Introduction

A small river winds through some marshy flats with mangroves on the wedge of the water and bare patches of ground with no trees,
Mill Creek in the centre ground has fringing mangroves, with a salt marsh community on the landward side, subject to less frequent tidal inundation, within the Georges River estuary.

A salt marsh is an ecosystem that is integrally part of a bigger system, that of estuarine or intertidal wetlands. A salt marsh is an environment in the upper coastal intertidal zone between land and salty or brackish water, dominated by dense stands of halophytic (salt-tolerant) plants such as herbs, grasses, or low shrubs. These plants are terrestrial in origin and are essential to the stability of the salt marsh in trapping and binding sediments.

Important ecosystem

Salt marshes play a large role in the aquatic food web and the exporting of nutrients to coastal waters. They also provide support to terrestrial animals such as migrating birds as well as providing coastal protection.

Undervalued ecosystem

Salt marshes are particularly endangered within the estuaries found along the coast of NSW. They have been little understood and little valued in the past. Consequently they have been filled, often with garbage, reclaimed and therefore lost as functioning ecosystems, particularly within rapidly urbanizing contexts. They are now often invaded by mangroves colonizing landwards. For these reasons they are now listed in NSW under the Threatened Species Act as endangered ecological communities.

Location

Low lying swampy looking area with red coloured grass and trees and shrubs only growing in the distance where the grass is green.
Saltmarsh plants must be hardy to survive the salty environment such as this one at Empire Bay.

Within NSW, the following areas are known to contain salt marsh:

  • all of the northern NSW coastal rivers

  • the Lane Cove River, Homebush Bay, Botany Bay and Georges River in the Sydney region

  • the Shoalhaven River, Currenbene Creek, Merimbula Lake, and Pambula Lake on the south coast.

Distribution

The distribution of salt marsh is within a vegetation zonation which depends on elevation and hydrology. Seagrasses are wholly inundated (flooded) by the estuarine waters in rivers and bays. Moving landwards within the intertidal zone; mangroves are followed by salt marsh, then freshwater rushes and reeds, casurina forest, then Eucalypts. Coastal riverflat forests occur commonly on land adjoining estuaries. The distribution of salt marsh is determined by the level of inundation by the tides and levels of water salinity.

Google Earth

An example of a salt marsh ecosystem lies in the area near the Kooragang Wetland Rehabilitation Project on Google Earth. To move out of street view level and look at the wider area click the yellow person icon in the bottom right hand corner. This resource will only work in the Google Chrome browser.

Significance

Shows how there is mangrove right next to the river then behind that is a band of salt marsh a 10 or so meters wide and then big trees start growing further inland.
Saltmarsh is a fragile system and has been declared an endangered ecological community under the Threatened Species Conservation Act.

Plants

The salt marsh flora is considerably more diverse than mangrove flora in temperate southeast Australia (Adam, 1994*). Characteristic plants include:

Occasionally mangroves are scattered through the salt marsh. Tall reeds may also occur, as well as salt pans.

Animals

Salt marshes are the spawning grounds of crabs, with this spawning triggered during spring maximum tides. Crab larvae are an important food source for fish in estuaries. Nineteen species of estuarine fish use salt marsh habitat and the larvae of rare butterflies, including the Salt Pan Blue (Theclinesthes sulpitius) and Painted Skipper (Hesperilla picta) butterflies depend on it. Salt marshes are part of an estuarine buffer zone that filters run-off and soaks up flood peaks.

*Adam, P. (1994) Saltmarsh and Mangrove. Cambridge University Press, Cambridge pp. 395-436

Conservation

close up view of mangrove seedlings poking out of the mud where wallabies could easily eat them.
Amongst the many threats to salt marshes is the invasion of mangroves. Swamp wallabies were known to graze on mangrove seedlings, and it is believed that since the decline in their numbers since European settlement, mangroves have been able to spread without this limiting control.

In 1982 the south east Australian bioregions contained only 354 square kilometres of salt marsh, yet it was recognised as the most biodiverse type of salt marsh in Australia, with the greatest species richness and incidence of species endemic to Australia. It can be invaded by mangroves at its wet edge, and by other wetland species and human encroachment at its dry edge, and is thus being squeezed out. In the Sydney region the decline of the swamp wallaby has been suggested as a contributing factor as they were known to limit the landward encroachment of mangrove seedlings by grazing on them.

Threats to salt marsh

Major threats are:

  • infilling for development for example roads, residential, waste disposal, agriculture, industrial and recreational uses

  • modification of tidal flows as a consequence of artificial structures being erected

  • alteration of salinity and increasing nutrient levels resulting from the discharge of stormwater into salt marshes, which favours wetland species (other than salt marsh) that like the ‘freshwater’ input (such as Phragmites)

  • weed invasion especially by Spiny rush (Juncus acutus)

  • physical damage from human disturbance, such as trail bikes, domestic and feral animals

  • invasion by mangroves

  • inappropriate fire regimes

  • dumping of rubbish and pollution from oil or chemical spills from shipping or road accidents, catchment runoff of nutrients and agricultural chemicals.

Environmental Education Centres

open area with pools of water and thick grasses. a few small mangrove trees grow amonst the grass. There are big trees in the distance.
Saltmarsh at Empire Bay

The Georges River EEC is located on the Georges River, at the heart of one of the most urbanized river catchments in Australia. Despite this more than 40% of the land area of the catchment remains natural, and much of that is in protected reserves.

Excursions

Our excursions and fieldtrips are organised quite flexibly to take advantage of both these more pristine areas and smaller remnants of swamp, bushland, freshwater and saltwater environments. We focus on river quality monitoring, bushcare and sustainable resource management and practise to foster an ethic of care and citizenship in schools and students who participate in our programs.

Further Environmental Education Centres

Coastal and estuarine Environmental Education Centres are able to assist schools with programs based on intertidal wetlands, including:

Further information

the trees and bush found at the edge of the saltmarsh, the ground cover looks very different to the thick grasses of the salt marsh.
Blackbutts are one of the trees that can be found in the areas fringing saltmarsh.

Useful links

Further reading

N. Santilan and R. Williams; Shortnote: ‘The decline of saltmarsh in south-east Australia; Australia; Results of recent surveys’, Sydney Olympic Park, 16 March 2007.