Australia: The Land Where Time Began

A biography of the Australian continent 

The Southern Ocean - Plankton (Now classified as protists)

It was previously believed that the ecosystems of the Southern Ocean were simple, now that some research has been carried out, it has been shown that the situation is more complex than expected, and much more research is needed to get a good understanding of the full nature of food webs and their interconnections.

The number of known Antarctic phytoplankton species is now more then 200, covering a wide range of sizes, life cycles, shapes and nutritional value to grazers. Diatoms are enclosed in intricately sculpted, glassy shells that come in many shapes and patterns.

For much of the year light available for photosynthesis is very low to completely absent in Antarctic waters. In summer, when the ice melts and the now uncovered water is able to receive some of the weak sunlight, the phytoplankton undergo intense blooms of short duration. The phytoplankton in Antarctic seas are in much lower numbers than in other parts of the world where the same nutrient conditions apply. This is known as the Antarctic Paradox - high nutrient-low chlorophyll. This results from the high wind strength powered deep mixing of water in the euphotic zone (where light penetrates in sufficient strength to power photosynthesis),  pushes the water, and the phytoplankton to depths of more than 100 m below the surface, were there is insufficient light for photosynthesis. The ability of the phytoplankton to photosynthesise at maximum levels is also hampered by the chronic iron deficiency of the Southern Ocean. It is under low light conditions that phytoplankton require more iron to photosynthesise.

Some of the phytoplankton photosynthesise as usual when light it available, but when the light level is too low for photosynthesis, they eat other organisms. Some of the zooplankton have developed the ability to retain some of the chlorophyll from the phytoplankton they consume in their tissues, enabling them to benefit from some level of photosynthesis.

Permanent Open Ocean Zone (POOZ)

This is an area of ocean that is rich in nutrients, at least partly because the primary production by the phytoplankton is kept low by the rough conditions of the water, deep mixing of the surface layers pushing much of the photosynthetic plankton below the photic zone. The phytoplankton of the POOZ are grazed by a number of groups of organisms, but not usually by Antarctic krill (Euphausia superba). The phytoplankton are divided into 3 size categories, microplankton - the largest, nanoplankton - middle sized, and picoplankton - the smallest. The phytoplankton of the POOZ are mostly of the nanoplankton category.

The iron deficiency of the Southern Ocean has prompted a suggestion that the high levels of CO2 in the atmosphere in the atmosphere could be reduced by adding iron to the Southern Ocean to encourage photosynthesis by the phytoplankton. A problem with this suggestion is that it is not presently known if the extra CO2 from the atmosphere would eventually be sequestered on the seafloor as expected or instead lead to increased levels of respiration as non-photosynthetic organisms bloomed, possibly adding even more CO2 to the atmosphere. (White, 2003).

Seasonal Ice Zone (SIZ)

This zone is covered with ice in winter, but is mostly open water in summer. Contributing at least 40 % of the phytoplankton biomass of the Southern Ocean, it is the most productive zone in this ocean. It also contributes about 60 % of the pelagic primary production annually. As the ice melts in summer, retreating to the south, makes the water in this zone less saline. Algal blooms of Phaeocystis antarctica and large diatoms are the dominant planktonic life forms. These are grazed by large numbers of krill and other organisms, and the krill is fed on by baleen whales, crab-eater seals, penguins and other sea birds.

Coastal and Continental Shelf Zone (CCSZ)

In this zone, most of the ice remains even at times of maximum retreat. There are short, intense phytoplankton blooms. In this zone, Antarctic kill are replaced by smaller species of Euphausia, and there are fewer mammals than in the SIZ. There is a rich benthic fauna in this zone, most of which feed on the rain of organic debris from the surface waters, as well as patchy seaweed and a microflora of algae. The benthic fauna is much reduced in parts of the zone that has been scoured by ice and glacial debris. It has been found by studying the benthic fauna in this zone in McMurdo Sound that regular ice-scouring occurs to depths of 15 m. Only fast-growing algal species are present in this zone, as the summers are short, and the animals are mobile foragers, such as sea urchins, starfish, worms, crustaceans and fish. Below 15 m there is a rich biota, comprised of filter feeders such as anemones, soft corals, molluscs, sea squirts, sponges, tube worms. Some sponges have algal symbionts that photosynthesise, available light being increased by the glassy spicules in the tissues that act as light tubes, conducting light to where it is needed by the algae, allowing them to photosynthesise at light levels that would normally be insufficient for photosynthesis.

There are many mobile scavengers, and great concentrations of burrowing animals are found on sandy and muddy seafloors. There are large proboscis worms (Parborlasia corrugatus) up to several metres long. They feed by enveloping their prey with their proboscis and protect themselves with acidic mucus. There are large numbers of microorganisms on and in the sediment. There are large numbers of micro-algae, some of which are attached to seaweeds and other organisms. Seaweeds are not common below 40 m and are completely absent by 100 m.

Around the coast of Antarctica the ice begins to form in March, reaching a maximum area of about 20 million km2 by September. Many microscopic organisms occupy the lower surface of the ice, in cracks in it and at the boundary between the ice and snow. On the lower side of the ice is an ochre-coloured concentration of diatoms. The small grazers of these congregations of diatoms can be found in cracks in the ice and other features. Some seaweed occurs in patches.

It is when the ice forms from March on that the water with high levels of salinity, the cold, salty water flowing north along the bottom of the ocean, the Antarctic Bottom Water, giving rise to deep abyssal currents that create the vertical circulation and mixing of the global ocean (White, 2003).

The melting of the ice in spring and summer releases quantities of fresh water that dilutes the ocean water, making a shallow, less saline layer that is high in nutrients, in which the algae bloom at a time when there is 24 hours of daylight. There is a belief that the Antarctic waters are extremely  productive. These blooms, though highly productive, are localised and short-lived.

In spite of ice covering most of Antarctica permanently, areas of soil have been found that can support life, communities of terrestrial plants, algae, lichens and even animals, minute invertebrates. Some soils have been found that contain up to billions of bacteria, Achaea and Eubacteria. As someone once said 'life finds a way' even in the coldest place on Earth.

Sources & Further reading

Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003


Author: M. H. Monroe
Last Updated 25/02/2011


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