Marine life
In the rhythm of light and ice
Like the land areas of the polar regions, the seas are also classed as extreme habitats. The Antarctic and Arctic Oceans have the coldest and most constant water tem-perature of all the world’s oceans. This temperature is below zero degrees for most of the year and seasonal fluctuation is usually less than five degrees Celsius. In very southerly ocean regions such as the McMurdo Sound, a bay that forms part of the Ross Sea in the Antarctic, the difference between summer and winter temperatures is in fact less than half a degree Celsius. The inhabitants of this region must therefore cope with very cold ambient temperatures throughout the year. For most of the time the water temperature is minus 1.8 degrees Celsius. Furthermore, seasonal changes have a more marked effect on the polar seas than on any other ocean: in summer the sun never sets, while in winter darkness reigns for months on end.
This switch between polar day and polar night has a profound impact on life in the Arctic and Antarctic Oceans. Sea ice forms as the days shorten, so that in winter it covers much of the ocean surface; in summer the ice melts again, retreating to a minimum area. The ebb and flow of light and sea ice determine the rhythm of life in the polar regions. Where the sea ice breaks up in summer, sunlight at last penetrates the upper layers of water and stimulates algae growth. At the same time, the melting floes release microorganisms and other life-forms associated with the ice; these disperse in the water, along with trace elements such as iron that have been encased in the ice or deposited on its surface in the form of dust over the winter.
Meiofauna
Meiofauna, also known as mesofauna, are a major category of benthic fauna. The term covers all bottom-dwelling organisms of between roughly 0.05 millimetres and one millimetre in size. Smaller organisms are classed as microfauna, while larger ones up to 20 millimetres in size fall under the heading of macrofauna.- In the spring and summer, sunlight, iron and other water-borne nutrients such as nitrogen, phosphorus and silicon compounds trigger large-scale algal blooms that provide the basis for the food webs of the polar oceans. In coastal waters in the Antarctic, algal density can peak at a level of 30 milligrams of chlorophyll per cubic metre of water. In winter, by contrast, there are so few algae in these same places that the water’s chlorophyll content may fall to less than 0.01 milligrams per cubic metre. In no other ocean are seasonal differences in biomass production so large. In autumn and winter the formation of sea ice inhibits life in the Southern Ocean. When the sea surface cools and turns to ice, key nutrients such as iron have usually been used up by algal growth in the summer. Any substances that remain sink to the sea floor, partly as a result of the thermohaline circulation of the water masses. This means that virtually no food is left in the upper metres of the water column. Algae stop growing and primary production ceases. Moreover, the sea ice shields the water column from the wind and thus prevents intermixing of the upper water layers. As a result of this lack of eddying, algae, faeces and other particles suspended in the water column fall to the sea floor, thereby drastically reducing the nutrient content of the column.
For most of the inhabitants of the polar seas, the seasonal succession of light and ice means that periods of abundance constantly alternate with periods of hunger. In addition, many organisms – particularly those that live on the bottom of the shelf seas – are always in danger of having their habitat destroyed by drifting icebergs or sea ice floating in the shallow waters. An iceberg ploughing across the sea floor in the Antarctic kills more than 99.5 per cent of the established macroorganisms and more than 90 per cent of the smaller meiofauna. In areas in which icebergs are plentiful this can happen more than once a year. In consequence, the biotic communities in these disturbance zones are usually very young and colonize the sea floor only patchily.
- 4.22 > Elephant seals, together with king penguins and other seabirds such as petrels and albatrosses, line the shore of Saint Andrews Bay on the north coast of South Georgia. Here the penguins form breeding colonies of up to 100,000 birds.
A question of iron
Although the marine fauna of the Arctic and Antarctic have much in common with each other, they are not identical. This is partly because of differences in the supply of nutrients and trace elements in the two regions. In the far north, rivers carry large quantities of suspended material into the marginal seas, thus providing the Arctic Ocean with the iron that is vital to living things; the deep south, by contrast, lacks such a reliable source of iron. Although the water masses of the Antarctic are nutrient-rich, they suffer from an almost universal lack of iron. In consequence, algal blooms form mainly in two areas: firstly, in the coastal waters and polynyas (areas of unfrozen sea within the ice pack), where iron comes from sources such as glacier meltwater, and, secondly, on the edges of the continental plate, where iron-rich water wells up from the depths. The largest of these upwelling zones stretches eastwards from the tip of the Antarctic Peninsula to South Georgia. It is a hotspot of life, home to the largest concentrations of krill in the Antarctic and a magnet for krill hunters such as whales, penguins and seals.
By comparison, the upper water layer of the central Arctic Ocean is relatively nutrient-poor. The summer ice melt and the pronounced stratification of the Arctic water masses as a result of the large quantities of freshwater discharged by rivers prevent deep, nutrient-rich water rising to the surface. Brief, intense algal blooms in the spring and summer therefore occur mainly near the edge of the ice and in the marginal seas. The Barents Sea, Chukchi Sea and Bering Sea are among the most productive marine ecosystems in the world, providing so much food for bottom-dwellers, fish, seabirds, seals and whales that huge colonies of these creatures can occur.
- 4.23 > Copepods not only make up the majority of marine zooplankton – with around 13,000 species, they are also the most species-rich group of crustaceans. Polar species are usually somewhat larger and more nutritious than their relatives in the mid-latitudes.
- Despite this, animal numbers in the Arctic are often only a fraction of the size of Antarctic populations. For example, in the area around the North Pole and the adjacent sub-polar regions there are just 13 bird species with a total population of more than a million, while in the south there are 24 polar and sub-polar bird species. The most abundant seal species likewise lives in the Antarctic: it is calculated that there are between 50 and 80 million crabeater seals (Lobodon carcinophaga) in the region, although the vast extent of their habitat means that population estimates are uncertain.
The large numbers of birds, seals and whales in the polar seas at one time led scientists to assume that more biomass is produced and passed on in the food web in these regions than is the case at lower latitudes. This was attributed to short food chains formed of a few key organisms. With regard to the Antarctic it was thought that almost all life hinged on diatoms photosynthesizing and being eaten by Antarctic krill, which were in turn hunted by all the larger animals such as fish, penguins, seals and whales.
This simplified point of view is outdated. We now know that the range of primary producers in the polar seas – in this case predominantly the algae – is just as diverse as in the mid-latitudes. Microbes, plankton and other microorganisms interact in complex ways. In addition, it is now recognized that, although krill undisputedly remains one of the key species, there are many feeding relationships in the Antarctic in which it plays no part.
Consideration of the food web in the two oceans reveals two striking features. Firstly, in the polar seas there are relatively few species that serve as food for the large predators. For example, 80 to 90 per cent of the zooplankton in the Arctic consists of fatty copepods, which form the most important link between the primary producers and larger consumers such as fish and baleen whales. In the Antarctic, this role is performed by krill, amphipods and copepods. Secondly, the hunters and consumers of the Antarctic pursue different prey from those of the Arctic. While seals, whales and seabirds in the Arctic Ocean eat mainly fish and organisms that live on the sea floor, the large predators of the Antarctic Ocean feed largely on krill and fish such as the Antarctic silverfish (Pleuragramma antarctica). Sharks, walruses and whales that search for food chiefly on the sea floor are completely absent from the Antarctic.
The survival tricks of cold-blooded sea-dwellers
The fauna of the polar seas consists largely of cold-blooded creatures which over millions of years have developed unique mechanisms for adapting to their extreme living conditions. However, because of the geographical isolation of the Antarctic and its longer glacial history, these mechanisms are more marked there than in the Arctic. Notable adaptive mechanisms in the cold-blooded creatures include:- slower growth, late sexual maturity,
- reduced activity,
- production of antifreeze proteins (especially in fish),
- reduction in red blood cells (also mainly in fish),
- incorporation of unsaturated fatty acids into cell membranes,
- weight savings by doing without calcium deposits in scales and skeleton,
- gigantism,
- smaller clutches with large eggs that contain food reserves to support the growth of the larvae, and
- live births and attentive care of the young.
- 4.24 > Ice algae and free-swimming phytoplankton in the water form the foundation of the food web in the Arctic Ocean. Using sunlight, carbon dioxide and nutrients, these organisms produce biomass that provides nourishment for all consumers, from zooplankton to bottom-dwellers, fish, birds, sea mammals and ultimately humans.
Temperature and lack of food as brakes on growth
Cold has a persistently adverse impact on the lives of cold-blooded sea creatures. Among other things, it affects their respiration and muscle function and hence their ability to move. It also slows their growth and development, and in consequence there is considerable similarity between the life cycles of polar species. Life in the cold oceans proceeds very slowly, and each developmental step takes longer than in the mid-latitudes. For example, the embryonic development of many cold-blooded sea-dwellers in the polar regions takes five to ten times as long as that of thermophilic species in the mid-latitudes. The embryonic stage is often followed by slower growth and delayed sexual maturity. Thus while fish in warmer regions are usually ready to mate after one to four years, the larger fish species in the Antarctic take between six and ten years to reach this point. The Patagonian toothfish (Dissostichus eleginoides) does not reach sexual maturity until it is 13 to 17 years old. Some Antarctic ascidians, bryozoans and sponges form an exception to this rule: they are also cold-blooded, but various studies have found that they grow or spread relatively quickly. Nevertheless, even these animals develop more slowly than related species in the mid-latitudes.
The delayed development of polar species is partly attributable to the fact that cold hinders the animals’ protein metabolism – i.e. the constant synthesis of proteins and their subsequent breakdown into amino acids in the cells. Continuous protein synthesis is an essential process in growing organisms, enabling protein to be transported to the organs and structural elements. At temperatures of zero degrees Celsius and below, however, it becomes difficult for cold-blooded creatures to maintain their protein metabolism and produce many fully functioning proteins. Researchers have discovered that in the cells of Antarctic species up to 80 per cent of the synthesized proteins are not re-used but are instead broken down again. In thermophilic species the corresponding figure is just 25 to 30 per cent. Polar species have a particularly high concentration of ubiquitin, a protein molecule that is responsible for identifying and removing damaged proteins. These faulty proteins are degraded in a particular part of the cell nucleus, the proteasome. In polar fish the ubiquitin is activated between two and five times more frequently than in fish from the mid-latitudes.
- As a result of this and other cellular features, the protein metabolism of cold-blooded animals in the polar seas yields relatively few proteins that can ultimately be used for growth. This means that growing is a particularly energy-intensive process for polar species and in many of them it therefore proceeds very slowly. On the other hand, the cold-blooded inhabitants of the Arctic and Antarctic Oceans reach an unusually advanced age. Larger species of fish such as the Patagonian toothfish, which lives in the Antarctic, have a life span of between 15 and 30 years. The Antarctic bivalve (Laternula elliptica) lives for up to 36 years. The ocean quahog (Arctica islandica), a species of clam found in the far north, is a record-holder: in 2006, British scientists found a specimen off the coast of Iceland that was 507 years old.
Researchers also attribute the slow growth of cold-blooded sea-dwellers to the extreme seasonal changes that affect the polar seas. Most cold-blooded animals only grow when they find food – but for many polar species sufficient quantities of food are available only during the periods of algal bloom that occur in summer. In the Antarctic, for example, more than 95 per cent of the species that feed on free-floating phytoplankton or algae growing on the sea floor stop eating in winter, while those that do continue to feed consume only a small percentage of what they would otherwise eat. One of the consequences of this is that the animals’ growth is almost entirely limited to the summer.
Radiocarbon dating
Radiocarbon dating, also known as C14 dating, is a method of determining the age of organic matter. Scientists calculate the relative quantities of the radioactive carbon isotope 14C and the non-radioactive isotope 12C in the sample; the resulting ratio indicates how many years have elapsed since the plant or animal died.Always in energy-saving mode
It is not only the development of cold-blooded creatures that proceeds slowly in the polar regions: their everyday lives are also spent in energy-saving mode. This means that they move particularly slowly and avoid unnecessary effort. For example, Antarctic fish do not chase after their prey in the water column but conserve their energy by lying in wait for them on the sea bottom. Adamussium colbecki, a bivalve found in the Antarctic, closes its valves only half as often as molluscs in warmer regions, and the predatory snail Trophonella longstaffi takes 28 days to bore through the shell of its prey and eat it – a task for which related species in water of ten to 15 degrees Celsius need just ten to twelve days.
Naturalists currently know of only two adaptations developed by marine creatures in the Antarctic to make up for the disadvantages of the temperature-related effect on movement and enable them to operate at a speed akin to that of their non-polar cousins. The first involves the swimming speed of Antarctic fish. They have around twice as many mitochondria in their muscle cells as related species in warmer seas. This enables Antarctic perch to generate so much energy that they can when necessary swim just as fast as comparable species from the mid-latitudes. The second example is supplied by the Antarctic bivalve Laternula elliptica: because its foot muscle is two to three times the size of that of related molluscs in the mid-latitudes and tropics, it can dig itself into the sea floor just as fast as they can.
Fats, frost protection and colourless blood
The strategies for adapting to life in the polar seas with which people the world over are most familiar are those adopted by fish. All species of fish that live in waters of below zero degrees Celsius prevent themselves freezing to death by producing antifreeze substances in the form of glycoproteins. These are found in all the fishes’ body fluids and they are not excreted by filtering organs such as the kidneys. Glycoproteins are sugar compounds that inhibit the growth of ice crystals in the fishes’ tissues. As soon as an ice crystal starts to form, the glycoproteins accumulate in this miniature crystal and prevent further water molecules docking on to it. The tiny ice/sugar complex that arises is then excreted via the metabolism. Using this protective mechanism, the fish reduce the point at which their body fluids freeze to below minus 2.2 degrees Celsius and are able to survive ambient temperatures of up to minus 1.8 degrees Celsius. This is an adaptive strategy that fish from the Arctic and Antarctic have developed completely independently of each other; it remains one of the best examples anywhere in the world of parallel evolution – the development of identical characteristics in unrelated species.
A second distinctive adaptive strategy occurs only in the Antarctic Ocean. The Southern Ocean is home to the icefish or white-blooded fish (Channichthyidae), a family of Antarctic fish. Sixteen species of this family of predatory fish have no red blood corpuscles and they also lack haemoglobin, the red pigment that gives blood its colour. Their blood is completely translucent. Haemoglobin is responsible for transporting oxygen in the bodies of many vertebrates, including humans. Each molecule of the pigment has four docking points for oxygen molecules; this is a particularly efficient method of transporting oxygen from the lungs, where it is taken in, to the parts of the body where it is needed. On the return journey the haemoglobin molecules pick up the carbon dioxide produced in the tissues and take it back to the lungs, where it is exhaled.
- 4.26 > Human red blood corpuscles are shaped like tiny biconcave discs and filled with the iron-rich blood pigment haemoglobin. Their shape enables them to transport oxygen very efficiently. The blood corpuscles also prevent the toxic haemoglobin escaping into the blood stream.
- Excellent as this gas transport system is, haemoglobin is not entirely beneficial. Free haemoglobin in the body can be toxic: that is why it is encapsulated in the red blood corpuscles in humans and many vertebrates. In addition, haemoglobin becomes less efficient at binding oxygen as temperatures fall. Under very cold conditions, the presence of many red blood corpuscles carrying haemoglobin can make the blood more viscous and hence harder for the arteries to transport – especially when miniature ice crystals are also floating in the blood, as is the case in the Antarctic fish.
To prevent this problem, many species of polar fish have in the course of evolution reduced the number of red blood cells in their bodies. The icefish have even managed to do away with haemoglobin entirely. They live only from oxygen that diffuses directly into their blood in their enlarged gills or via their skin. The oxygen then dissolves physically in the blood, which means that the oxygen molecules do not dock on anywhere but are transported as they float freely in the blood.
However, the amount of oxygen dissolved in the blood in this way is fairly small. Icefish such as the blackfin icefish (Chaenocephalus aceratus) have to make do with a quantity of oxygen in their blood that is about ten per cent less than that available to red-blooded Antarctic fish. Researchers now think it likely that this haemoglobin-free oxygen supply in icefish only works because the cold conditions in the Antarctic Ocean mean that almost all the fish there have a metabolic rate ten to 25 times slower than that of fish in warm seas of 30 degree Celsius, with the result that the Antarctic fish use less oxygen. Furthermore, the cold water masses of the Southern Ocean are very oxygen-rich, with an oxygen concentration that is almost twice that of tropical seas, making it easier for all inhabitants of the Antarctic waters to take in oxygen. If fish outside the Antarctic had at some point stopped producing haemoglobin, they would have died immediately, but under Antarctic conditions such creatures still have a chance of survival.
- Nevertheless, the fish with “white” blood display some specific characteristics that suggest that their circulatory systems must handle very large quantities of blood in order to provide a sufficient supply of oxygen. For example, by comparison with fish with red blood corpuscles, icefish have a heart so large that it pumps four to five times as much blood, arteries that are one-and-a-half times bigger in diameter and a blood volume two to four times greater.
The colder the habitat of cold-blooded sea-dwellers becomes, the more frequently they incorporate unsaturated fatty acids into their cell membranes. This enables the membranes to maintain their fluidity and hence remain fully functioning at low temperatures. Without this protective mechanism, the membranes would become gel-like in the cold and lose the permeability that is vital to survival.
Antarctic fish also have no swim bladder. So that they can nevertheless move with as little expenditure of energy as possible, they store lipids in their liver and other cells. These fats provide additional buoyancy. The fish also reduce their body weight by incorporating relatively little calcium into their skeleton and scales, replacing it where necessary with cartilage. As a result, the skeleton of the blackfin icefish appears almost transparent.
Polar giants
Although most of the cold-blooded inhabitants of the polar seas grow slowly, some of them – especially those in the Antarctic – reach a remarkable size. This has led researchers to coin the term “polar gigantism”. The sea spiders (Pycnogonidae) of the Antarctic are a good example: they achieve a diameter of more than 50 centimetres, while the largest sea spiders in moderate latitudes grow to no more than three centimetres across. The amphipods (flea crabs) of the Southern Ocean are up to nine times as long as their tropical cousins, and the cup-shaped glass sponges reach record sizes of two metres in height and 1.5 metres in diameter.
The factors behind this gigantic polar growth have long been hotly debated by scientists. The reasons that have been put forward include:- Low maintenance costs: Because temperatures in the Antarctic Ocean are low and the metabolic rate of most cold-blooded species is restricted, polar organisms generally need to expend less energy to maintain a large body than similar-sized animals in warmer regions.
- High oxygen content of the polar seas: This eases respiration and hence metabolism.
- High concentration of silicon in the water: This enables diatoms, glass sponges, radiolarians and other organisms to build up their silicon-based skeletons without expending excessive energy.
- Abundant supply of food in summer: The large algal blooms constitute a rich source of food that provides ideal growing conditions for animals with a slow metabolic rate and encourages growth-promoting competition between individuals.
- Seasonal fluctuations: In relation to their body mass, larger organisms need less energy than smaller ones. Bigger creatures can also lay down larger energy reserves, which puts then at an advantage at times when little or no biomass is being produced.
Large eggs, attentive parents
Polar fish lay a relatively small number of eggs, but these eggs are considerably bigger than those of thermophilic species. In addition, Antarctic fish are often surprisingly active in caring for their young: they deposit their eggs on rocks on the sea floor or in the oscula of glass sponges and guard them until they hatch.
The flea crabs and krill of the Southern Ocean also produce strikingly large eggs that are usually two to five times as big as the eggs of related species in lower latitudes. The trend towards larger eggs in the polar regions is apparent even within a species. For example, the eggs laid by the Antarctic crustacean Ceratoserolis trilobitoides in the Weddell Sea are almost twice the size of those spawned by the same species in the vicinity of South Georgia a little further north. Scientists attribute this phenomenon partly to the uncertain availability of food in the polar regions. While animal species in warmer oceans can be fairly certain that their offspring will find sufficient food and grow quickly, this is not the case in the polar seas where food is sometimes hard to come by and the low temperatures make for long development times. For these reasons the young are usually given larger food reserves at the start – that is, in the egg. In addition, the offspring are usually somewhat larger when they hatch, thus increasing their chances of survival at the most critical stage of their lives. Scientists have also discovered that the cold-blooded creatures of the cold, oxygen-rich polar seas have larger cells than related species in warmer waters that contain less oxygen: this is another simple reason why their eggs are larger.
After spawning and fertilization, the life cycle of many polar species again differs from that of their thermophilic cousins. While invertebrate species in warmer regions often pass through a larval stage during which they must actively search for food (planktotrophic nutrition), the young of many Antarctic species are provided with a yolk sac that supplies the larvae with food to last them until they reach the next developmental stage in their metamorphosis; this is termed lecithotrophic nutrition. The main reason for this is once again the extended development time as a result of the cold conditions in the polar seas. The less food the larvae find, the more slowly their already protracted development proceeds; this in turn means that there is a longer period during which the young are at risk of being eaten themselves. The larvae of the Antarctic starfish Odontaster validus, however, must search for their own food and may spend up to 180 days in the water column before they settle on the sea floor and complete their metamorphosis to young starfish.
- 4.28 > in spring and summer the many pores, pockets and brine channels of the Arctic and Antarctic Oceans support flourishing and species-rich communities of cold-adapted ice algae, bacteria, archaea, viruses, fungi and microorganisms. Scientists have identified more than 2000 species that live in or on sea ice.
Life in, on and under the sea ice
The sea ice of the Arctic and Antarctic provides a unique habitat for the flora and fauna of the polar regions – even in those parts of the polar seas in which ice cover is present only at certain times of year. Scientists have identified more than 2000 species of algae and animals that live in or on sea ice – the majority of them too small to be seen with the naked eye. In addition to these species there are numerous bacteria, archaea, viruses and fungi that are adapted to the cold. In consequence, researchers now believe that the sea ice harbours a biological community made up of several thousand species, the growth and reproduction of which underpins the survival of all the marine fauna of the polar regions.
At the start of this important food chain are the ice algae: when the seawater freezes, many of these become encased in the ice together with particles, nutrients, a whole host of bacteria and all sorts of microorganisms. Unlike meat and vegetables in the domestic freezer, though, the organisms themselves do not freeze; instead they survive on the underside of the ice or in the vast number of small pockets and channels full of brine and seawater that form in the sea ice. To exist in this extremely salty environment at temperatures as low as minus ten degrees Celsius in the Arctic and minus 20 degrees Celsius in the Antarctic, the ice-adapted microorganisms have altered the composition of the lipids in their cell membranes. This prevents the membranes hardening and ensures that the organisms can continue to absorb nutrients from the seawater. Protein production in the cells is also adapted to the cold so that all the processes vital to survival proceed as smoothly as possible even at low temperatures. Ice algae also form antifreeze proteins and lay down fat reserves in summer that enable them to survive the long winter. Despite these survival strategies it is nevertheless the case that the warmer and less salty the sea ice is, the better the sea-ice flora and fauna will flourish.
The ice-algae community consists mainly of diatoms, of which many different species occur in both polar regions. The number of ice algae present in a block of sea ice depends on how much light penetrates the ice, on its salt content and on the nutrients that are encased in the ice or available in the water beneath. Multi-year sea ice usually contains more species of algae than young ice. These relatively old floes also function as a sort of seed bank – especially in the pack ice, which frequently consists of newly formed ice, year-old ice and multi-year floes. In the spring, as temperatures rise and the ice becomes more porous, algae from the multi-year ice migrate to the younger ice and start an algal bloom there.
4.29 > Viewed from below, the Antarctic pack ice forms a rugged landscape in which algae colour the underside of the ice greenish brown in places where the most light penetrates.- Ice algae flourish mainly in the lowest layer of the ice, close to the water. Species such as the Arctic diatom Melosira arctica also colonize the underside of the ice; in spring they sometimes form algal mats that trail downwards into the water column for up to two metres. Bacteria, on the other hand, are found in almost all layers of the sea ice, although they cluster in the lowest layer and on the ice surface.
The species community of the sea ice spends the long, dark winter in a relatively torpid state. But in the spring, when the sun once again climbs above the horizon, the algae in the lowest ice layer quickly grow and multiply, drawing the nutrients that they need from the seawater. As soon as the ice algae start to bloom, tiny algae eaters such as copepods, amphipods and krill larvae fall on the growing mountain of food. Some of the algal build-up sinks downwards and is devoured on the sea floor by sea cucumbers and other bottom-dwellers.
When the feasting starts in the many niches of the ice, the first zooplankton hunters are already lurking directly below the ice. In the Arctic these species include predatory flea crabs such as Apherusa glacialis and Gammarus wilkitzkii. But they also need to be careful, because alongside the flea crabs there will be polar cod and Greenland cod hunting for zooplankton under the ice. The fish mainly seek out amphipods and copepods, but they also eat mysids (opossum shrimps). The polar cod actually spawns in the labyrinth of the pack ice. Its millions of young spend the first year of their life concealed in the nooks and crannies of the ice. With the pack ice, they migrate from the spawning areas north of Siberia to the central Arctic. By diving under the ice, scientists have also discovered that jelly-like zooplankton such as comb jellyfish can occur in dense clusters under the ice. These animals seem to gather mainly in areas where the sea ice projects particularly deeply into the water column, thus causing upwelling and downwelling of the water.
Mammals and birds have two strategies for gaining access to the larder under the sea ice. They may use holes and cracks in the ice to break into the feeding grounds; this method of hunting is used in particular by various seal species of the Arctic and Antarctic Oceans. Alternatively, they may wait for the ice-free summer. This is the method adopted by Arctic mammals such as beluga whales and the big baleen whales. They spend the winter outside the sea-ice zone, not travelling north until the ice slowly retreats and large algal blooms form in the marginal ice zone.
Polar bears hunt seals on the surface of the sea ice, thus forming one of a number of end points in a food web the existence of which is directly linked to the sea ice. The life style of each member of this web is so precisely adapted to polar conditions that these species would have little chance of survival elsewhere. For all of them, the shrinking of the Arctic and Antarctic sea ice means a loss of vital habitat.
Antarctic krill – the mass phenomenon
A keystone species of the polar regions that depends directly on the sea ice for its survival is the Antarctic krill (Euphausia superba). This bioluminescent crustacean, sometimes referred to as a light-shrimp, is a type of zooplankton that has garnered many superlatives. Its body length of up to six centimetres makes it one of the largest creatures of its type in the Southern Ocean. Individuals can live for up to eleven years and in terms of biomass they are one of the most abundant species on the planet. It is estimated that there are 133 million tonnes of Antarctic krill in the circumpolar regions, excluding larvae. Only humans weigh more in total. The term “krill” comes from the Norwegian word for whale food. It used to include other zooplankton species such as winged snails (Pteropoda) and jellyfish, but in everyday language the word is now used to refer only to Euphausia superba.
Antarctic krill occur only in the Southern Ocean and are thus one of the many endemic species of the Antarctic. There are five other species of crustacean in Antarctic waters, including Euphausia crystallorophias, the ice krill or crystal krill. This species lives mainly in the very cold marine shelf regions in the south, while Euphausia superba prefers deep sea areas further north with warmer average water temperatures of between zero and three degrees Celsius. The habitat of Euphausia superba is thus limited to somewhat more than half of the extent of the Southern Ocean – more precisely the area between 51 and 74 degrees south. Scientists have identified six large concentrations – one in the northern Weddell Sea and the Scotia Sea, one off Enderby Land, one around the Kerguelen Gyre, two smaller accumulations in the north of the Ross Sea and one population in the Bellingshausen Sea to the west of the Antarctic Peninsula.
- 4.30 > Using data from net catches and other sources, scientists have found that populations of Antarctic krill are distributed unevenly around the southern continent. Particularly large swarms occur in the northeast of the Antarctic Peninsula, off the coast of Enderby Land, around the Kerguelen Gyre, in the Ross Sea and in the Bellingshausen Sea.
- Because of this patchy distribution, the krill is not the link between primary producers and higher consumers in all parts of the Southern Ocean. Scientists have now identified three zooplankton communities in the Southern Ocean and their corresponding keystone species. The zooplankton in the northern part of the Antarctic Ocean are dominated by the salp Salpa thompsoni and the amphipod Themisto gaudichaudii. In the southern part, on the other hand, it is mainly the ice krill and the Antarctic silverfish (Pleuragramma antarctica) that occupy the key positions in the food web, with the Antarctic krill playing an important but nevertheless subordinate role. In the middle, however, the Antarctic krill and its close relative, the euphausid Thysanoessa macrura, plus a number of copepods, are the main prey of hunters such as fish, whales, seals, penguins and other seabirds.
In the regions where they flourish, the krill occur in swarms of up to 30,000 animals per cubic metre of water. In the Antarctic summer the krill swarms usually occupy the upper 50 to 150 metres of the water column. At the start of winter in April, however, they often sink to a depth of about 200 metres, but they have also been sighted at depths of 1000 to 3500 metres.
The eggs of the crustaceans, which the females lay from January to March, sink to a depth of more than 2000 metres. In the deep sea, free-swimming larvae emerge from the eggs; as the summer draws to a close the larvae rise higher and are carried by the surface currents in the upper part of the water column. Thus the krill larvae that hatched in the Bellingshausen Sea migrate within 140 to 160 days to the waters around South Georgia.
- The young krill survive their first winter by hiding in the niches, hollows and cracks of the sea ice, feeding in autumn on the ice algae and in winter mainly on copepods and other microorganisms. Formation of the sea ice early in the winter appears to be an important factor, enabling the larvae to find protection and food for as long a period as possible. When the ice melts in spring, triggering the growth of algal blooms, the crustaceans complete the final stage of their development and become young adults.
The survival prospects of the mature krill are less heavily dependent on the sea ice. Some crustaceans survive the dark part of the year by ceasing to eat and reducing their metabolic rate by up to 50 per cent. During these fasting periods the animals may actually shrink. Other creatures look for alternative sources of food: they eat zooplankton that are still floating in the water column, or they sink to the sea floor, where they eat plant and animal remains that have trickled down. It is the length of the day that determines when the crustaceans’ metabolic rate and feeding behaviour switch to the winter pattern. This information is derived in part from laboratory studies which showed that under winter light conditions the animals ate very little, even if plenty of food was floating in the aquarium.