Discover Antarctica's Marine Life With Nat Geo


It is probably surprising to most people that life abounds in the seas surrounding Antarctica. The reasons for the abundance of life in Antarctic waters are threefold: 1) the sea water is cold (cold water holds dissolved gases, such as carbon dioxide and oxygen, much better than does warm water); 2) the sea waters are turbulent and produce strong upwellings near the continent that keep essential nutrients, such as phosphates, carbonates, and nitrates, as well as minerals, in suspension where they can be easily utilized by the immense growths of phytoplankton; and 3) the long hours of daylight during the Summer months promote a tremendous amount of photosynthesis that cultivates the algal blooms which form the huge base in the Antarctic food chain.

The phytoplankton of Antarctica consists of about 99% diatoms (unicellular plant-like organisms with cell walls made up of silica), with the rest being primarily dinoflagellates. The Antarctic Convergence is actually a biogeographic boundary, since one finds different populations of planktonic organisms, fishes, and even birds, on either side of it. North of the convergence the sea floor is primarily calcareous silt formed from the empty shells of countless Globigerina protozoans. South of the convergence the sea floor consists almost entirely of the siliceous remains of diatoms. Where the Antarctic Surface waters and the SubAntarctic Surface waters come together at the convergence, the sudden meeting of the two different water temperatures incapacitates or kills much of the plankton and brings it to the surface. For this reason seabirds often flock along the convergence.

The biological productivity in Antarctic waters is the highest in the world. This productivity can be determined by two methods of measure. The first is the Standing Crop of Phytoplankton which is a measure of the amount of chlorophyll in a given sample of surface water. The second is the Yield, or Water Productivity, which is figured by assessing the amount of Carbon 14 assimilated by a given sample of plants. Both the standing crop and the yield are highest near the islands and along coastal areas because of upwellings and turbulence, and lowest in the mid-oceanic regions. The inshore waters of the Antarctic Peninsula contain a standing crop that is as much as 10 times greater than neighboring waters, while the yield in the peninsular region is as much as five times greater than that of surrounding waters. The standing crop in Antarctic waters varies from 0.5 to 10 milligrams of chlorophyll per cubic meter of surface water. The average yield in Antarctic waters is 0.9 gram of carbon assimilated beneath each square meter of surface water per day (the average yield for the other oceans of the world is 0.15 g C/m2/day).

The Antarctic Ocean (the ocean water south of the Antarctic Convergence) contains only about one twentieth of the world's sea water, but it contains one fifth of the world's marine biological production of carbon (3 trillion kg, or 3.3 billion tons of per year). Antarctic phytoplankton reaches its lowest standing crops and yields between April and July, when the sun is low or below the horizon, sea ice spreads, and the planktonic populations descend to subsurface layers. In October, after the ice starts to break up and drift, algal blooms begin and spread south as the ice recedes.

The Antarctic Ocean food chain is quite simple, compared to other marine ecosystems. There are relatively few trophic levels, because there are relatively few species. Simply put, the immense growth of drifting phytoplankton (the first link in the marine food chain) is eaten by zooplankton, of which the shrimp-like krill is the most prominent.  Of course there are many species of invertebrates besides krill, but most of them are bottom dwelling animals and seldom seen (apart from what our Undersea Specialist can show us). A large percentage of the known species of fishes, squids, sea birds, seals, and baleen whales feed primarily on the abundant krill stocks.  Or, there may be just one trophic level between krill and a large predator…that is, an elephant seal eats squid that eat krill. Of course, many predators are opportunistic feeders and will consume a multitude of prey species, not just krill, but it is interesting to note that even killer whales and leopard seals will feed on krill when it is abundant or other prey is not available.

The squids are abundant and make up an important, but little-studied part of the Antarctic ecosystem. There are some 20 species of squids found here, usually inhabiting deep waters. Most species are small in size, being less than 40 cm (16 in) in length, and feed primarily upon krill. A few species are much larger (the giant squid grows to a length of 15 m or 50 ft), and act as major predators upon fishes. In turn, squids are extremely important in the diets of seals and toothed whales, as well as the larger fishes and sea birds.

Almost all the fishes to be found on an Antarctica cruise are bottom dwellers, and because surrounding waters are very deep many of the species are rarely encountered. For our convenience, however, the fishes may be categorized into two environmental groupings: deep-sea fishes and coastal fishes. The coastal group contains the better known species, such as ice fishes, eel pouts, Antarctic cods, Antarctic herrings, Antarctic shads, Antarctic perches, and dragon fishes, and accounts for about 60% of the species and perhaps 90% of the individuals. The common names applied to the various groups found here are a bit misleading, because they are not closely related to the true eels, cods, herrings, shads, and perches of the Northern Hemisphere.

As expected, the species diversity of the fishes is low in Antarctic waters (only about 150 species have as yet been identified), but the number of individuals in these rich waters is high. For reasons dealing with osmotic excretory processes,marine fishes must maintain a body salinity lower than that of the surrounding sea water. The presence of dissolved salts lowers the freezing point temperature of sea water from 0°C (32°F) to about -1.9°C (28°F). In order for fishes to survive in this near freezing sea water, they must concentrate something other than salts in their blood and tissues to lower their own freezing point temperature to at least that of sea water. The ions of salts, such as sodium ions, potassium ions, and chloride ions seem to work very well. Some species even produce glycoproteins, which act like ‘antifreeze’ and inhibit the formation of ice crystals within their tissues. A few species apparently live their entire lives under the fast ice.  The cold water environment may be the reason why so many species appear rather sluggish and it also probably accounts for the high number of bottom-dwelling species. Even so, they are able to maintain considerable activity in these low temperatures because of the presence of very efficient metabolic enzymes. There are a few free-swimming species, such as the Antarctic herrings and shads, and both these groups feed on plankton.

The ice fishes all lack the oxygen-carrying pigment hemoglobin, which is common to all other vertebrate animals. Therefore, they have no red blood cells, and this gives them a pale and nearly colorless appearance. The cold waters hold a high level of dissolved oxygen and the ice fishes take it in through their gills as do other fishes, but it is transported dissolved in the blood plasma (not attached to specialized oxygen-carrying molecules). Likewise, the muscle tissues of these fishes lack myoglobin and this only adds to their whitish or pale look. 

Continue your reading adventure with some amazing Arctic cruising!

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