The Southern Ocean food web is the flow of food energy among organisms living in the Southern ocean, a major marine system that entirely surrounds the Antarctic continent. It is one the most biologically productive and physically dynamic systems on the Earth, and it plays a critical role in the global carbon cycle and the regulation of climate.
The food web is characterized by prolific seasonal blooms of phytoplankton dominated by different diatom species, correspondingly large numbers of primary consumers such as the Antarctic krill, and some of the world greatest concentrations of marine mammals and seabirds. Notable among these is the blue whale, thought to be the largest (and among the loudest) animals that have ever lived on Earth. The unusual productivity in a harsh physical environment with a short summer season is due to a unique and complex combination of physical and biological forces. The Antarctic Circumpolar Current is the Earth’s most powerful current and the only one to circumnavigate the entire planet. In addition to its remarkable marine life, the Southern Ocean also plays an important role in global deep-water ocean circulation and in climate regulation.
The Southern Ocean is defined as the ocean surrounding Antarctica up to a northern boundary at 60° South. It is approximately 20.3 million square kilometers (7.8 million square miles), about twice the size of the U.S.A., making it the world's fourth largest (following the Pacific, Atlantic, and Indian, but larger than the Arctic Ocean.) The Southern Ocean's lowest point is 7,235 meters (23,737 feet) below sea level in the South Sandwich Trench. Its sea temperature varies from -2°C to 10°C (28°F to 50°F).
One of the ocean’s most distinctive features is the Antarctic Circumpolar Current (ACC), the only current that flows completely around the globe. The ACC flows eastward through the southern portions of the Atlantic, Indian, and Pacific Oceans. The current transports more water than any other current, moving about 130–140 Sverdrups (Sv) eastward at Drake Passage between Cape Horn and the Antarctic Peninsula (A Sverdrup is the basic unit of volume transport used in physical oceanography and is equal to one million cubic meters of water flowing per second. One Sv is about the average amount of water that flows in all of the world's rivers.)
The ACC has relatively constant environmental conditions along its streamlines, a critical feature that supports high biological productivity.
The Southern Ocean also exchanges waters with the adjoining oceans, and is particularly important in global thermohaline circulation as it connects to three other oceans and cold, deep waters form in the Ross and Weddell seas. Thermohaline circulation describes the global, density-driven circulation of the oceans. Thermo represents heat and haline describes salt, the two principal determinants of the density of sea water.
The Southern Ocean ecosystem is also influenced by the large-scale presence of ice, including permanent ice shelves close to the continent, and the seasonal advance and retreat of sea-ice further north. Ice cover provides a barrier between atmospheric and oceanographic processes, while melt water from retreating sea-ice affects the saline and thermal stratification of the ocean.
Diatoms–zooplankton–predators food chain
The Southern ocean food chain is among the most prolific the world. It is anchored by a distinctly seasonal pattern of phytoplankton "blooms", especially diatoms. Diatoms are unicellular or chain-forming phytoplankton that use silicon (Si) in cell wall construction. Diatoms are a major component of phytoplankton in the Southern Ocean, representing about 40% of total primary production.
The high rates of photosynthesis observed in the Southern ocean require large amounts of nutrients such as silica and iron. These nutrients are supplied through the process of upwelling: an oceanographic phenomenon that involves wind-driven movement of dense, colder, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The Southern Ocean is the largest upwelling region in the world comprising 20% of the world oceans.
These condition support some of the planet's highest rates of marine primary production. Large areas of the Southern Ocean average more than 500 grams of carbon per square meter per year (500 gC m-2 yr-1). Pockets of super-productive waters have rates exceeding 1000 gC m-2 yr-1. By way of contrast, the average rate of primary production in the open ocean as a whole averages about 50 gC m-2 yr-1.
Diatoms in the Southern Ocean play an essential role in the biological carbon pump, which refers to the processes by which marine organisms consume dissolved inorganic nutrients and CO2 during photosynthesis and "package" these elements into organic material, which is then exported, largely by gravitational settling, into the deep sea. Because the biological carbon pump partly determines the equilibrium CO2 concentration in the atmosphere, primary production in the Southern Ocean plays an important role in the global carbon cycle. Diatoms may account for as much as 40% of the ocean primary production, and thus gives them the potential to affect changes in atmospheric CO2 concentrations on a variety of time scales.
The second level in the Southern Ocean food chain is anchored by the Antarctic krill , a shrimp-like crustacean that lives in large swarms and reaches huge numbers in Antarctica. Antarctic krill can grow up to a length of 6 cm and can live for 5-6 years. Krill form massive "super-swarms" that can reach a density of 30,000 individuals per square meter. They are one of the largest protein resources on Earth and are fished with large nets for human consumption. The total weight of Antarctic krill is between 50 to 150 million metric tons, making it among the most prolific forms of life on the planet in terms of biomass.
The third level in a typical Southern Ocean food chain is occupied by a variety of krill eaters such as birds, penguins, seals, and baleen whales. The top of the food chain belongs almost exclusively to the Orca (killer) whale (Orcinus orca), the most common predator of the large krill eaters. The leopard seal (Hydrurga leptonyx) is a main carnivore on penguins and thus is considered a major top predator as well.
This basic food chain in the Southern Ocean and Antarctica supports the largest concentration of marine life on the planet. The biggest mammals, and the largest number of marine mammals and seabirds, are commonly seen around Antarctica in the austral (Southern Hemisphere) summer. The food web has small number of trophic levels and a large number of apex predators. The latter are predators at the top of the food web that are not normally preyed upon.
There are two principal reasons for the high density of apex predators. The first is the large phytoplankton blooms, dominated by different diatom species, that occur in the summer. Second, the path from primary production to apex consumers is very short due to the dominance of large diatoms in phytoplankton blooms. Short food chains make for efficient transfer of food energy to apex predators. In addition, diatoms at the base of the food chain provide a large quantities of food energy for apex predators. This occurs because the chain starts with particles more than 20 times larger than the picoplankton that dominates much of the ocean’s photosynthesis in regions that have very low concentration of nutrient (picoplankton have cells between 0.2 and 2 µm).
Notable among the region's wildlife is the largest animal ever believed to have lived, the blue whale (Balaenoptera musculus). Blue whales grow to be about 80 feet (25 m) long on average, weighing about 120 tons (109 tonnes). The largest specimen ever recorded was a female 94 feet (29 m) long weighing more 174 tons (158 tonnes). Amazingly, these enormous mammals eat tiny organisms, like plankton and krill. They can do so because the prodigious productivity of the Southern Ocean food web. The whales alternatively swim then gulp a mouthful of plankton or krill The water is then forced through baleen plates hanging from the upper jaw. The baleen acts like a sieve and traps the food.
The marked seasonal fluctuations in the availability of food contributes to the short food chain, and also drives key biological processes. There is a short period of only two to three months (or less in the highest latitudes) in the summer when conditions are favorable for primary production. The extreme seasonality in production also means that there is little capacity to build-up long food chains involving many steps to the highest trophic levels.
Consumers must be able to make full use of the short summer periods to breed and survive during the low production periods of winter . These conditions favor the two extremes of smaller species that can develop quickly in response to favorable conditions and large-bodied predators that are often highly mobile. The large, mid- and higher trophic level species, such as penguins and seals, have relatively long lifespans (often greater than 10 years) and are highly mobile (foraging over hundreds to thousands of kilometers), and many move away from the area during the periods of low production.
The Southern Ocean plays a critical role in the global carbon cycle and the regulation of climate. It is the largest ocean region in which surface waters directly connect to the ocean’s interior currents, thereby providing a pathway into the deep sea for carbon dioxide released from human activities. In the recent past the Southern Ocean had been absorbing 15 percent of all human carbon dioxide emissions. However, it has been absorbing less carbon dioxide from the atmosphere since 1981, even though emissions to the atmosphere have increased more than 40 percent due to burning of fossil fuels.
The decline in carbon dioxide absorption is due in part to the documented increase in windiness in the southern hemisphere, which is the unexpected product of the depletion of the ozone layer and global warming. The depletion of stratospheric ozone hole has led to a strong cooling of the Antarctic stratosphere, which in turn results in a strengthening of stratospheric winds. The increased winds are churning the Southern Ocean and bringing up the stored carbon dioxide from the deeps up to the surface. As the surface becomes more saturated with carbon dioxide, it absorbs less and less from the atmosphere. If this trend continues, it could lead to a further increase in atmospheric carbon dioxide by a few tens of parts per million, making it more difficult to slow climate change.
Numbers of Antarctic krill have dropped by about 80% since the 1970's. The most likely explanation is a dramatic decline in winter sea-ice. The Antarctic Peninsula, a key breeding ground for the krill, has warmed by 2.5°C in the last 50 years, with a striking decrease in sea-ice. Primary production is particularly high along the sea ice edge, because the ice is thinner, which allows more sunlight to penetrate, and because ocean mixing processes along the continental shelf cause an upwelling of nutrient-rich deep water. Increased nutrients support increased primary production along the ice edge. The ice edge is an absolutely critical habitat, a nursery, for larval krill who feed on the algae. Given the pivotal role that krill play in the Southern Ocean food web, a decline in their numbers would ripple through the entire system.
The Convention on the Conservation of Antarctic Marine Living Resources (CAMLR) came into force in 1982, as part of the Antarctic Treaty System. It was established mainly in response to concerns that an increase in krill catches in the Southern Ocean could have a serious effect on populations of krill and other marine life; particularly on birds, seals and fish, which mainly depend on krill for food. CCAMLR attempts to manage the krill stock so that its biomass will not fall below a level considered appropriate to meet the food requirements of other species in the food-web.
- Convention on the Conservation of Antarctic Marine Living Resources (CAMLR) home page
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