Arctic Species in Climate Change

One of the most prominent victims of climate change is the polar bear, which is already listed in the “vulnerable” category on the International Union for Conservation of Nature’s (IUCN) Red List of Threatened Species. In contrast to ice-dependent mammals like the ringed seal, it’s not yet considered to be endangered worldwide. But its stocks could dwindle in response to further ice retreat. Moreover, many small and unassuming animals, which are important food sources for other species, depend on the ice. In some regions, certain amphipods living on the underside of the ice, and nematodes living in the ice, have become rare.

One of the most important changes concerns the sea ice, which many animals in the High North depend on. As air and water temperatures continue to rise, the ocean’s frozen canopy is not only becoming thinner and less extensive; the ice isn’t growing as old as it once did, and is becoming more mobile, breaking up earlier in the spring and forming again later in the autumn. In addition, thanks to melting sea ice and more intense precipitation, today more freshwater is flowing into the Arctic Ocean. This freshwater lies like a lid atop the deeper, saltier water layers. As a result, fewer nutrients from the ocean’s depths rise to the surface, while less oxygen is transported below. Another effect of climate change: the increasing acidification of the ocean through carbon dioxide dissolved in the water. This change is especially detrimental for all species forming shells or skeletons of calcium carbonate, as the sinking pH values make it harder for them to do so. 

Many inhabitants of the Arctic use the sea ice as their habitat, nursery or pantry. The underside of the ice is home to a highly specialised biotic community of tiny organisms like bacteria, ice algae and crustaceans. These serve as direct or indirect food sources for larger marine organisms like fish, seabirds, seals and whales. Furthermore, for many seals the ice is an important place to rest, while some also use it as a nursery. And the ocean’s frozen shell offers good hunting grounds for polar bears.

Ice algae include all species living below ice floes or in the brine channels permeating the sea ice. Granted, these specialists can grow and use photosynthesis even in the dim light they find there. But when the thinning ice lets through more sunlight, initially their production will increase tremendously. In addition, they are increasingly growing in meltwater ponds, which form on the ice in the summer and freeze over in the autumn. In many regions of the Arctic, the melting sea ice will likely result in higher productivity among the ice algae in the short term. Yet this mass production likely won’t last all summer, since the Arctic Ocean lacks the nitrogen required for the production of organic matter. Moreover, the ice algae, which are adapted to weak light, could be subjected to stress when there is too much light, and could shut down their production in response. And when the sea ice ultimately disappears entirely, these adapted species simply lose their habitat.

Observations made by AWI experts have revealed that this effect can change the entire food web in the Arctic Ocean in just two months – and reaching from the water’s surface to 4,000 metres below. Thanks to algal blooms on the surface, in the short term, crustaceans, fish and other marine fauna have more to eat, although the quality is most likely to be lower. And since the blooms eventually sink down to the seafloor, it produces a veritable paradise for sea cucumbers, feather stars and other benthos. As a result, they grow better and reach sexual maturity earlier.

When the ice algae no longer have a habitat, it could spell trouble for the many species that directly or indirectly depend on them. An AWI study has shown that ice algae cover 50 to 80 percent of the carbon needs for the ice fans among the amphipods, copepods and conchs. And even for species living in deeper waters, this figure is 20 to 50 percent. Free-floating algae will be able to partly compensate for this loss of nutrients, but in some places they most likely won’t be enough to fill all the empty stomachs. Especially not in the autumn, when they have long-since drifted down to the seafloor.

According to many experts, polar bears will especially suffer from the loss of sea ice. After all, it’s where their migratory routes, breeding grounds, and in some regions, their nurseries lie. But most importantly, their hunting grounds. Though the animals can definitely spend part of the year on land, they chiefly hunt on the ice. They mainly prey on seals. And even though polar bears are excellent swimmers, they don’t stand a chance of catching a seal in the water – which is why they lie motionless beside ice holes, sometimes for hours, until one of the marine mammals comes up for air. Then they strike with breath-taking speed. But the regions in which this strategy has the best chances of success are now growing smaller and smaller. And on land, there are few alternatives for the hungry hunters. Since reindeer can easily outrun them, at least over longer distances, the bears, which can weigh up to 800 kilogramme, often have to make due with rodents, birds or eggs when hunting on land. Some have specialised in raiding landfill, which can lead to dangerous encounters between bears and humans.

The polar cod (Boreogadus saida) is a cold-water survival specialist. When the water temperature drops below zero, special glycoproteins keep its blood from freezing. As long as the Arctic Ocean remains frigid, the fish has a clear competitive advantage over fish species that don’t produce their own antifreeze. But as temperatures rise, the tables could be turned, putting competitors from warmer regions in the advantage – especially since the polar cod can develop metabolic problems when temperatures get too high. Laboratory tests conducted at the AWI have shown that these fish don’t grow as well in four-degree (Celsius) water as in water at the freezing point. In addition, polar cod are at risk of losing their nurseries, at least in some regions: the fish lay their eggs under the ice in winter, and their young are especially sensitive to heat. Many juvenile fish spend their first two years in and below the ice; its caves and channels offer them protection and an ample supply of plankton to feed on. It remains unclear whether or not parts of the population could get by without this refuge.

Whereas cold-loving species like the polar cod (Boreogadus saida) and Greenland cod (Arctogadus glacialis) are likely to increasingly have problems, the Arctic Ocean could offer new opportunities for less demanding species. The Atlantic cod (Gadus morhua), for example, is comfortable at water temperatures ranging from four to ten degrees Celsius and can readily adapt its metabolism, circulation and other bodily functions to changing conditions. It’s also flexible when it comes to food, salinity and water depth, which has allowed it to migrate farther north from the North Sea and the North Atlantic, where it finds favourable living conditions e.g. off the coast of Svalbard. AWI researchers have even confirmed the presence of individual Atlantic cod in the central Arctic Ocean, recently.

Many of the ca. 240 known fish species in the Arctic Ocean are also interesting for commercial fishing. To date, the fleets are especially concentrating on shallow coastal areas like the Barents Sea north of Norway and Russia. Now that species like the Atlantic cod are appearing there more frequently, the catch sizes have grown over the past few years. However, in future the polar cod’s distribution range most likely won’t extend so far to the south as in the past. And at least initially, there will be limits to how far north the fishing fleets can expand their activities: although the high seas of the Arctic will become more and more accessible as the sea ice dwindles, on 25 June 2021 an international agreement entered into force that prohibits commercial fishing in the high seas of the central Arctic Ocean, initially for the next 16 years. In the interim, research will be conducted into the ecological impacts of allowing commercial fishing there. Further, in light of the central Arctic Ocean’s low nutrient content, the profitable exploitation of its marine resources shouldn’t be expected any time soon.

In fact, Fram Strait between Greenland and Svalbard has become a preferred travel route for new residents of the Arctic. Though warm water from the Atlantic has always flowed into the Arctic Ocean there, over the past few decades climate change has made it even warmer, paving the way for many Atlantic species to venture northward. These newcomers include the conch Limacina retroversa, the amphipod Themisto compressa and various species of copepods and jellyfish. Even the orcas have now extended their hunting grounds farther north.

The newcomers from southern waters are changing food webs in the High North: new predators, prey and competitors are emerging, which can have far-reaching impacts. The Arctic amphipod Themisto libellula, for example, is traditionally an important prey species for fish, seals, seabirds and whales: consuming just one of the little creatures, which can measure up to six centimetres long, puts 40 milligrams of fat in a predator’s stomach. But if these native treats are supplanted by the originally Atlantic species Themisto compressa, animals feeding on crustaceans will have a hard time, since the new species only grows to two centimetres and contains three milligrams of fat at most. As a result, fish, birds and the like have to eat much larger quantities to cover their energy needs. What this will all mean for biotic communities in the Arctic in the long run is hard to say. 

Quite possibly, in the short term. For instance, there are indications that polar cod off the coast of Greenland have reacted positively to the earlier breakup of the ice and slightly higher water temperatures. But it’s questionable how long these positive effects will last if temperatures continue to climb.

Especially for fauna living in coastal regions, it will be virtually impossible. Though there are certainly lower temperatures to the north of their previous distribution ranges, these waters are also home to deep-sea regions that can’t offer them suitable habitats. For example, there are no viable spawning grounds for the polar cod. In contrast, various species of zooplankton could also cope well in these regions. However, they’ll most likely only be able to reach a fraction of their previous stocks; there simply aren’t enough nutrients in the central Arctic Ocean to support higher productivity.

In fact, there are a number of similarities: both regions are home to species that can’t easily cope with rising temperatures or other effects of climate change. In addition, sea ice is essential to ecosystems in both regions. And just like in the Arctic, if the Antarctic Ocean grows warmer, there are limits to how far poleward local organisms can migrate, since the Antarctic continent forms a natural barrier. Yet there are also differences that will most likely mean more dramatic impacts in the Arctic than in the Deep South.The Arctic Ocean stretches from 75 degrees N to the North Pole, while the ice-covered Southern Ocean only stretches from roughly 60 to 70 degrees S. As a result, the North undergoes much more drastic changes in light conditions in the course of the year. The endless dark of the Polar Night, in which algae can’t grow and there is therefore less food for fauna, goes on much longer there. This is yet another reason why migrating to colder regions to the north isn’t an option for many Arctic species and newcomers alike: they simply can’t endure the long starvation phase, especially when there is no sea ice to supply them with algae. Furthermore, there are differences in the nutrient supply: the Arctic is lacking in nitrogen, which means that, regardless of the temperature and light conditions, it will never be a Garden of Eden. In contrast, the nutrient supply is fairly good in the Antarctic, although a lack of iron severely limits algal growth in broad expanses.