The Southern Ocean, which is entirely surrounded by sea, is a mass of water and ice covering approximately 36 x 106 km2 which represents about 10% of the world's oceans. The northern limit of the Southern Ocean is set by the Antarctic Polar Front (Antarctic Convergence), identifiable as a 40 km wide ocean band where a 2-3° C change in temperature of the sea surface occurs. This relatively abrupt change in water temperature occurs where dense, cold polar water meets and flows under less dense warmer water from the tropics. The Antarctic Polar Front separates the Southern Ocean from the Atlantic, Pacific and Indian Oceans which border it to the north. Water in the Southern Ocean moves eastwards (clockwise) around Antarctica carried by the Antarctic Circumpolar Current, the largest ocean current in the world. A counter-current, the Antarctic Coastal Current, flows westward closer to the continent.
The Southern Ocean Food Web
The Southern Ocean food web describes the flow of energy within the Southern Ocean ecosystem from microbes to the top predators. Marine phytoplankton (the primary producers) use energy from the sun to make simple organic molecules by photosynthesis. The phytoplankton fix about 50 tonnes of carbon (from CO2) per 100 km2 of sea surface each year. Because of their dependence on light, phytoplankton growth is restricted to surface waters where there is little sea ice (which would otherwise limit light penetration), and it is influenced by seasonal variations in sunshine. A small amount (<10%) of the organic molecules produced by photosynthesis are recycled by other microorganisms which may drop to the sea floor where they fuel bottom-dwelling organisms. The major proportion passes through the food web at the hub of which is the Antarctic krill (a key herbivore) and a variety of zooplankton. Predators of krill such as the baleen whales, crab eater seals, penguins, fish and squid are the primary predators of the food web. Higher order predators include the killer whale, leopard seal and birds such as the wonderful skua.
The Antarctic krill, of which there are several species, is a shrimp-like
crustacean growing to about 5 cm in body length. Krill (the name is derived
from a Norwegian whaling term meaning "small fry") are widely distributed
around the Antarctic continent.
Euphausia superba dominates north
of the pack ice, while the smaller E. crystallorophias is the principle
krill species under the ice. E. superba form dense swarms, typically
hundreds of metres across and 20 or so meters deep. The largest swarms
can spread over several square kilometres and may extend to a depth of
200 metres. Swarms of such dimensions may contain up to 10 x 103
tonnes of krill which makes them an extremely abundant food supply. It
has been estimated that there are over 6 x 1011 individual krill
with a biomass of around 250-600 x 106 tonnes but, because of
their patchy distribution, these figures may not be particularly reliable.
Although labelled as the dominant herbivore in the Southern Ocean food
web, krill are actually omnivores feeding on the zooplankton as well. The
life history of E. crystallorophias is not well known and further
study of this species remains a great challenge to scientists.
|The Antarctic krill
- Euphausia superba.
For more information about krill visit Uve Kils' website from where this image is sourced.
Krill display a number of interesting adaptations, particularly behavioural ones such as swarming and movement in the water column which may be related primarily to their feeding habits. Perhaps their most profound adaptation relates to their survival during winter when food supplies are relatively scarce. Instead of building up large fat reserves, krill reduce their size capitalising on their own body proteins as a source of energy. This reduction in size presents a special problem for animals which have exoskeletons. The problem is overcome in krill by retaining the ability to moult into adulthood. The ability to increase or decrease size depending upon food supply enables the krill to survive during the winter. This is further enhanced by their omnivorous rather then strictly herbivorous character thus maximising available food over the winter months.
The modern Antarctic fish fauna is exclusively marine, with 203 bottom-dwelling species and 75 mesopelagic species being recognised (reviewed in Eastman, 1993). This represents about 1% of all known fish species. The fauna is highly endemic (about 88% of the species are restricted to Antarctica) and of the bottom-dwelling species, 104 (51%) belong to a single suborder, the Notothenioidei. The fossil fish fauna shows that Antarctic fish have undergone a considerable reduction in diversity since the early Tertiary, but the reasons for this are not clear, particularly since there are no notothenioids in the fossil record to date. The dominance of notothenioids presumably reflects the presence at some early stage in their evolution of an ancestral species which overcame the difficulties of survival at low temperatures. Subsequent speciation and evolution of this ancestral species resulted in the current notothenioid fauna (monophyletic radiation). The suborder Notothenioidei comprises six families: the Nototheniidae (Antarctic cod), the Harpagiferidae (spiny plunderfishes), the Bathydraconidae (dragonfishes), the Channichthyidae (icefishes), the Bovichthyidae (thornfishes), and the Artedidraconidae (plunderfishes).
|The nototheniid - Trematomus
After whom was this fish named?
How was he involved in early exploration of the Antarctic?
During their evolution the Antarctic fishes have adjusted to cope with the Antarctic environment (reviewed in Macdonald and Montgomery, 1990). None possess a gas-filled swimbladder (used for buoyancy) which suggests that the ancestral species was probably a bottom dweller. Some of the modern species such as Pleurogramma antarcticum have become secondarily pelagic, decreasing their densities by incorporating lipid into their tissues and developing a cartilaginous skeleton with hollow vertebrae. Many notothenioids have pelagic larvae which can feed directly on plankton. Since these are low in the food chain, the larvae benefit from a larger, potentially more available food source.
Although obviously cold (-1.86° C at high latitudes to +5° C near the Antarctic Polar Front), the waters of Antarctica are relatively stable (seasonal variation is typically < 0.2° C) and local fish species have evolved to become intolerant of temperature changes. The concentration of salts in sea water depresses its freezing point to about –1.9° C, while that in fish blood depresses it to about –0.8° C. To survive at near freezing temperatures fish contain various types of antifreeze in their blood, reaching a concentration of about 35 mg/ml. Fish of the species Pagothenia borchgrevinki contain 8 glycopeptide antifreezes of similar structure of different molecular weight (2,600-33,700 daltons). Each has a repeating tripeptide (3 amino acid) backbone (threonine-alanine-alanine) with a disaccharide extending from the threonine residue. The antifreeze glycopeptides act by binding to small ice crystals in the fish to prevent further growth. Although present in most body fluids, there is no anti-freeze present in the urine. This is a result of specialisation of the kidneys which do not contain glomeruli. Filtration of the blood takes place by active secretion through the walls of the kidney tubules, thus preventing loss of anti-freeze glycoproteins.
|The Nototheniid -
After whom is this fish named?
What was his contribution to Antarctic exploration?
The viscosity of blood increases at lower temperatures, increasing the energy required to pump it around the body. Many Antarctic fish are adapted to this situation by having fewer red blood cells (the predominant blood cell type), thus effectively thinning the blood. In temperate fish the corresponding reduction of red blood cell haemoglobin would significantly lower the amount of oxygen carried to the tissues, but the increased solubility of gases at low temperature allows Antarctic fish to carry more oxygen dissolved directly in the blood. The solubility of oxygen in sea water at 0° C, for example, is 0.83 vol. % per atmosphere of air, while its solubility at 20° C is 0.53 vol. %.
The channichthyids (icefish) are unique among vertebrates in that they have a deleted gene for haemoglobin and thus they are totally dependent on the increased solubility of gases at low temperatures to transport oxygen in their blood plasma. The oxygen carrying capacity of icefish blood is about 0.7 vol. %, which is only 10 or so of the amount carried by the blood of haemoglobin-containing Antarctic fish. Transport of oxygen in icefish is facilitated by a number of anatomical and physiological adaptations including a large heart (similar in weight to that of a small mammal) with a large stroke volume (6-15 x that of other teleosts) increasing cardiac output at low heart rate and low ventral aortic pressure. They also have a high blood volume (2-4 x that of other teleosts), large diameter blood vessels to lower resistance, low blood viscosity, well vascularised gills and a scaleless skin to maximise the extraction of oxygen from the sea water, and a relatively low average metabolic rate minimising the demand for oxygen. The icefish are so well adapted they appear not to be compromised by their lack of haemoglobin and they can lead active, predatory lives.
There are a considerable number of avian species which visit Antarctica, but relatively few breed on the continent. Of those that do, discussion will be restricted to the skuas and the penguins.
There are five currently recognised species of skua. The two which dominate on the Antarctic continent are the South Polar skua (Catharacta maccormicki) and the brown skua (C. lonnbergi). The South polar skua is widely distributed over the mainland, whereas the brown skua is restricted to the Antarctic peninsula. Interbreeding is possible where the distributions of the two species overlap.
Lieutenant Evans (who went south with Scott) wrote that their greatest
value to the early explorers was their tasty and nourishing flesh, reminding
him somewhat of wild duck. He was either possessed of a wonderful imagination
or extraordinary digestive powers.
|An adult south polar skua (Catharacta maccormicki ) with chick.|
The south polar or Antarctic skua is also known as McCormick’s skua, after Dr McCormick (the surgeon on Ross’ expedition), who history records as the first person to shoot one. It is a handsome bird with a light-brown head and breast progressing to a darker brown back and posterior. Its webbed feet are clawed and it has a vicious looking curved beak. It is undoubtedly the most southern existing animal species, one bird apparently following Amundsen almost to the South Pole itself. Those nesting near penguin colonies feed mostly on unattended penguin eggs and chicks when in season, but most feed at sea on Pleurogramma antarcticum. Skuas nesting near research stations are known as skilled scavengers. Young (1994) provides an excellent summary of the relationship between penguin and skua, predator and prey.
The south polar skuas on Ross Island leave the region over winter, migrating to Japan and the Alaskan coast, and return in late October. The first eggs are laid in mid-November and the first chicks hatch about a month later, leaving the colony as fledglings in late March. Two eggs are generally laid, and both usually hatch, but within a day or two of hatching the second chick is attacked by the stronger first chick and driven from the nest where it falls as prey to other adult skuas. This display of siblicide is apparently not shown by subantarctic skuas, and may be related to food availability. The skua is the scavenger of the south, congregating around scientific bases seeking food scraps. They also haunt penguin rookeries seeking eggs, but also attacking chicks. At sea, the skua feeds on Antarctic silverfish Pleurogramma antarcticum.
|Sibling south polar skua chicks.
The first born (and the larger of the two) is harassing its younger sibling which will be driven ultimately from the nest. Once excluded it will fall prey to other adult skuas which are always on the lookout for an easy meal.
There are 17 species of penguins distributed over six genera, all of which are found in the family Spheniscidae. They are all flightless, pelagic seabirds and vary in size from the Little Penguin (Eudyptula minor) which weighs just over 1 kg and is around 40 cm tall to the Emperor (Aptenodytes forsteri) which is a hefty 30 kg and reaches 115 cm in height. Only two penguins are restricted to the Antarctic, the Adelie (Pygoscelis adeliae) and the Emperor.
The Adelie penguin (Pygoscelis adeliae) at about 5 kg is
among the smaller of the penguins. It is easily identified by its distinctive
white-rimmed eyes set in a black head. It was named by the French explorer
Dumont d’Urville after his wife. It is the most numerous of the Antarctic
penguins and breeds further south than most, the most southerly colony
of about 4000 birds being at Cape Royds on Ross Island (77°
30’ S, about 1350 km from the South Pole). The largest colony is at Cape
Adare where there are about 400,000 birds.
|Adult Adelie penguin ( Pygoscelis
The Adelie is easily recognised by the white ring around its eyes.
The first Adelies come ashore around the third week of October and egg laying peaks in the middle of November. At the start of the breeding season there is still a great deal of solid ice between the birds and their nesting sites and distances of 20-40 km may have to be traversed over the ice mass. Egg incubation lasts about 34 days, with most chicks thus appearing in mid-December. The adult birds survive the early weeks of the season without feeding, the females through to egg laying and the males through the first 15-20 days of incubation. Food for the chicks, mostly krill, is provided after hatching by their parents which spend large parts of the day hunting at sea. Adults typically travel 20-30 km a day in search of krill swarms which they collect in short dives of 10-40 m depth, occasionally extending to 170 m. Females have been tracked to over 340 km from the colony, but distances around 10 km are more common when rearing chicks. Within two weeks the chicks have put on enough weight to insulate themselves against the weather and they can wander around the colony. The first chicks are ready to go to sea in late January, having lost their coat of fluffy down, and in late-February the adults begin to moult to get a new set of protective feathers. Before moulting begins the adult Adelies return to the sea for 2-3 weeks.
The poor Adelie provided a source of blubber for fuelling the fires of the early explorers and, along with its eggs, also made its way to the dinner table. The indefatigable Lieutenant Evans wrote that the adults tasted "quite like hare, and (are) much improved by red currant jelly". The Adelie was not to everyone’s taste, however, and Frederick Cook (a member of de Gerlache’s 1897 expedition) described it more like "a piece of beef, odiferous codfish and a canvas-backed duck roasted in a pot with blood and cod liver oil for sauce". Somehow or another this latter description seems more likely!
The Emperor penguin (Aptenodytes forsteri) is a big bird, over a metre tall and weighing around 30 kg. They can dive deeper than any other penguin reaching depths in excess of 450 m to find their preferred food of fish and squid. They also range widely in search of food, often travelling 150-1000 km in a single foraging trip. There are about 400,000 Emperor penguins breeding at a number sites on the Antarctic continent. The breeding season of the emperor penguin is so long that it has been advanced into the winter. At Cape Crozier, breeding birds congregate in March/April, the eggs are laid May/June, and the chicks hatch in July/August. Soon after laying her single, large egg (460 g), the female returns to sea for the next 64 or so days, leaving the male to incubate the egg alone in the most hostile environment imaginable. As well as having an extensive range of physiological adaptations to cope with the extreme conditions, the Emperors huddle together at about 10 birds per square meter to share body heat. The Emperor does not make a nest on the bare ice, but instead incubates the egg between a loose outer fold of abdominal skin and a patch of bare belly skin which is highly vascularised.
|An adult Emperor penguin.|
In order to shed some light on the embryonic development of the Emperor, three members of Scott’s last expedition (Edward Wilson, Henry "Birdie" Bowers and Apsley Cherry-Garrard) undertook "the worst journey in the world" travelling about 110 km on foot from their hut at Cape Evans to an Emperor penguin colony at Cape Crozier (Cherry-Garrard, 1922). They left Cape Evans on 27 June 1911 and made camp at Cape Crozier on July 15, manhauling 343 kg on two sledges. Their journey took place in the permanent dark of an Antarctic winter in temperatures which dropped to -61° C and in tremendous gales which at one stage ripped their camp apart and blew away their tent. Without their tent, the three explorers would almost certainly have died of exposure on their return journey. By a stroke of extraordinary good luck they were able to find it in undamaged condition when the gale subsided. The three ultimately made it back to base camp at Cape Evans, although Wilson and Bowers were later to perish with Scott on the ill-fated assault on the Pole.
Like all animals living in Antarctic, birds are adapted to cope with the extremes of the environment. This is demonstrated clearly by the penguins which have a thick thermoprotective layer of subdermal fat to maintain body heat while also streamlining the body for swimming. Their short, overlapping feathers trap insulating air when diving and promote insulation even under windy conditions. Heat loss through the legs and feet is minimised by keeping them tucked up against the warmth of the body and by having heat exchangers in which heat is transferred between intertwined arteries and veins. Many penguins such as the emperors also huddle together in extreme conditions to retain body heat. Their efficiency in heat retention is such that they have evolved special adaptations for those occasional warm sunny days that do occur in Antarctica. These adaptations take the form of blood vessels which come close to surface on the almost featherless inside surface of their flippers. Thus in sunny conditions, or when otherwise hot, penguins can radiate metabolic heat by raising their flippers to expose the undersides.
The seals (Order Pinnipedia) are distributed over three families: the families Phocidae (true seals) and Otariidae (the eared seals) are found in Antarctica, whereas members of the family Odobenidae (the walruses) are not. There are six species of Antarctic seals. These include the Antarctic fur seal (Arctocephalus gazella) which represents the Family Otariidae, and five phocids represented by the elephant seal (Mirounga leonina), the Weddell seal (Leptonychotes weddelli), the Ross seal (Ommatophoca rossi), the leopard seal (Hydrurga leptonyx) and the crabeater seal (Lobodon carcinophagus).
The Weddell seals (Leptonychotes weddelli) are an ice-breeding
species which reach about 3 m in length and which weigh up to 400-500 kg.
There are about 800,000 Weddell seals in Antarctica where they live all
year round on the fast ice. They are the only mammal which over-winters
in the Antarctic, humans excluded, and they breed further South than any
others. At White island, about 20 km from the barrier edge, there is an
apparently self sustaining population of Weddell seals cut off from the
seasonally open water of McMurdo Sound. Assuming that Weddell seals can
travel no more than 12 km under the water on one breath, and given that
there are apparently no breathing holes on the Ross Ice Shelf in this area,
the inevitable conclusion is that they must be an isolated population.
DNA testing is required, however, to resolve this possibility.
|Weddell seals ( Leptonychotes weddelli ) on the sea ice in McMurdo Sound.|
The Weddell seal feeds mostly on fish and squid, diving to depths greater than 700m to find them. Dives can last over 60 min, but most feeding dives are short (<30 min) and shallow (200-400 m). The average meal size is a whopping 23 kg (that’s 257 quarter pounders!). A Weddell seal was once seen to catch a 1.5 m long Dissostichus mawsoni estimated to weigh 31 kg which was brought to the surface and consumed over three hours. The hunting territo
All the Antarctic seals display similar general adaptations to the local environment, but each species also has unique adaptations reflecting their occupancy of distinct ecological niches. Weddell seals feed primarily on fish and have peg-like molar and pre-molar teeth to grip their prey. During the winter they live under the ice (where it is warmer) and they thus need to keep their breathing holes from freezing over. This is achieved by having their canine and second incisor teeth modified for ice cutting which is undertaken by dragging the upper teeth (which project forward) from side to side on the rim of the ice hole. This action wears down the upper canines and second incisors, and tooth wear with associated ulceration is probably an important factor contributing to the relatively short life span of Weddell seals. Crabeater seals, found on the pack ice in the north of the Ross Sea, have remarkable five-pointed molars which interlock when their jaws are closed to form a strainer capable of sieving krill. The common name "crabeater" is thus misleading as they eat krill and not crabs. Leopard seals, also common on the pack ice, have interlocking, tricuspid molars ideally suited for sieving krill, but they are also accomplished predators preying on penguins, other seals (especially young crabeaters), fish and squid and have long and slender canine teeth. Their lower jaw is relatively massive as is common in generalised predators.
More generally, the Antarctic seals have a thick insulating layer of blubber which is poorly vascularised, thereby further minimising heat loss to the surrounding environment. Like penguins, the flippers of seals also have anatomical heat exchangers in which each major artery is surrounded by a network of veins so that heat from the former is absorbed by the latter and not lost to the environment. During warm weather or strenuous exercise the heat exchanger can be partially bypassed by directing more blood into surface vessels in the skin thus enhancing the loss of heat to the environment.
Seals such as the Weddell, which has been closely studied, are consummate divers (Kooyman, 1981). They can dive to depths in excess of 600 m and can remain immersed for over 60 min. Several adaptations contribute to this extraordinary capability. These include the ability to store large amounts of oxygen in their body tissues as a result of their size, and of the relative abundance of the oxygen-binding pigments haemoglobin (in the blood) and myoglobin (in muscle). Weddell seals can also regulate their heart rate and their blood flow to different organs and tissues to conserve oxygen usage during diving. Being such excellent divers raises the question of how they avoid getting "the bends". The bends occur when nitrogen from the air, dissolved in the tissues under pressure, bubbles out when the pressure is reduced. It seems that seals avoid the bends by minimising the absorption of nitrogen by partially collapsing their lungs before diving, thus purging inhaled air and limiting its contact with the blood. During deep feeding dives the lungs of Weddell seals collapse completely as their thorax is crushed by the extreme pressure, but the ribs, being cartilaginous and flexible ,are able to deform and resist breakage. Weddell seals hunt their prey under the sea ice in limited light conditions. They have adapted to this niche by evolving large eyes with the anatomical features typical of nocturnal animals. An excellent summary of physiological adaptations in the Antarctic seals is provided by Macdonald and Montgomery (1990).
A number of different whale species are found in Antarctic waters. These include the sperm whale (Physeter macrocephalus) and the orca or killer whale (Orcinus orca) among the toothed whales, and the sei (Balaenoptera borealis), fin (Balaenoptera physalus), minke (Balaenoptera acutorostrata), humpback (Megaptera novaeangliae), right (Balaena glacialis) and blue (Balaenoptera musculus) among the baleen whales. The latter group of whales is named after the horny plates (baleen) lined with bristles which sieve the krill and smaller planktonic organisms on which they feed. The blue whale is the largest of all whale species reaching 30 m in length and weighing up to 180 tonnes. The killer whale, however, is indisputably the top predator of the Southern Ocean ecosystem, feeding on squid, fish, seals, penguins and other whales. They can reach 9 m in length and weigh up to 8 tonnes.
Life on the land
The Antarctic continent is host to a small number of fungal, plant and animal species, including lichens, mosses, mites, springtails and midges. Lichens proliferate because there is little competition from mosses, and because of their ability to withstand drought and cold.
|The Collembolan ( Gomphiocephalus
This small springtail ( barely 1mm long ) is just visible against the pale rock background.
This scanning electron microscope image reveals finer detail not normally visible by eye.
The Terrestrial Food Web
On land, the primary producers which form organic material from carbon dioxide by photosynthesis include cyanobacteria (formerly known as the blue-green algae), diatoms, algae, lichens, mosses and liverworts. Living among them are the decomposers, microorganisms such as filamentous fungi which feed on the organic material made by the primary producers. Single celled protists, microscopic animals such as rotifers and tardigrades, and larger mites and collembolans graze on the primary producers and the decomposers. Cyanobacteria, various microorganisms, as well as certain algae and lichens can actually live and grow inside rocks, such as the sandstone of the Dry Valleys, where they form what are known as "endoliths".
The ponds and lakes which are found in Antarctica usually have a benthic
community dominated by a carpet of cyanobacteria, and a planktonic community
dominated by unicellular algae.