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 wetlands1. A salt marsh is an environment in the upper coastal intertidal zone between land and salty or brackish water, dominated
by dense stands of halophytic2 (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.
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 hydrology3. 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 forest4, 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 Earth5. 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.
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 sulpitius16) and Painted Skipper (Hesperilla picta17) 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
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 bioregions18 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 mangroves19 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 Phragmites20)
weed invasion especially by Spiny rush (Juncusacutus21)
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.
The Georges River EEC22 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:
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.
