Saline solution a serious threat to wetlands
Tuesday, 10 April 2001
On 2 February 1971, an international convention came into being. Known as the Ramsar convention after the Iranian city where it was adopted, it committed its signatories to the conservation and wise use of wetlands, primarily to provide habitat for water birds.
Australia is a signatory to the RAMSAR convention, and one of our important wetlands lies around the mouth of the Murray River. For years, Dr David Paton of the Department of Environmental Biology has been studying the estuarine waters of the River, Lakes, Murray Mouth and Coorong.
"A line of sandhills called Younghusband Peninsula runs for 150 kilometres from the mouth to Kingston," said Dr Paton. "Behind it is this great lagoonal system like a big appendix. About 50 km along its length it gets very narrow and breaks into two. The southern lagoon is the bottom half of it," he said.
This lagoonal system is known as the Coorong. As you move along it from the mouth, its salinity increases well beyond the level of seawater.
"It's a natural laboratory," said Dr Paton. "You can test the performance of organisms under different salinity regimes. The southern lagoon may be 2 times saltier than seawater in winter time but 3 times saltier in summer because of evaporation. It fills up in late autumn and winter with freshwater runoff, so you get a seasonal change," he said.
This high salinity means that the southern lagoon is a hypersaline sytstem; one that Dr Paton describes as "really simple."
"High salinity causes all sorts of problems for animals," said Dr Paton. "Near the mouth you'll find marine worms, but not 30 kilometres back from it, " he said. " They've been replaced by other organisms like tiny ostracods; distant relatives of shrimps."
These peculiarities of the hypersaline system govern the food sources of many birds protected under the Ramsar convention. "In the high salinity you'll have plants like Ruppia tuberosa," said Dr Paton. "A flowering plant that is a lifeline for birds."
Ruppia tuberosa is a small aquatic plant with flowers that float on the water. They shed their pollen there, so the flower heads must be kept above the Coorong's water, which may form waves 30 cm high. Each flower head has a spiral like a loose spring. As the waves come and go, it rises and falls, remaining on the surface.
The plants produce pollen and then seeds, but more important for plant-eating birds are the starchy food stores called turions. Other birds depend on fish adapted to the hypermarine system.
"One fish prominent there now is the hardihead," said Dr Paton. "It's commonly known as whitebait, a small 10cm fish which is very important for a number of birds such as horny headed grebes and fairy terns," said Dr Paton. "The southern lagoon has the biggest population of fairy terns in country," he said.
" If you suddenly change the system from high salinity where these whitebait do very well, you bring in predatory fish and the whitebait fish population drops, affecting the fish-eating birds."
Changes to the salinity of the southern lagoon are not just an academic threat. There are plans to cut drains through to the lagoon in order to clear water from areas of the south east.
"It is part of process to rehabilitate large areas of agricultural land from dryland salinisation," said Dr Paton. "The land has lost its productivity because of the loss of native vegetation," he said. "Those plants are relatively deep-rooted, and they keep the water table low. Since the water table is saline, taking off those deep rooted plants allows the saline water to rise to the surface, and we now have salinised and unproductive lands."
The drains are intended to solve this problem by carrying the surface water into the Coorong but, if they do, they will carry it into the hypermarine system of the southern lagoon. Despite the salt loads in the drained water, it cannot match the hypermarine salinity of the Coorong water, and will gradually dilute it.
"In five to ten years the hypermarine south lagoon system will have changed to an estuarine system," said Dr Paton. "How will plants and animals respond? I expect lots of fresh water birds will be using it, but few hypermarine birds, and Ruppia gives us a clue to what will happen," he said.
Ruppia tuberosa grows around the lagoon edges in water between .3 to .8 metre deep. Dr Paton has taken plugs of the plant and placed them placed at intervals along the Coorong to mimic the salinity changes it will face.
"It does best if placed back where it came from," explained Dr Paton. "In lower salinity it does not do as well. In the north lagoon, which mimics what the salinity will be like if the drains go through, it grows fantastically but gets swamped by all the other things growing and never reproduces, so it disappears," he said. "The north lagoon has virtually no Ruppia; it has other species without turions which don't support birds."
Dr Paton has other examples of how the plants, animals and microorganisms of the southern lagoon will be affected by the drains. He believes that they are an inadequate solution to the problem that they are supposed to fix.
"If I were to deal with dryland salinity in the South East," said Dr Paton, "I 'd keep the problem where it is, not transfer it down the chain. Maybe we should trash one property to make storage for this water we are trying to get rid of," he said.
"The real problem is not salinity but vegetation clearance," he said. "We should be challenging whether we should be continuing to clearing."
"The solution is to use much more strategic revegetation to absorb the water before it gets into the ground water system," said Dr Paton, "To eliminate dryland salinity with a sustainable outcome instead of something that will continue to destroy natural assets."
Dr Paton's research will feature in a 30-part radio series on the Murray, to be broadcast on Radio 5UV in September.
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