The most important living organisms in the Blue Lake are stromatolites. According to divers the stromatolites living in the lake can be over 10m tall and wide. But what are they?
Mia Thurgate, an expert who has studied these strange features and dived the Blue Lake, describes them in layperson’s terms as “layered deposits of minerals, such as calcium carbonate, that have formed by the action of living organisms”.
Typically, the organisms involved in constructing stromatolites are microscopic algae, although larger algae and bacteria may also play a role. These microbes tend to occur in dense communities which form a ‘skin’ or mat on the surface of the stromatolite. Under the surface of the Blue Lake, the stromatolites form columns of rock, covered in thin films of pink and green.
The microbes that build stromatolites change the chemistry of their surrounding environment so that minerals are precipitated out of the water. Microbes must continually push their way up through the layer of precipitated minerals to prevent being smothered. As this process repeats over time, layers of minerals are formed, so that the internal structure of the stromatolite starts to resemble the growth rings of trees. These layers are called laminations, and are the internal feature that distinguishes stromatolites from other mineral deposits formed by microbes.
Stromatolites first appeared on Earth 3.5 billion years ago. Evidence from the fossil record shows they were the dominant form of life on Earth between 2,800 to 540 million years ago, reaching their greatest diversity around 1000 million years ago. Most of the microbes which construct stromatolites are photosynthetic, that is, they use energy from sunlight, together with carbon dioxide and water, to produce carbohydrates and also give off oxygen, which is liberated into the atmosphere.
Some scientists believe stromatolites were amongst the earliest living organisms on Earth that could photosynthesize. It may be that the through this activity, stromatolites fundamentally changed the Earth’s atmosphere. Before they were present in large numbers, there was almost no oxygen in the atmosphere, but once stromatolites came to dominate, oxygen levels were greatly increased.
This may have provided the environmental conditions that allowed more complex life forms such as animals to evolve.
Early stromatolites were mainly found in shallow marine environments across the world. Today, they are relatively rare and mainly restricted to alkaline (such as the Blue Lake) or hypersaline waters, and also in hot spring environments. These are harsh environmental niches that few other organisms can tolerate. The reason for the decline in stromatolites and their limited present-day distribution patterns is unclear. The oceans of the world are today much less alkaline than they were in the past, so it may be that changing environmental conditions played a role.
Unfortunately for us, the stromatolites in the Blue Lake are mostly found below the water surface so they are best seen if you are diving. So what do the stromatolites under the Blue Lake look like? Mia Thurgate, who has observed at least eight different types of stromatolite diving just five locations around the Blue Lake reports:
“Many stromatolites in the Blue Lake rise out of the sediment-covered lake floor or are found attached to the sheer rock walls of the volcano. They form columns of various shapes and sizes, conical mounds, and very large reef-like structures which may be over 12m long. The largest stromatolites are found at depths of 5-10m below the surface, and typically form huge tower-like structures up to 10m tall. Isolated turrets and spires have developed on top of many towers, so that the whole structure resembles a strange rocky castle. The smallest stromatolites are usually only a few centimetres long and are mostly found in heavily shaded areas on the walls, or in very deep waters”.
Stromatolites extend almost continuously from the surface of the Blue Lake to depths of 45m. At depths of greater than 10m below the surface, the shapes and forms of the stromatolites begin to change. In the deeper lake waters, the stromatolites form low mounded walls and canyon-like structures, in others areas they look like scattered rubble. Curiously, some of these deep water structures are covered in aquatic mosses and sponges that don’t seem to be present in the shallower waters.
The Blue Lake stromatolites are not the only examples of stromatolites in the South East. There are also stromatolites forming in at least eight sinkhole (cenote) lakes in the region. One group of cenotes containing stromatolites is found just south of Mount Gambier, and a second group is located near Mount Schank. And there are relatives of the stromatolite, thrombolites, growing in saline lakes near Robe. Thrombolites are microbial formations that have a clotted internal organisation as opposed to stromatolites that have a laminated structure.
Scientists use the term ‘microbialite’ as a catch-all phrase for all the different deposits formed by algae and other microbes, no matter what their internal structure looks like. Our region may well be a global ‘hotspot’ for microbialite diversity as over 20 different types are present in the lake environments of the South East, more than most other places on earth.
Thurgate, M.E. 1996, ‘The Stromatolites of the Cenote Lakes of the Lower South East of South Australia’. Helictite, 34 (1): 17-25. Thurgate, M E 1996, ‘Stromatolites of the Karst Lakes of the Mount Gambier Region. What are they and why should we care?’ Journal of Australasian Cave and Karst Management Association Inc. vol 23 pp. 29-33
Thurgate, M E 1998, ‘The Stromatolites of the Blue Lake Crater, Mount Gambier, South Australia’ in possession of the Author
Dr Phillip Playford as quoted on ABC, 2007, Stomatolite Fact Sheet, 8/12/2000, Gardening Australia ABC Web Page, viewed 25/1/07, http://www.abc.net.au/gardening/stories/s226982.htm
Government of Western Australia 2007, What are stromatolites?, Department of Industry and Resources, Geological Survey Web Page, viewed 25/1/07, http://www.doir.wa.gov.au/GSWA/D8826E646FC746C7BCE99DB5CF5F7C01.asp