A Likely Outcome Should The Lower Lakes Become Estuarine
By Mike Young (ret. DEH) and Ken Jury, ret. Senior Journalist (Marine & Aquatic Ecology). Goolwa, SA.
The objectives of allowing tidal access into the lower lakes by manipulating the barrage gates are only partly visualised by many people with an interest in the areas long-term future.
Many see the concept of letting seawater into the lakes as a single event to cover the floor of the lakes.
That is not our expectation!
The intent is to introduce tidal flow to the lakes which would, in time scour accumulated silt from the vicinity of the mouth (that we are paying heavily to dredge), and create stronger flows through all of the historic channels, going from the mouth area back into the lakes.
How much scouring occurs, and in which channels, and even when the tide is coming in or going out, can all be manipulated by thoughtful opening and closing of modified barrage gates.
As channels enlarge, and clear of silt, greater volumes can access the lakes providing the tidal rise and fall so important to the biological health of a tidal estuary system.
Channels and tidal flats would form within the lakes, and in time would be colonised by the biota (total animal & plant life of the site) which forms the all-important food chain base.
Most of the biota known to the Coorong could be expected to colonise the lakes!
When the food is there, everything else makes use of it! Fish, birds, crabs molluscs, etc.
The real savings are that it is not then necessary to try and bleed freshwater from the Murray River to maintain the wide expanses of the lakes, where very large amounts of water are lost to evaporation and soakage every year.
Of all River Murray water that enters South Australia each year, the volume taken by evaporation from Lake Alexandrina is more than what the river generally contains between the border and Wellington.
By holding that amount of river water back from entering the lakes, we can maintain healthier wetlands along the river and still have some left over to regulate portions of seasonal flow of fresh into the lakes, to maintain the ‘estuarine’ conditions there.
Most estuarine life responds to periodic low salinity (‘fresh’) flows to stimulate life cycles. The Eastern Mount Lofty drainage would likely contribute periodically, during seasonal rain periods.
But, the most significant gain, outside of water conservation benefits in the core business of RAMSAR – is the provision for a continuance of traditional feeding areas for intercontinental migratory birds.
The increase in inter-tidal zone feeding areas for waders would be enormous!
The intent of RAMSAR is to provide assured feeding areas for migratory waders. However, they rarely benefited from the freshwater systems that we, in the past had turned the Lakes into.
It recently has been a reed-dominated system with a steady pool level, with no inter-tidal feeding zones, and certainly shunned by most waders. Some still tried to make do in the Coorong, but man has killed the greater proportion of that anyway. Most of the birds on or around the lakes themselves were sedentary or nomadic birds oriented towards open water and reeds.
There is a good deal of concern by the scientists and the community with regard to the Coorong. Particularly about the declining, narrow inter-tidal zone for wader feeding.
For fish populations, the explosion towards the middle and top of the food chain base in the tidal lakes would be enormous. There are numerous species that would inhabit the estuarine lakes. Species such as the marine/estuarine Black Bream and Mulloway would also take up residence with many coastal pelagic sea species not far behind.
Other examples by common name would be four species of Mullet generally found along the southern coasts. They include Coorong mullet, Yelloweye mullet, Sand mullet and the very large Sea or Jumper mullet, juveniles of which often enter estuaries during their developmental stage.
Other species would include Flounder, Australian Salmon, Flathead, Anchovy, Marine Hardyhead, Long-finned Goby, Congolli and the amazing Short-headed lamprey, a dual marine and freshwater species that would only migrate with the aid of a fish-way or ladder through a permanent weir at Wellington. Similarly, migrating Galaxias, sometimes known as native trout are a freshwater species that rely upon tidal areas to spawn. This amazing specie is born at sea and then returns to its freshwater environment to reach maturity.
Whether we experience further catches of King George whiting from the confines of the upper Coorong/Murray Mouth is uncertain. Yes, this quality species has almost become a regular in this area, much to the surprise of many. Oddities of the marine and freshwater world do occur. No less a report of a good size garfish landing on the deck of a yacht near the Hindmarsh Island Bridge a few months back.
Not surprising, salinity levels in the Goolwa channel are now regularly extreme; and would support some estuarine species Whether this was a Southern Sea garfish (Hyporhamphus melanochir) or perhaps a River Garfish (Hyporhamphus regularis), is unknown.
Either way, both are fine table fish! Should the lakes become an estuarine feature, then sea garfish would likely become a draw-card as a table fish for anglers and possibly for the benefit of commercial fishers as well.
We should also expect various crustaceans, marine worms, molluscs as well as the various rays, skates and so on. All have a role in the food chain that would develop and benefit by becoming a resource again. While all estuarine systems rely on periodic fresh-water flows, they are also reliant on tidal flow with rise and fall, just like a pulse.
Today’s blue waters of the northern Coorong continue to frame the distant beauty of its impressive backdrops. Extending this colour and charm into the lakes, will action a rejuvenation of its channels, its shores and its life-forms throughout. The food chain will revitalise, and the flora and fauna will return. There will be much more to see, to find and to enjoy!
By Mike Young (ret. DEH) and Ken Jury, ret. Senior Journalist (Marine & Aquatic Ecology). Goolwa, SA. May 2009