often drops to two degrees below zero. Most fish can’t survive under such conditions – but some icefish species produce special-purpose “antifreeze” proteins, which prevent ice crystals from forming in their bodily fluids. In addition, they are the only vertebrates that have no red blood cells or haemoglobin in their blood. This colourless blood is less viscous and harder to freeze. But it also has a serious drawback: without red blood cells, oxygen transport in their bodies is far less effective. Accordingly, in the course of their evolution, these fish have developed extremely large hearts and wide arteries, a large amount of blood, and high blood pressure. Thanks to these adaptations, the icefishes are among the most successful and widespread denizens of the Antarctic.

Penguins are one of the most energy-efficient members of the animal kingdom. They possess extremely dense plumage, in which up to twelve feathers grow on a single square centimetre. Below the topcoat, their bodies are covered in fine down, between the fibres of which an insulating layer of warm air forms. This entire heat-trapping shield is waterproof, as the animals constantly rub themselves with the oil produced by special glands. Moreover, a complex thermal exchange process between their veins and arteries ensures that cooler blood flows to their feet and wings, while warmer blood is pumped to the core; this reduces heat loss through the extremities. And when emperor penguins huddle together during the winter brooding, their bodies’ surface temperature rises 0.6 degrees Celsius from the physical contact alone.

When it comes to climate change, the Antarctic displays two contrasting trends. For example, after the Arctic, the Antarctic Peninsula is the second-fastest warming region on Earth: over the past 50 years, mean average temperatures there have risen by 2.6 degrees Celsius – and even more in winter than in summer. Experts believe the rising temperatures may be connected to a shift in wind systems, due to which more warm air is transported to the Antarctic Peninsula than in the past. As a result, the sea ice is dwindling, the ice shelves extending from the land into the sea are crumbling, and the glaciers are retreating. Conversely, in East Antarctica, there is an equilibrium of shrinking and growth among the ice shelves.

Similar to the Arctic, the Antarctic is home to numerous species that have adapted to life in, on or below the ice. For example, of the six seal species native to the Southern Ocean, only the Antarctic fur seal and southern elephant seal breed on land; the crabeater seal, Ross seal, leopard seal and Weddell seal do so directly on the sea ice. In addition, countless penguins depend on stable ice cover as a platform for resting – emperor penguins even brood their eggs on the ice. Further, the nursery of one of the most ecologically important fish species in the Southern Ocean is below the ice: the Antarctic silverfish, a staple for many penguin and seal species, lays its eggs there in a metre-thick layer of delicate ice crystals. And the ice on the Southern Ocean is extremely important for the food web; tiny algae grow on its underside, providing food for amphipods, krill and other small herbivores – which are in turn food sources for countless larger marine fauna species.

These small, shrimplike crustaceans, which feed on microalgae and zooplankton, have perfectly adapted their lifecycle to the annual growth and shrinking of the sea ice. The larvae hatch in autumn and soon have to endure the long Antarctic winter. They manage to do so thanks to the plentiful algae on the underside of the ice. Studies have revealed that young krill can meet two-thirds of their carbon intake requirements from these ice algae alone in winter. At the same time, the cracks and crevices in the ice offer a safe refuge where the small creatures can hide from predators at night. As the ice dwindles in response to climate change, this young krill will likely have a harder time surviving the winter – which is one of the reasons why the stocks of these ecologically vital crustaceans have significantly declined in the waters off West Antarctica since the 1930s.

Though the young have a harder time surviving as the ice dwindles, the krill can certainly also be found in ice-free regions. Apparently there, too, they find something to eat during the winter, demonstrating their flexibility. As such, there’s no immediate risk of the krill going extinct. But there may not be such massive swarms of them in the future. According to estimates, there are currently between 300 and 500 million metric tons of krill in the Southern Ocean. If these stocks decline substantially in the future, food could become scarce for many other species. And a larger percentage of this popular prey might then no longer be found in their traditional regions, but in colder waters farther south. We are already observing a migration of krill from the Antarctic Peninsula toward the Amundsen Sea, where the warming is less pronounced.

The krill, a small marine organism, certainly lives up to its name, a Norwegian word that roughly translates to “what whales eat”. And in fact, the massive whales use the bristles (baleens) in their mouths to harvest huge quantities of krill from the sea. Without these tiny morsels, even the world’s largest animal – the blue whale, which can reach lengths of over 30 metres – couldn’t survive. But the tiny crustaceans, measuring up to six centimetres long, offer more than just a hearty meal for whales; they are widely considered to be the most essential food source in the Southern Ocean, since all the larger fauna in this habitat directly or indirectly depend on them. The Antarctic silverfish, for example, is a passionate krill fan, and is itself on the top of the menu for many seal and penguin species. The same is true for many species of squid, which are preferred food species for penguins. In addition, the crabeater seal, chinstrap penguin and Adélie penguin feed on krill. As such, they play a vital part in the marine food web and are essential to the wellbeing of marine life.

When the air and water temperature, sea-ice extent, available food, and the conditions in their traditional nurseries change, it has highly complex effects on penguins. Some species are already claiming new breeding grounds, while others have been forced to move closer to the South Pole, losing part of their distribution range in the process. For example, the Adélie penguin has profited from warmer climate phases for thousands of years: when the sea ice shrank and the glaciers retreated, it uncovered more of the stony ground for it to build its nests on. But the warming has now reached such a scale and speed that it’s doing the birds more harm than good. For example, rapidly rising temperatures on the western Antarctic Peninsula have led to declining numbers of the Adélie penguin, whereas the stocks in other regions are stable or even growing. In contrast, the gentoo penguin is a climate change “winner”. In areas where the Adélie penguin is in decline, this species often grows in number. The gentoo needs open water to hunt; also, since it eats more fish and squid, it’s less dependent on krill, which tend to be found near sea ice.

At this point, no-one can estimate how rising temperatures and dwindling sea ice will affect the stocks of Antarctic seals: for the majority of species and regions, there aren’t enough long-term studies to reliably identify trends. One of the few exceptions: the elephant seals on Marion Island in the southern Indian Ocean. For more than a decade, a team from the University of Pretoria and the AWI fitted countless specimens with satellite trackers, which allowed them to discover an interesting connection: when they hunt lanternfish and squid in warmer waters, these massive seals dive much deeper, possibly because they have to follow their prey into deeper and cooler water layers. Accordingly, when waters continue to warm due to climate change, hunting may become far more difficult for them. In addition, there are indications that climatic changes and a correspondingly limited food supply could reduce the survival rate for young elephant seals. On the other hand, the rising temperatures could also mean e.g. more ice-free stretches of beach on the western Antarctic Peninsula, where adults could moult and breed. As such, the outlook remains uncertain, even for the relatively well-researched elephant seals.

For most other species, such estimates can only be based on their needs. Consequently, the highly specialised crabeater seal, which directly depends on the sea ice and krill, will likely be harder hit by climate change than e.g. the Weddell seal, which has a broader range of prey. Especially near the western Antarctic Peninsula, the crabeater’s “restaurants” will likely shift farther from the coastal shelf and more to the south.

The changed ice conditions appear to already be affecting the behaviour of marine mammals: that was the main finding of a study in which experts from the AWI and the Helmholtz Institute for Functional Marine Biodiversity (HIFMB) in Oldenburg recorded the voices of various Antarctic seal species in the Weddell Sea. As the study shows, the retreating ice is already producing audible effects – during low-ice periods, those areas where normally numerous seal voices can be heard are suddenly silent.

Similarly to seals, it’s difficult to judge how whales will respond to climate change. Experts assume that changes in the sea-ice conditions will also have direct and indirect consequences for these marine mammals. There are species like the Antarctic minke whale that tend to retreat to ice-covered waters in winter. As such, it’s quite plausible that they will have to shift their distribution ranges to the colder regions far to the south. In contrast, other whales tend to avoid thick ice cover; in the future, they could stay longer in regions that were previously “off-limits” to them most of the year.

In addition to ice cover, above all, the distribution of prey will likely be affected by climate change. For instance, where the krill stocks decline, sometimes more salps appear. These gelatinous tunicates prefer warmer waters and less ice. Though whales do eat them, they much prefer krill, which could lead them to migrate to krill-rich regions. Moreover, the marine mammals’ annual cycle in the Antarctic could be more frequently disrupted: as climate change progresses, there will likely be more El Niño events. And this climatic anomaly has a major influence on the distribution of humpback whales. Using underwater microphones, an AWI team determined that these whales usually spend all year in the Weddell Sea – except under El Niño conditions, when their voices go silent. Presumably the krill stocks decline in the Weddell Sea at such times, while growing elsewhere.

First of all, such events have always been hazardous for animals living on the seafloor. In shallow coastal waters, the freshly calved icebergs plough the ocean’s floor, killing any organisms that don’t manage to get out of harm’s way. As climate change progresses, this could happen more often. On the other hand, the conditions below ice shelves are similar to those in the deep sea: given the darkness and meagre food supply, life doesn’t exactly flourish there. But that can rapidly change when the ice breakup allows more sunlight to reach the water. This could be seen e.g. in the Weddell Sea, where the Larsen A ice shelf collapsed in January 1995. Twenty years later, more species lived there than ever before. Even slow-growing organisms like glass sponges colonised large areas in the course of just a few years. And with them came a host of other marine fauna like sea stars, krill and icefish, corals, seals and Antarctic minke whales.

In the course of 50 years, Fourcade Glacier on King George Island has retreated more than a kilometre inland, creating a new, ice-free habitat for algae and other forms of marine life off the coast of Potter Cove. But this also has a downside: in summer, the glacier releases large amounts of meltwater, containing a wealth of fine sediments, into the ocean. Although this fertilises the ocean with iron, which is important for algal growth and chronically in short supply in the region, the sediments also cloud the water, which means the algae have less light for growth. When the algae die, their remains sink to the seafloor, where they are broken down by microorganisms. This reduces the oxygen concentration and a thick layer of sludge forms on the ocean floor, attracting species that feed on the algae remains or fauna prey species and can get by with little oxygen. In contrast, bivalves, ascidians and sponges, all of which filter their food from the water, don’t have a chance there. And the krill have also disappeared from Potter Cove, forcing species like the Adélie penguin to swim farther from shore to fill their bellies.

Scientists have observed efforts to avoid the progressive warming among e.g. southern elephant seals. The huge seals are increasingly shifting their territory to colder regions farther south, abandoning those farther north. Similar behaviour has been seen among e.g. chinstrap penguins and Antarctic minke whales.

Under climate change, generalists that can cope with a broad temperature range and which rely on various food sources have far better cards than cold-specialised species. For example, lanternfish are among those denizens of the Southern Ocean that will likely cope with the new conditions relatively well.

For land-based plants, the Antarctic is hardly a paradise: they can only grow in the few ice-free areas, and even there, the cold, dryness, extremely short vegetation periods and other adverse conditions make surviving a challenge. Only two species of blooming plant manage to thrive despite in this harsh environment: a grass called Antarctic hair grass, and Antarctic pearlwort, a member of the carnation family. Otherwise, the vegetation chiefly consists of mosses and lichens. Under climate change, the ice continent is gradually becoming greener. For instance, the growth rates of certain mosses on the Antarctic Peninsula have quintupled in the past 50 years. And the two species mentioned above have not only spread substantially since roughly 2009; they’re also growing much faster and in higher concentrations than in past decades.

The Antarctic Treaty, which entered into force in 1961, stipulates that the uninhabited Antarctic between 60 and 90 degrees South latitude can only be used for peaceful and above all scientific purposes. This regulation forms the basis for protecting the ice world at the southern end of the planet. And in 1991, the signatories ratified the Protocol on Environmental Protection to the Antarctic Treaty. It prohibits the commercial mining of natural resources and stipulates that the Antarctic, together with its flora, fauna and a certain level of air and water quality, is to be preserved for future generations. According to the Protocol, areas of particular ecological, scientific, historical or aesthetic value can also be declared Protected Areas. For example, the breeding colonies of penguins and other birds, the Amundsen-Scott South Pole Station, and the historic hut used by polar researcher Ernest Shackleton enjoy this status. In addition, there are special Marine Protected Areas (MPAs) in the Antarctic, which are established on the basis of the Convention on the Conservation of Antarctic Marine Living Resources (CCAMLR). The first-ever MPA, declared in 2009, was a 94,000-square-kilometre area near the South Orkney Islands; in 2016 it was joined by 1.55 million square kilometres in the Ross Sea. Three further MPAs are currently planned: one on the Antarctic Peninsula, one in East Antarctica, and one in the Weddell Sea, which, with a total area of more than three million square kilometres, would be the largest of its kind in the world. The scientific data needed as the basis for proposing an MPA in the Weddell Sea (Phase 1) was collated and analysed by experts from the AWI.

Protected Areas can help ensure the survival of species put under stress by climate change. Granted, MPAs cannot directly protect marine life from the impacts of climate change; however, they can offer refuges where further anthropogenic influences on the ecosystem are reduced or entirely eliminated, helping the species in question adapt to the changed conditions. In the future, the MPA in the Weddell Sea could offer a unique haven for cold-adapted species: given its ice cover and favourable currents, it will likely be one of the last regions of the Southern Ocean to feel the impacts of climate change. In order to be effective, Protected Areas should be as large as possible. Only then can the survival of e.g. penguins be ensured when their food sources shift to other regions.

It’s not just seals, penguins and whales that are interested in Antarctic krill; human fishing fleets pursue them as well. The tiny crustaceans can be used as fish feed for aquaculture, or as ingredients for nutritional supplements or wound creams. They are mainly caught near West Antarctica, in the northern Weddell Sea, and off the Antarctic Peninsula, where krill fishing has already transformed from a summer to a winter activity. But it’s hard to say how large future catches will be. In some regions, they will likely grow; in others, krill fishing will likely die out. The second important fishing species in the region is the Antarctic toothfish, whose stocks currently seem to be stable. But since we know so little about its lifecycle, and since its young may live under the increasingly sparse sea ice, the fishing industry may yet have an unpleasant surprise coming.