Saltmarsh is found on intertidal mud-flats in southern, eastern and far-western Victoria. The rainfall varies from 650 to 900 mm a year. About 25% of the area once supporting Saltmarsh in Victoria falls on public land most of which is represented in conservation parks and reserves. About 30% of all Saltmarsh has been permanently cleared for coastal or marine development.
Saltmarsh ecosystems develop as a series of irregular bands radiating
towards the sea. The inland components are usually less than 30 cm
tall and consist of a range of succulent and semi-succulent herbs,
grasses and sedges, often interspersed with salt-water, brackish or
freshwater ponds. The outer band consists almost entirely of low,
succulent herbs beneath and between patches of small succulent shrubs (Sclerostegia,
Halosarcia), usually to 1 tall. The most seaward edge of Saltmarsh
is often bordered by a shrubland, to 3 m tall, consisting of a single
species of Mangrove (Avicennia marina).
The landward edge of Saltmarsh is usually bordered by
Coastal Scrub or
Heathland with scattered
individuals of the large tussock grass Austrostipa stipoides
Saltmarsh is botanically the least diverse of the Victorian ecosystems - at least for vascular plants. In many places a single species (Avicennia - a mangrove, Sclerostegia - a succulent shrub, Sarcocornia - a succulent herb) may dominate the vegetation to the exclusion of all others and there are rarely more than 10 species in any area of less than a hectare. In most places the band of Saltmarsh is relatively narrow but in some parts of Westernport and Corner Inlet (south and east of Melbourne) large expanses, a kilometre wide or more, may line the coast. Most of the plants, and certainly those that make up the bulk of the biomass, are shrubs, herbs, grasses or sedges which have small, relatively inconspicuous wind-pollinated flowers and produce small seeds that are dispersed by wind or water. A reproductive strategy which doesn't involve animal vectors is necessitated by the regular inundation of most species with sea-water, even on calm days. Insect and bird pollinators are attracted to plants by bright, often perfumed flowers that also provide a nectar reward. Obviously both perfumes and sugar solutions will be washed away by the tides and few flowers can tolerate being submerged in the sea.
In contradiction to this norm the largest Saltmarsh species, Grey Mangrove, does have recognisable (although not large) flowers which provide nectar and pollen to attract insects (flies, bees, beetles and honey bees - Mangrove honey is rare but is sometimes sold by beekeepers). Although Mangroves grow on the outer edges of Saltmarsh they are seldom completely submerged at high tide and their flowers are normally restricted to the upper leaf axils. In addition the flowers have no delicate petals, just four robust sepals, and only one or two in each cluster are open at any time while the rest are tightly closed and impervious to water and salt spray. The final part of the reproductive cycle is also different from most other Saltmarsh species in that the Mangrove produces a 3 cm long, egg-shaped seed which remains on the plant and germinates into a fat-leafed seedling before being released into the sea.
A consequence of the reproductive strategies of Saltmarsh plants is that the majority of birds found in the ecosystem are species that feed on marine organisms, but there are exceptions. The most significant of these is the Orange-bellied Parrot (Neophema chrysogaster) an endangered winter migrant from Tasmania which never strays far from Saltmarsh and feeds almost exclusively on the seeds of glassworts, grasses, sedges and some herbs. The more abundant, White-fronted Chat (Epthianura albifrons), a small black and white bird, is another exception and it feeds largely on flies, mosquitoes, midges and small beetles that often swarm in large numbers in Saltmarsh.
Obviously sea-water is the major limiting ecological factor in determining which plants and animals can survive in Saltmarsh. The ecosystem is, like Riparian Forest, an ecotonal one which straddles both terrestrial and marine environments. During periods of high tide most of the animals found on the muddy substrate will be truly marine and many of these will burrow into the mud or cling to the base of some plants when the water recedes. Between the tides terrestrial animals may forage amongst the vegetation. Wading and wetland birds are almost always present. The permanent plant life, however, is essentially terrestrial in origin but with a tolerance for wet and saline conditions.
Vascular plants that live in Saltmarsh need to be able to tolerate salt and water in large quantities. Some deal with saline conditions by succulence and isolating salt in structures within the leaf and stem tissue, some will excrete salt onto the leaf surfaces and others simply tolerate high salt concentrations in their tissues. Most plants which are periodically submerged in water have mechanisms for reducing the requirements for oxygen during these times and may have hydrophobic surfaces to repel water. Mangroves, which have their roots permanently in water, utilise a specialised, aerial root structure called a rhizophore to channel atmospheric oxygen to the plant at low tide. Sedges and rushes that grow in Saltmarsh transport oxygen to their roots via hollow vessels (aerenchyma) in their stems and leaves.
Saltmarsh is a worldwide ecosystem and in Europe has been utilised as grazing land for centuries. There are clear similarities between Victorian and northern hemisphere Saltmarsh principally in the preponderance of succulent herbs (glassworts) and salt-tolerant, sward-forming grasses. In Victoria the glassworts, like most of the herbs, are perennials while in Europe and North America they tend to be annuals. In addition the northern saltmarsh grasses are much bigger and more robust (Spartina) than the Victorian species (Distichlis, Puccinellia, Sporobolus).
Shortly after European settlement of Victoria the local Saltmarsh was used as pasture for livestock in the hope that it would perform that function in much the same way as the northern hemisphere ecosystem. Unfortunately what followed was the degradation of the vegetation and local settlements, without the protection of the Saltmarsh, were also exposed to harsher effects of the sea. By the early 1900s there were active programs to restore the pasture by planting Spartina imported from England. The species that were brought to Victoria were initially the sterile hybrid Spartina x townsendii and later (in the mid 1900s) the fertile hybrid Spartina anglica, both are extremely invasive plants but the latter is usually more aggressive and may supplant the former. Both species arose from the introduction, in 1870, of the North American Spartina alterniflora to English saltmarsh where it hybridised with the local species Spartina maritima to create the robust, sterile hybrid. In later years a polyploid, and hence fertile, form of the hybrid arose (anglica) and English saltmarsh was never the same again. The new, bigger species invaded areas that the local species couldn't and also replaced it on its home territory. This characteristic made it a popular grass for stabilizing saltmarshes in many countries, including Australia, and there is now an international industry associated with its distribution. The major problem with Spartina anglica is, of course, that it changes the composition of Saltmarsh dramatically. It has long been regarded as a pest plant in England and other countries because, amongst other things, it degrades the native saltmarsh as a habitat for shore birds. In Victoria there are no native Spartinas and, as the native grasses are so small by comparison, the alteration to Saltmarsh is greater. Fortunately the deliberate introductions have not been as successful as those for other coastal species - for example Marram Grass (Ammophila arenaria) - but there are dense swards of the species in areas such as Corner Inlet.
Saltmarsh in Victoria has often been regarded as useless land and serious attempts to clear it away to allow access for ports, jetties and marine vessels have been going on for some time. In more recent times, however, the importance of Saltmarsh for protecting shorelines, as feeding grounds for local and migratory birds, as nurseries for a wide range of marine animals and as ecosystems of significance in their own right, has become better understood. The substantial areas of Saltmarsh (and Mangrove) in Westernport are now amongst the biggest, least disturbed and most diverse versions of the ecosytem in southern Australia.
© Paul Gullan, Viridans Biological Databases