Australia: Wetlands no longer a shore thing

Geoff Maslen The Age 3 Jul 12;

SOME parts of Victoria's coast have lost more than 60 per cent of the salt marsh and mangrove areas that flourished before European colonisation, according to Australia's first large-scale study of coastal wetlands by Victoria University researchers.

Backed by a $470,000 grant from the National Heritage Trust's Strategic Reserve, the team spent months exploring Victoria's coastlines, studying the different types of wetland vegetation and poring over maps drawn by early surveyors to obtain accurate details of the wetlands that had disappeared.

Professor Paul Boon, from the university's institute for sustainability and innovation, undertook the project with colleagues from a range of government and consulting organisations. He says salt marshes, mangroves and other types of estuarine wetlands once covered nearly 300 square kilometres of Victoria's coastal areas.
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Those that remain are critical habitats for a large number of birds, including those stopping over on cross-hemisphere migrations or locals such as the critically endangered orange-bellied parrot, which winter on the marshes.

"The wetlands are also particularly important for providing habitats for small invertebrates and fish which, when they mature, then move into the estuaries on out-going tides," Professor Boon says.

"Since the arrival of Europeans, however, large areas of the salt marshes and mangroves have been displaced as land was cleared for agriculture, the development of cities and towns, ports and industrial complexes."

His study found the western shore of Port Phillip Bay has suffered a 50 per cent depletion of its wetlands, the Lonsdale Lakes area on the Bellarine Peninsula a 60 per cent loss and the Powlett-Kilcunda and Anderson Inlet sectors of the South Gippsland coast up to 65 per cent. Some inlets, such as Shallow Inlet on the South Gippsland coast, lost all their mangroves after Europeans arrived.

"The marshes which originally occurred in inner Melbourne and around the port have been destroyed by the growing city while those of Altona retain much of their area but have been corralled within a largely suburban landscape," Professor Boon says. "The Cheetham wetland at the mouth of Skeleton Creek has been almost entirely converted to salt evaporation ponds and the upper marsh has been replaced by the Sanctuary Lakes housing development."

He says that on the Western Port coast at Lang Lang "phenomenal land erosion" is taking place and sweeping salt marsh and agricultural land into the bay. The sediment is also affecting sea grasses, which have suffered over the past three decades because of increasing turbidity. Suspended material in the water prevents light reaching the sea grasses and, because they can't photosynthesise, they die. Yet the sea grasses are essential for recreational fishing and protecting the sea bed against erosion.

Worse is to come for the wetlands that remain. Climate change is inducing sea level rises around the world while increasingly frequent and bigger storm surges will carry seawater further inland. Professor Boon says these will be just as important for their ecological as well as their social and economic significance.

That's because, if predicted rates of sea-level rise are realised, much of Victoria's public land that now support inter-tidal vegetation will be inundated. When that happens, conservation of salt marshes and mangroves will require large areas of currently freehold or leasehold land to be set aside so they can shift inland.

Around coastal cities and towns, however, salt marshes and mangroves cannot shift to escape the rising seas because of stone walls, expensive houses, industrial estates or land that has been cleared for agriculture. Professor Boon says this means the salt marshes will be trapped between increasing sea levels and a mangrove community that has become more vigorous.

Mangroves are limited in southern Australia by low winter temperatures and frosts. But, as water and air temperatures rise, it is likely mangroves will extend their range. He says in New South Wales mangroves are already pushing into other areas, possibly because of higher temperatures but also because more sediment is flowing down rivers — a result of more episodic storms and bushfires.

"Salt marshes will be caught between a rock and a hard place — literally," he says. "I did my PhD in Queensland on sea grasses, then came to Monash to do post-doctoral research on salt marshes in the mid-1980s yet there has been little investment in research into coastal wetlands in southern Australia since then.

"That's in contrast to the millions spent on the Murray-Darling Basin scheme and recent findings from the United Kingdom that coastal wetlands are roughly three times as valuable as an equivalent area of inland freshwater marsh."

"Our project showed how important salt marshes and mangroves are, especially for preventing soil erosion: one of the best ways to protect the inland from storm surges is to have a wide belt of active wetland vegetation that muffles its energy. Another is the way the marshes intercept nutrients coming off the hinterland into coastal areas; if this doesn't happen, algal blooms occur in the sea and indirectly cause the loss of sea grasses which are shaded out. And a third one is providing habitat for our own and the migratory birds as well as all the invertebrates and young fish."

Professor Boon says the problem for the salt marshes and mangroves is that they belong to a part of the environment where traditionally refineries, housing estates and ports are located, where channels are dredged for shipping — all without adequate consideration given to the ecological implications.

"Estuaries have fallen between two stools: there is a lot of strong marine research in Australia as well as a lot of strong freshwater work whereas the estuaries and coastal wetlands have missed out. Management authorities used to look at the hinterland for agricultural purposes and freshwater areas for irrigation impacts but no one looks at the coastal lands. Yet these are the regions where most of us live or where we take our holidays and that's the part that's going to hit by climate change."

Professor Boon has not long returned from an international conference on wetlands in America. There he heard a report by Dr Stefan Rahmstorf from the Potsdam Institute outside Berlin, who is widely respected for his "level-headed predictions and models" of sea level rises. Dr Rahmstorf now says sea levels will rise by an average of at least a metre before the end of the century while a rise of at least 1.5 metres "is not out of the realms of possibility".

"But the situation depends on a large number of local factors, too, such as whether the coasts themselves are rising or falling because of continental plate movement and other regional influences," Professor Boon says. "Most of northern Australia is sinking as the Australian tectonic plate collides with Asia whereas in the south, as in Gippsland, the coast may be subsiding because of anthropogenic factors such as oil extraction. That would give us a double whammy — rising sea levels and a subsiding coast ..."

* The Victoria University report can be downloaded at vu.edu.au/institute-for-sustainability-and-innovation-isi/publications