Like the continent which it surrounds, the Southern Ocean is characterised by extreme conditions. Water temperatures can be as low as -1.8°C near the coast of Antarctica and reach no higher than 3.5°C at the boundary with warmer surrounding Oceans. Also, the high latitude of the Southern Ocean means that daily light conditions vary greatly with season. Each year after the southern hemisphere’s summer solstice in December, the daily period in which there is sun light gradually decreases until it reaches it’s minimum in June and there is 24 hour darkness in the Antarctic circle. After this, the sunlit period of each day begins to gradually increase with time until it reaches its maximum during the summer solstice and sun never sets in the Antarctic circle.

Given these conditions, you would expect the Southern Ocean to be deserted. It can clearly be seen that on land, there is a general decline in the abundance and diversity of life as you move from the tropics to the poles. In fact, Antarctica is the least populated of the world’s continents. It has little biomass and few complex animals that are regarded as completely terrestrial or freshwater, the largest being the midge. However, if you compare this with the surrounding ocean, you will find one of the most extreme differences in ecosystem biological richness in the world. The Southern Ocean is one of the world’s most productive marine environments and it’s home to 9700 known species, including blue whale! Even large animals which temporarily inhabit the shores of Antarctica, such as penguins and seals, rely entirely on this rich ecosystem for food.

 

File:Antarctic blue whale.jpg
Antarctic blue whale: Oregon State University (photo by Paul Ensor, with support from Canon New Zealand Community Sponsorship Programme)

But why live in the Southern Ocean?

The Southern Ocean is characterised by the formation and melting of vast areas of sea ice on a seasonal basis. This is one of the main drivers of a physical process called upwelling. This involves nutrient rich deep water being brought upwards towards the surface, where it massively benefits tiny organisms called phytoplankton.

Phytoplankton create energy by photosynthesis, a process which requires sunlight. So in the Southern Ocean they can only survive close to the surface, where sufficient light reaches them during the well lit season. Nutrients such as nitrogen, phosphorus, silica and iron are also incredibly important for the growth and survival of these organisms. The upwelling of these nutrients to depths inhabited by phyotplankton means that some of the world’s most immense aggregations (or blooms) of phytoplankton occur in some areas of Southern Ocean. For example in the Ross Sea, a bay in the coast of Antarctica, blooms can extend up to 200km (pictured below). These blooms are restricted to the Austral Spring and Summer months, when the daily period in which there is sunlight is sufficient for phytoplankton to photosynthesise. Despite this the Southern Ocean, which only makes up 10% of the World’s Ocean, accounts for ∼20% of the global transfer of atmospheric CO2 to the ocean (Phytoplankton are the main reason that this happens). As in many other ecosystems, phytoplankton form the basis of the food web and these intense blooms lead to a feast for the rest of the organisms in this environment.

 

Bloom in the Ross Sea
True-color image of a phytoplankton bloom in the Ross Sea on January 22, 2011: NASA image courtesy Norman Kuring, Ocean Color Team at NASA Goddard Space Flight Center.

 

The Southern Ocean Food Web

Much of the phytoplankton in the Southern Ocean is eaten by grazing animals, including crustaceans such as copepods and Antarctic krill. Antarctic krill (Euphausia superba) have shrimp like bodies and can grow over 5cm in length. They feed upon phytoplankton in an incredibly efficient manner, filtering particles out of the water at high speed. They scrape away at the underside of sea ice to feed upon phytoplankton that live within it, whilst using the ice as a form of shelter as juveniles. Therefore it is unsurprising that the Southern Ocean, with its intense phytoplankton blooms, contains a colossal amount of krill. The average abundance of Antarctic krill in this ecosystem is estimated to be 500 million tonnes and they can form dense swarms over a km long, creating a major food source for a wide variety of animals in this ecosystem. Blue whales, which like other baleen whales are adapted for filtering small pieces of food from the water on a huge scale, consume up to 3600 kg of krill per day. The crabeater seal only eats krill and is the most abundant seal species on the planet. Antarctic krill are also a key part of the diet of squid (which are very abundant and diverse in the Southern Ocean) and fish species in this area. Some animals which eat krill are eaten by other animals. For example penguins eat fish and squid (aswell as krill) and killer whales eat seals and squid.

Antarctic Ocean (Southern Ocean) foodweb (Illustration courtesy of the British Antarctic Survey)

The future of life in the Southern Ocean

The temperature of the Southern Ocean is rising. This is causing a number of destructive environmental changes, including a significant reduction in the extent of sea ice. This is therefore changing the extent of seasonal sea ice formation and loss, a major reason why the Southern Ocean ecosystem is so full of life. The reduced extent of sea ice is also directly affecting animals which rely on the sea ice for breeding, grazing and hunting purposes and shelter from predators. Direct anthropogenic disturbances, such as intense fishing are also to blame for declining populations of species in the Southern Ocean. A reduced abundance of one species, particularly one which supports many species throughout the food web, such as the Antarctic krill could dramatically change the species composition of the whole ecosystem (this is further explained in the video below).

The importance of Antarctic krill and impacts that threaten their future (Australian Antarctic Program)

 

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