The disputed natural history of the Lower Murray
Most of the water the Murray Darling Basin Authority plans to take from irrigators under its new plan will be sent to the Lower Lakes in South Australia ostensibly to keep the River Murray’s mouth open and the Lower Lakes full of fresh water. An alternative would be to let the Southern Ocean flood through the River’s mouth and fill Lakes Albert and Alexandrina with sea water. This salt water solution is being resisted on the basis the lakes were naturally fresh but that is not what the microfossils in the sediment of the lakes indicate.
It is not contested that the lakes formed approximately six thousand years ago when the Southern Ocean broke through the modern coastal sand barriers of Sir Richard Peninsula and Younghusband Peninsula filling the interdunal areas. That point of entry, now known as the Murray River’s mouth, remains dynamic and has moved 6 kilometres over the past 3000 years.
Sea levels have fluctuated throughout geological time. The ice sheets during the last peak in glaciation contained about 55×106 km3 more ice than today and sea levels were on average about 130 metres lower than they are today.
At that time the River Murray extended across the continental shelf and its mouth was about 180 kilometres to the south west of its present location. The climate was much colder, drier and windier, and where the Murray enters the sea today there were distinctive desert dunes running parallel to the distant coastline.
As the ice melted, sea levels rose reaching a peak 6,000 to 7,000 years ago when sea levels were 1-2 metres higher than their present position after which there has been a sea level fall of about two metres.
The distance seawater will flow naturally onto a landmass is affected not only by the extent of sea level rise but also by the height of the land barrier which may change with tectonic processes that can cause uplift or subsidence. Studies by Professor Bourman and co-workers from the Universities of Wollongong and Adelaide have shown that the Lower Lakes area has been subsiding over the past 125,000 years at a rate of 0.02mm per year.
The Lower Lakes have also been subject to seismic activity with what is referred to as ‘regional warping’ occurring from the Mount Gambier volcanic region towards the base of the rising Mount Lofty Range which is immediately south west of the Lakes. Residents of the Lower Lakes experienced an earthquake on May 10, 1897, and then again on September 19, 1902. These tectonic movements may have accelerated coastal erosion processes along the entire Encounter Bay shoreline, which shows a general landward encroachment over the last few thousand years.
John Cann and co-workers from the Universities of South Australia and Adelaide have studied fossil foraminifera – tiny protozoa with shells of calcium carbonate – preserved in the sediments of the Lower Lakes to discriminate episodes of seawater incursion from periods of high river flow. Comparing the occurrence of species typical of freshwater with species typically found in the sea, they concluded that the Lower Lakes had a maximum marine influence 5,255 years ago and a maximum freshwater influence 3,605 years ago. The period of maximum freshwater influence is thought to coincide with the period when the Murray Mouth was greatly restricted or closed because climatic conditions in the catchment were much drier.
Dr Cann and co-workers conclude that the change in the foraminifera complex over the most recent 2,000 years indicate a general trend of increasing marine influence, up until the construction of the five large steel and concrete barrages that now block the natural ebb and flow between the Lower Lakes and Southern Ocean.
This conclusion that the lakes were getting saltier for most of the last 2,000 years is not obvious, however, from studies of another group of microfossils, tiny plants called diatoms that occur in sediment cores.
Diatoms are unicellular algae common in rivers, lakes and the ocean with particular species unique to freshwater and others to saltwater.
Jennie Fluin from the University of Adelaide, and co-workers from that University, and also CSIRO, studied the diatoms in a sediment core from the southern section of Lake Alexandrina. They found that between 7,000 years and 2,300 years ago, a strong marine influence was present. But they conclude that the change in the species abundance over the last 2,000 years shows a general decline in relative abundance of marine species. The data are equivocal, however, and only in the very top section of the core, in a distinctive light grey mud, perhaps deposited after the construction of the barrages, does a species with a low salt tolerance become common.
In summary, studies of the natural history of the Lower Lakes indicate a marine origin, that during long periods the waters of the lakes were salty, and that during other periods they were fresh. It is unclear how salty or fresh and for how long. The area is characterised by environmental variability.
Our society places a premium on restoring degraded and polluted places to their natural state. It is clear from the scientific literature that the Lower Lakes have a marine origin and that they could be healthy if filled with water from the Southern Ocean rather than taking fresh water from upstream which has been government policy at least since construction of the barrages.
That the salt water solution is resisted, and that the Murray Darling Basin Authority insists in its new plan that even more water be taken from irrigators to keep the lakes fresh, suggests that this key institution is more influenced by politics than science.
Filled with seawater, and with regular tidal flushing, the Lower Lakes would possess a different, but not necessarily less natural, or less healthy, assemblage of plants, animals, fish and microorganisms.
Jennifer Marohasy is a biologist and adjunct research fellow at Central Queensland University
Jennifer Marohasy “The Murray: a fresh perspective”
Bob Bourman “Life of the Murray”