Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Jonas Ball, Sinclair Knight Merz Pty Limited, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06750 7
Numerous freshwater lakes, ponds and pools are found on Macquarie Island, where the almost constant rain ensures a plentiful water supply. The water bodies range in size from small ponds in waterlogged peat areas to moderately large and deep lakes in fault-controlled valleys. Short, steep streams connect the lakes to the ocean. The surfaces of most of the lakes freeze for short periods during winter, with water temperature ranging from 0C at these times to 8C in summer. Many of the lakes have abundant emergent vegetation, and are home to collections of microscopic animals that have more similarities to other Subantarctic islands, including Heard Island and South Georgia, than the Australian mainland.
The glaciated cone of Big Ben, an active volcano 2745 m high, makes up the greater percentage of Heard Island. Numerous lakes and bogs are located in the ice-free areas around the coast of the island. These can divided into two main types: pool complexes in peat, which have many similarities to string bogs in alpine Tasmania and boreal forests of the Northern Hemisphere; and large, deep freshwater lakes which occupy depressions left by retreating glaciers. These glacial lakes are often separated from the ocean by narrow sandbars, some of which have been breached in recent years resulting in intrusion of saltwater into the originally freshwater lakes.
Big Ben, Australia's only active volcano on Heard Island - one of Australia's new World Heritage properties inscribed for its geological and biodiversity values.
Source: Ken Green/Australia Antarctic Division (2804D6).
Even though Antarctica is largely a continent of ice, there are a surprising number of Antarctic lakes. Most of the lakes are located in coastal ice-free areas such as the Vestfold, Larsemann and Bunger Hills, where they are dominant features of the landscape. These lakes, which can be up to 10 km2 in area and over 150 m deep, are of two main types: fresh and saline. Summer meltwater run-off from the Antarctic ice sheet into the ice-free areas has resulted in the formation, in some cases, of large freshwater lakes which form part of short drainage systems to the ocean. The water in some of these lakes is amongst the purest naturally occurring water in the world, and has particularly low levels of nutrients and therefore productivity. The saline lakes were formed when seawater was trapped in basins during uplift of the land at the end of the glacial period approximately 10 000 years ago. Since isolation from the ocean, these lakes have evolved as a result of the balance between dilution by meltwater and evaporation. The most saline of these lakes, Deep Lake in the Vestfold Hills, contains over 250 g salt per litre, and rivals the Dead Sea as one of the saltiest lakes in the world. Remarkably, some of these saline lakes are only a few kilometres from the meltwater-fed freshwater lakes.
Many of the saline lakes are meromictic or stratified, which means that the water in the lakes does not mix completely during each year. This behaviour is due to an increase in salinity, and therefore density, with depth, and leads to the depletion of oxygen in the deeper, unmixed waters, and production of hydrogen sulfide gas. These conditions are particularly favourable for the preservation of sediments that reveal the history of the lake and climate change over its lifetime. There are very few meromictic lakes in Australia. All the lakes except for the most saline freeze to a depth of up to two metres during winter, but this ice insulates the lakes from freezing completely. Water temperatures under the ice in meromictic lakes can be surprisingly warm due to solar heating, reaching 20C in some cases.
Perhaps the most fascinating of all the lakes in Antarctica are located under thousands of metres of ice in the continent's interior. These subglacial lakes were discovered during radio-echo sounding traverses to map the thickness of the Antarctic ice sheet. The largest, Lake Vostok, which is located beneath Vostok Station under nearly 4000 m of ice, has an area of 14 000 km2 , and a maximum depth of over 500 m. No drilling program has yet been undertaken to confirm the existence of these lakes, but this could occur soon.
The biology of Antarctic lakes is often quite simple, but is of considerable interest. The lakes are dominated by microbial processes (i.e. involving single-celled organisms), and the largest animals in the lakes are small crustacea, including cladocera (water fleas) and copepods. Fish are absent. A major scientific question is the source of the biota of the freshwater lakes, as at the height of the previous ice age it is thought that there were no areas of exposed land on Antarctic, and therefore no land-based lakes. The biota either survived in lakes on the ice itself, or has been transported from continents and islands to the north by winds, migratory animals or ocean currents. The source of the biota in the saline lakes is clearer - organisms that were present in the seawater trapped to form the lake. Subsequent changes in salinity and water temperature have placed pressure on the organisms, and only those that can adapt to change survive.
The biology of the subglacial lakes is as yet unknown, but they are thought to contain life (probably limited to bacteria) that has been isolated from the atmosphere for perhaps 400 000 years.
The main environmental pressures on most of the lakes come from scientists studying them. Protocols for sampling the lakes have improved dramatically over the last 20 years, and are now aimed at reducing the possibility of chemical contamination by, for example, fuel spills, or biological contamination by the introduction of organisms from one lake into another. Other threats are more subtle, and involve deposition of pollutants, such as lead and persistent organic chemicals, either from nearby scientific stations or from temperate areas. For a small number of lakes in the immediate vicinity of scientific research bases, pressure can also come from the use of the lakes as a potable water supply, pollution, and disturbance to the drainage basin by vehicles, construction and pedestrian traffic.