Mia Thurgate

ACKMA Journal No. 23. June 1996. Pages 29 – 33

Several years ago, while diving in some of the sinkholes (cenotes) of the Mt Gambier region, South Australia, I noticed that the walls of these features were covered in dense bands of regularly arranged columns of rock, most of which were covered in a thin film of pink and green matter.  Puzzled as to what these columns could be, I embarked on a detective exercise.  At first I came up against a brick wall - people either had not paid much attention to these columns before or otherwise had assumed they were some kind of weird solutional feature.  Eventually, one of my colleagues passed on a geological paper describing features called stromatolites which had been recorded in a single sinkhole in South Africa.  Further detective work along this line and some sampling of the columns convinced me that the features in Mt Gambier were also stromatolites.

DEFINING STROMATOLITES

So what is a stromatolite?  The scientific definition is full of jargon and rather long-winded.  The simplest way 1 can come up with to describe these features is that they are layered deposits of calcium carbonate and various other minerals which have been created by the action of living organisms.  Typically, the organisms involved in stromatolite construction are microscopic algae, although larger algae and bacteria may also play a role.  The algae tend to occur in dense communities which form a "skin" or mat on the surface of the host stromatolite.  The communities in these mats are made up of many different species of organisms, not all of which re involved in stromatolite building.

The organisms which are stromatolite builders create layers of sediment in two main ways.  Some algal cells or colonies are surrounded by a sticky gelatinous coating which can trap and bind particles of sediment that are suspended in the surrounding water.  Over time, sufficient particles are trapped in this way to form a mineral crust, which the living organisms must grow out from to prevent burial.  Over many hundreds or even thousands of years, successive layers of this crust build up to form a stromatolite.

In other cases, particularly where there is little water movement, the biological processes of the algae create a micro-environment that promotes the precipitation of dissolved minerals out of the surrounding waters.  In these cases, the precipitated minerals will often form crystals around the algal cell structure, which are eventually left behind as layers of sediment as new algal cells grow above the mineral layer.

THE STROMATOLITES OF THE MT. GAMBIER REGION

Stromatolites are found in their tens of thousands on the walls of the sinkholes of the Mt. Gambier region.  To date, I have only studied Gouldens Hole and the Black Hole stromatolites in any detail, and there are at least six other sinkholes in the region which also support living stromatolite communities.  It seems most likely that precipitation is the dominant method of formation for these deposits, if for no other reason than there is little water movement in these environments to stir up sediment.

Typically, the stromatolites are arranged in regular bands around the sinkhole walls, extending from just above the current water level to depths of 15 to 20m below the surface.  Each individual stromatolite structure is offset from its neighbours to minimize shading of those below. The active or "growing" surfaces of the stromatolites face away from the sinkhole wall, and even when located beneath an overhang, point towards the direction of the surface and the source of sunlight.

From an examination of the living communities on the surface of the stromatolites, it seems that there is very little overlap in the composition of the microbes found at each cenote.  In other words each cenote appears to have a unique stromatolite flora.  Each sinkhole contains many different types or forms of stromatolites.  Overall, I have recorded at least 10 different forms from three cenotes, and there is the potential to find more. 

In Gouldens Hole, at least eight different stromatolite forms can be identified.  The larger forms include large, unbranched finger-like columns that are wide at the base and tapered at the tip; long, conical columns that resemble baseball bats with a narrow base and rounded wide tip; broad, branching, curved columns, and; elongated dome-shaped stromatolites which are interlinked to form reef-like rims along parts of the sinkhole wall.  Some of the larger stromatolites are in excess of 6 m long from base to tip!

The stromatolites of the Black Hole are generally similar to those found in Gouldens Hole, however, there appear to also be several stromatolite forms which are unique to the Black Hole. These include multi-branching columns that resemble inverted chandeliers, and simple paddle-shaped columns which typically grow in isolated patches.  The "chandelier" forms are often up to 9 m tall, and other stromatolite forms commonly reach lengths in excess of 5 m.

The sinkhole lakes are not the only environments that support stromatolites in the Mt. Gambler region.  Actively-forming stromatolites have also been found in the volcanic Blue Lake, just outside of the city of Mt. Gambler.  While the origins of this lake are not related directly to karst  processes, the crater rim sits on top of the local Gambler Limestone, and the waters of the lake  are derived directly from the karst aquifer system that also feeds the sinkhole lakes.

To date, I have dived at five locations around the Blue Lake, and at each site found huge reefs, towering columns and domes of active stromatolites which can be up to 12 m long.  The Blue Lake stromatolites are best developed at around 5-10 m below the surface of the lake, but do occur in continuous bands down to depths of 45 m. Generally, the size of the stromatolite structures becomes smaller with depth, but the clarity of the water in the Blue Lake and the seepage of carbonate-rich groundwater through even the deepest parts of the lake have allowed the development of quite large structures even below 40 m. Given that the shoreline of the Blue Lake is around 5 km near the surface, we have probably barely realised the potential of what the  lake might contain.  To date, I have observed at least 8 different types of stromatolite forms in the Blue Lake, and only one of these forms corresponds to that found in the nearby sinkhole lakes.

WHY SHOULD WE CARE?

Having provided a brief description of the stromatolites of the lakes around Mt. Gambler, some of you may be wondering - why should we care?  After all, aren’t stromatolites just a bunch of rocky protrusions decorating the walls of features that not many of us get to see?  Besides which, what do they have to do with cave and karst managers?

From a scientific viewpoint stromatolites have many values, and if you'll excuse the parochial attitude the stromatolites from the Mt. Gambler region are something of which we should be particularly proud.  Stromatolites were once one of the most abundant life-forms on the planet, with a fossil history dating back to 3.5 billion years ago.  For reasons which are still not clear, stromatolites declined rapidly in abundance around 570 million years ago so that in modern times, actively-forming stromatolites can be considered to be relatively uncommon, if not rare. 

Today, scientists are particularly interested in studying modern stromatolites, because it is believed they hold the key to understanding the ancient environments of the Earth.  The stromatolites of the Mt. Gambler region may well prove to have a vital role to play in this reconstruction.  One example of this is that the domes, reefs and columns of the Blue Lake display structural variations that are similar to fossil stromatolite forms.  It then follows that if we can begin to understand how the modern-day examples are forming, we can understand how ancient stromatolites developed and how they interacted with their environment.

Regional environmental history may also be alluded to by studying the Mt. Gambler stromatolites. Dr. Jacob John of Curtain University in Western Australia, studied some of the algal scrapings from stromatolites in the Mt. Gambler sinkholes and found evidence of marine organisms within the stromatolite communities.

Does this mean that the sinkholes have been previously inundated by the sea, and if so how often, and how have these organisms since adapted to a freshwater existence?  Hopefully, further research along these lines will give us an interesting perspective of the history of these karst features.  At the moment, we are unsure of the age of the stromatolites, but research is currently underway to solve this riddle and provide further clues on the past history of the area.

Compared to living stromatolites elsewhere in the world, the Mt. Gambler examples are important for a number of other reasons.  The Blue Lake stromatolites are the first record of these features in a deep, groundwater-fed lake of volcanic origin.  There are also stromatolites in a volcanic crater on Satonda Island, Indonesia, but this lake is hypersaline and the environmental and biological inputs to stromatolite construction are not directly comparable to the Blue Lake.  The Blue Lake stromatolites are amongst some of the largest stromatolites in Australia, with many individual structures reaching in excess of 10 m. This contrasts descriptions from other parts of the world, where stromatolites generally only reach a few metres in height.

The Black Hole, Gouldens Hole and the other sinkholes represent the second known occurrence of stromatolites in sinkhole lakes.  The only other similar record is from the Wondergat Sinkhole in South Africa.  In comparison to the Black Hole and Gouldens Hole, the Wondergat stromatolites are much smaller Oess than 1 m in height), and they are structurally less diverse because only two distinctive stromatolite forms have been recorded in Wondergat.

Why are the Mt. Gambler stromatolites so large, extensive and diverse in form?  There are as yet no simple answers to these questions.  Whatever the reasons for this proliferation of stromatolite growth, the Mt. Gambier lakes are unique in the Australian context, and will probably have interesting links and comparisons with stromatolite occurrences in the world context.  Obviously, a great deal more study is needed to achieve anything like a good understanding of these communities.

CONSERVATION AND MANAGEMENT ISSUES

Having outlined a few of the many  reasons why stromatolites are significant, there are several conservation and management issues to be addressed.  The single largest threat to the stromatolites comes from groundwater pollution.  Nutrient levels in the groundwater around Mt. Gambler have been steadily rising since the 1970s and contributing to an increase in the abundance and persistence of organisms which compete with the stromatolite algae for resources.

In the last 20 years or so, the sinkhole lakes have experienced dense blooms of blue-green algae in the water column during summer.  Anecdotal evidence suggests that these blooms are linked to rising nutrient levels in the groundwater.  The blooms are a problem for the stromatolites because they effectively block out most of the sunlight entering the sinkhole lake.

Stromatolite-building algae, like all plants, depend on sunlight for their growth and reproduction. The reduction of light levels in the sinkholes is likely to impede stromatolite growth, and if it continues for a long enough period, could kill off the stromatolite algae.  Added to the loss of light due to summer blooms is the problem of the increase of larger algae.  In Gouldens Hole, larger algae such as Cladophora, form large, dense sheets which often shroud the stromatolite beds for much of the year.  In Lake Clifton, Western Australia, it has been suggested that a similar situation may eventually cause the death of the entire stromatolite community by smothering the algal communities which are responsible for stromatolite growth.  Can we prevent such an outcome in the Mt Gambler region?  I honestly don't know the answer, but hopefully by letting divers, cavers, researchers and managers know of this threat, something can be done to halt the threat.  At the very least, integrated catchment management and vigilant monitoring are needed.

This situation is compounded by accidental damage by divers and, I hate to admit it, the collection of samples for study.  The numbers of divers using the sinkholes is steadily increasing, and unfortunately for the stromatolites, it is in the sinkholes that novice cave divers tend to be trained.  It is understandable, if not regrettable, that novice cave divers are too busy trying not to drown to be worrying about a few rocky lumps on the wall, but damage does occur.  I’ve seen divers accidentally knock off stromatolites with their fins, or dislodge pieces that were being used as hand holds during decompression stops.  Diver education during training courses should include an appreciation of the delicacy of the stromatolites to avoid undue damage.  This type of educational input is already underway on coral reefs where similar damage occurs, so it would make sense to implement similar programs in cave diving courses.

As for collecting, it is always an ethical dilemma as to how much to take, where from and why. Perhaps those of us involved in these activities should be prepared to account for all samples taken for research by keeping a register of the number of samples we take.  This could be submitted with progress reports which are usually required for collection permits.  Sharing of samples between groups of researchers would also seem appropriate to minimize the amount of material collected.  In various biological fields such as entomology, there are established codes of ethics which have been developed to minimize the likelihood of over-collecting.  I would certainly be interested to undertake discussions with interested parties along these lines.  After all, stromatolites and the other animals and plants that share the same environment are often as specialized and fragile as those of terrestrial caves, and are just as worthy of protection.

SOME USEFUL REFERENCES

Gomes, N.A. 1985.  Modern stromatolites in a karst structure from the Malmani Subgroup, Transvaal Sequence, South Africa.  Trans.  Geol.  Soc.  South Africa, 88, pp. 1-9.

Kazmierczak, J. and S. Kempe. 1990.  Modern Cyanobacterial analogs of Paleozoic stromatoporids. Nature, 248, pp. 1244-1248.

Thurgate, M. 1995.  Sinkholes, caves and spring lakes: An introduction to the unusual aquatic ecosystems of the Lower South East of South Australia.  SAUSS Occ. Paper No. 1.

Walter, M.R. (ed). 1976. Stromatolites. Elsevier Scientific Publishing Co. Amsterdam.

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