Mangroves and Saltmarsh

Mangroves

In the past mangrove forests were considered to be smelly eyesores and were usually reclaimed. They were often destroyed when mudbanks were dredged. Mangrove forests are now regarded as key habitats.

Mangroves favour sheltered shores on tidal flats in estuaries and bays, the sheltered bays of offshore islands and on some coral reef cays. Mangroves are capable of growing in salty water thanks to several adaptations, including being able to tolerate salty sap, having thick waxy leaves that exclude salt, secreting salt through pores, and by concentrating salt in leaves and then shedding the leaves periodically. You can tell a mangrove forest straight away. It is in the muddy intertidal zone and the forest floor at low tide is a mass of root systems. The Grey Mangrove (Avicennia marina) has spiky vertical roots, called ‘pneumatophores’ that protrude from the mud. These roots allow the plant to breathe in these poorly aerated soils. They also create a large interlocking platform of roots that anchor the tree in the soft mud. They can handle a cyclone as big as Darwin’s 1974 Cyclone Tracy.

Mangroves grow in the tropics and the temperate zone. In the Australian tropics, 37 species occur. The number of species decreases further southwards. In temperate Victoria and South Australia, only the Grey Mangrove grows. Australia has the third largest area of mangroves in the world; approximately 22% of the total coastline is covered in mangrove forest.

Some mangrove trees are ‘viviparous’, which means that seeds germinate on the tree before falling into the water and dispersing with the tides and waves. If the seed lodges on an unstable new mud bank it can quickly become established. Spreading seeds on the tide doesn’t exactly spread the plant far and wide. Mangrove populations from different catchments are often isolated. Populations can be genetically distinct within each estuary.

Mangrove forests are regarded as key fish habitats for species like mullet, bream, whiting, luderick, flathead, prawns and crabs. They are especially important as nurseries for juvenile fish. Mangroves also provide habitat for other forms of wildlife including birds like the threatened Mangrove Honey Eater. Mangrove trees produce large amounts of organic matter, about a kilogram for every square metre of forest floor. The fallen material is grazed by many small animals. They also protect vulnerable low lying areas from storm damage. They also trap and stabilising sediment, and filter nutrients and contaminants from runoff, helping to maintain water quality.

Mangrove species are often zoned parallel to the shoreline. The zone where each species lives is determined by tide levels and soil conditions. The River Mangrove generally likes the mean sea level mark. The Grey Mangrove and the Red Mangrove are often found growing in a zone behind the River Mangrove. The Large-Leaved Mangrove and the Milky Mangrove are usually found growing in and above the mean high water mark. Milky Mangrove expels a white sap on breaking the stem or leaves which is irritating and poisonous on contact.

Having exposed roots, mangroves are easily trampled by human feet, vehicles or stock. They are also vulnerable to coastal development, harbour works, pollution, drainage works, boat wakes on narrow waterways, climate change and sea level rise. Saltmarsh Coastal saltmarsh has similarities with mangroves. Both habitats involve the colonisation of intertidal sediment flats by salt tolerant plants. In this case we are talking about low lying ground cover of shrubs, sedges, rushes, reeds, grasses and succulent herbs. They are even sometimes found on the high tide line behind mangroves. The ground is usually a mix of plant cover and bare indentations where excess salt collects.

Saltmarsh

Coastal saltmarsh can range from narrow belts of growth to huge tidal flats stretching over hundreds of square kilometres. Like mangroves, they are home to a range of animal and plant species. Saltmarsh also supports crabs, prawns, molluscs, and insects. At high tide they provide food for larger fish and birds. Crabs have been shown to be a significant food source for fish and over 40 species of juvenile fish have been recorded inhabiting tidal saltmarsh areas. Birds love it too and saltmarsh provides summer feeding and roosting habitat for many wading birds, such as the Sharp-tailed Sandpiper and Marsh Sandpiper.

Saltmarsh plants reproduce three different ways, by flowering and producing seed, by cloning from a fragment of detached plant, or by spreading underground via a network of root-like rhizomes. As with mangroves, coastal saltmarsh have been long considered to be useless wastelands. The plants are fragile and easily damaged, so they are often scarred by the wheel ruts of off-road vehicles and the footprints of humans, stock and feral animals. Many areas have been reclaimed, or covered in invasive pest species. It has been estimated that since 1950, most estuaries in southeast Australia have lost over a quarter of their saltmarsh with some estuaries losing up to 80 per cent. It is protected as an endangered ecological community in many States.

Saltmarsh covers 13,500 km2 of coastal shoreline around Australia. The largest areas of saltmarsh are found in northern Australia (Queensland, Northern Territory and Western Australia). The greater species diversity is in southern Australia. In the tropical areas of northern Australia there are less than 10 species of saltmarsh plants, while over 250 species of saltmarsh and foreshore fringing vegetation have been recorded in New South Wales, most of this south of Jervis Bay. Research has also indicated a trend of landward movement of mangroves into the even rarer saltmarsh environments in the estuaries of south-east Australia and this poses a threat to saltmarsh habitat. This migration is due in part to increased sedimentation rates along the coast, which creates habitat more suitable for mangroves.

Some viewing boardwalks for mangrove and saltmarsh

We are emphasising the use of controlled boardwalks as opposed to making your own way, as visitation will trample areas not adequately managed.

NT

Mangroves in the Northern Territory represent 42% of Australia’s mangrove communities. Mangroves also penetrate up to 100 km inland because of the greater influences of tide and low-lying nature of the riverside land. Most of the coastline is affected by low wave energy, supporting mangrove growth on 42% of the 10,953 km coastline.

Darwin, East Point – Lake Alexander boardwalk, but you will see them at nearly every boat ramp, foreshore mudflat and river crossing.

QLD

Climatic conditions vary widely in a State that covers the tropics to the subtropcal. The north-east coast is sheltered by the Great Barrier Reef creating ideal conditions for mangroves on the mainland coast. In the south-east, the coast is further protected by an chain of sand islands forming extensive, sheltered coastal channels ideal for mangrove development. In the north-west, the expansive shallow gulf of the Gulf of Carpentaria provides a broad further sheltered enclave. There are also extensive tidal flats at numerous river mouths. Mangroves in Queensland are present in around 18% of the 13,347 km coastline.

Gold Coast –  Tallebudgera Creek Conservation Park, Burleigh Heads Gold Coast, Coombabah Lakes Conservation Area Gold Coast, Beree Badalla ‘Mangrove Tree haven‘ Wetland Reserve, Currumbin Gold Coast, Elanora Wetlands Habitat Gold Coast, Paradise Point, Phil Hill Environmental Park.

Brisbane - Gardens Point, Boondall Wetlands near Deagon,

Stradbroke Island, Myora Springs

Sunshine Coast – Maroochy Wetlands Sanctuary, Bilai Environmental Education Centre

NQ- Daintree National Park, Marrdja Botanical Boardwalk Cairns, The Jack Barnes Bicentennial Mangrove Boardwalk

WA

Western Australia’s mangroves survive in climates that vary from tropical to almost temperate humid conditions in Leschenault Inlet at Bunbury. Mangrove species richness shifts from 19 species in the north to none in the south. In the Pilbara region, the coast is a complex of deltas, limestone barrier islands and lagoons, with mangroves in this region form relatively diverse often stunted fringing stands.

Bunbury, Mangrove Cove Mangrove Boardwalk, Lake Clifton

Mandurah, Samphire Cove Nature Reserve

Perth, Alfred Cove Reserve, City of Melville.

NSW

Mangrove habitat within NSW is very small but may actually be expanding. A recent survey in 2005 recorded 125 km2 of mangroves in 86 estuaries and embayments. Some of the recorded increase may be a result of improved mapping, but a rise in sea level of about 10 cm over the past 100 years, plus increased sedimentation rates due to land clearing and erosion, will have played a part. There are only 72 km2 of saltmarsh spread in fragments around 114 estuaries, mostly in the Hunter/Central Rivers region. Most of the species diversity is in the north. Bruguiera gymnorhiza grows from the Tweed River to the Clarence River; the Red Mangrove, Rhizophora stylosa, grows from the Tweed River to the Macleay River and finally, the Milky or Blind-Your-Eye Mangrove, Excoecaria agallocha, grows from the Tweed River to the Manning River.

Sydney – Hornsby Heights near Sydney, Crosslands Salt Marsh Walk, Berowra Valley national park Homebush Bay, Sydney,

Badu Mangroves boardwalk Bicentennial Park Hunter River estuary,

Kooragang Wetland saltmarsh

Merimbula Top Lake Boardwalk & Walking Track Husskinson Jervis Bay,

Lady Denman Harbour Marine Reserve

VIC

Recent estimates indicate there are around 63 square kilometres of mangroves in Victoria. Victorian mangroves are restricted to sheltered shores often in association with saltmarsh species such as Sarcocornia quinqueflora (Beaded Glasswort) and Sclerostegia arbuscula (Shrubby Glasswort).

Pt Cook, Cheetham Wetlands, Skeleton Creek – salt marshes

Geelong, Limeburner’s Bay Conservation Area,

via Hovell’s Creek Trail Hastings, Western Port, Bittern Coastal Wetlands Boardwalk Western Port. Warneet mangrove boardwalk Western Port. Blind Bight coastal trail at Warneet Nature Reserve. Inverloch, Screw Creek Nature Walk.

SA

Mangroves in South Australia are confined to sheltered shores in the Gulf of Saint Vincent and Spencer Gulf, and protected bays on the Eyre Peninsula. They range from extensive and dense forests to sparse and isolated stunted trees. The total area of mangroves in South Australia is around 156 km2, and there is only one species, Avicennia marina, the white mangrove.

Eyre Peninsula, Arno Bay Garden Island and St Kilda, Adelaide

TAS

Saltmarsh can be seen in small fragments unprotected by boardwalks. Many spots like Cloudy Lagoon and Southport Lagoon are inaccessible. There are no mangroves. Lauderdale, Ralph’s Bay, has fragments of salt marsh by the roadside with lots of shorebirds nearby that should not be disturbed.