Western Australia Monsoonal Wet Summers to Cool Wet Winters
Western Australia’s mangroves border the Indian Ocean and Timor Sea, spanning a diversity of climatic and geographical settings. Within this range of environments, mangroves occur as a complex ensemble of habitats and plant types quite different from elsewhere in Australia. Climate varies from tropical subhumid around Port Warrender, through tropical semi-arid and arid along the coast bordering the Great Sandy Desert and Pilbara coasts, to almost temperate humid conditions in Leschenault Inlet at Bunbury. Tidal ranges vary from macrotidal, exceeding 10 m in the north, to microtidal in the south. In response to such abiotic differences, including temperature, species richness shifts from 19 species in the north to none in the south where each species has a particular southern limit of tolerance. Mangrove habitats further occupy different geological regions that influence distinct differences in growth form. In the Pilbara region, the coast is a complex of deltas, limestone barrier islands and lagoons, with a variable suite of substrates. As a result, mangroves in this region form relatively diverse fringing stands, albeit often stunted in stature but at times quite extensive in area. By comparison, the area bordering the Great Sandy Desert, with no substantial riverine input, has a series of tidal flats and barrier sits with lime mud and quartz sand substrates. Here, the few mangroves present occur as stunted and gnarled shrubs in isolated enclaves.
Uniquely variable growth conditions
A high diversity of habitats spanning more than 20 degrees of latitude. Mangroves are influenced by micro to macro tides of 10 m, and seasonal climates ranging from tropical subhumid like Port Warrender (pictured) through tropical arid and semi-arid like the Great Sandy Desert and Pilbara, to temperate humid at Leschenault Inlet, Bunbury. Photo: Vic Semeniuk
Arid climate specialists
Some of the most arid coastal areas in Australia, occur in an unusually heterogeneous mix of influences within the range of species-rich Australasian mangrove plants. The local species uniquely form a distinct arid mangrove community in the west, with at least one arid area specialist, Camptostemon schultzii, the Kapok Mangrove. Photo: Vic Semeniuk
Hot spots of diversity and structure
A latitudinal gradient in climate combined with heterogeneous geological settings have resulted in a patchwork of mangrove assemblages. These decrease in diversity south, but there remain characteristic (remnant) hotspots of diversity, structure and physiognomy in response to coastal hydrology, climate and freshwater seeps.
- Acanthus ebracteatus subsp. ebarbatus - KM
- Acrostichum speciosum - KM,PI
- Aegialitis annulata - KM,PI
- Aegiceras corniculatum - KM,PI
- Avicennia marina var. eucalyptifolia - KM,PI var. marina (PI,MW,SW)
- Bruguiera exaristata - KM,PI
- Bruguiera parviflora - KM
- Camptostemon schultzii - KM,PI
- Ceriops australis - KM,PI
- Diospyros littorea - KM
- Excoecaria agallocha var. ovalis - KM
- Lumnitzera racemosa - KM
- Osbornia octodonta - KM,PI
- Pemphis acidula - KM
- Rhizophora stylosa (KM,PI,MW)
- Scyphiphora hydrophylacea - KM
- Sonneratia alba - KM
- Xylocarpus granatum - KM
- Xylocarpus moluccensis - KM
- Camptostemon schultzii
Mangrove biodiversity and spatial extent is greatest in the tropical north where human pressures are relatively low.
Localised losses of mangrove habitat to petroleum, solar salt production and mining industry facilities, and development of coastal areas.
Notable knowledge gaps of mangrove ecosystems, their health, and the natural changes with severe storms, sedimentation and climate change.
Mangroves occur naturally in most coastal areas of Western Australia (modified from Ozestuaries 2006). Five regions are based on the types of mangrove habitat determined by varying climates and tidal ranges, plus their geological and physiographic settings.
Kimberley (KM) - Macro Tides and Monsoonal Rains
Cambridge Gulf and Bonaparte Archipelago, to Buccaneer Archipelago near King Sound, to Roebuck Bay near Broome and Cape Bossut – 9 local catchment areas, spanning tropical areas with semi-arid to subhumid climates, flushed with king tides to 10 m in range. This northern region has up to 19 mangrove taxa, and a total mangrove area of ~1,850 km2.
Pilbara & Great Sandy Desert (PI) – Broad Arid Flats and Barrier Islands
Cape Bossut to Exmouth Gulf – 5 local catchment areas including islands of the Dampier Archipelago, spanning tropical areas with arid climates. An unusual inland mangrove site of Avicennia marina occurs near Anna Plains, bordering the Great Sandy Desert. Camptostemon schultzii is notable also because it occurs further south on this arid west coast than the wetter Australian east coast. The region has 9 mangrove taxa, and a total mangrove area of ~636 km2.
Mid West (MW) - Ephemeral Arid Embayments
North West Cape to Geraldton – 6 local catchment areas plus the Houtman-Abrolhos Islands, spanning subtropical arid to semi-arid climates. Lake Macleod is the largest inland mangrove area in Australia where Avicennia marina fringes a vast salt lake. This central region has up to 2 species of mangroves, and a total mangrove area of ~14 km2.
South West (SW) - Sandy Coastal Dunes
Moore River to Esperance – 16 local catchment areas in total. Climate is generally temperate subhumid to humid. This southern region has just one mangrove species, Avicennia marina, in one estuarine lagoon, Leschenault Inlet around Bunbury, with a total mangrove area of ~1 km2. The state capital, Perth on the Swan River Estuary, is the only mainland capital to have no mangroves in the vicinity.
Nullarbor Plain (NU) – Desert Exposed Southern Coast
Esperance to Eucla – one broad, exposed and dry coast with no appreciable catchment flow paths. Climate is arid. No mangroves in this region.
Mangroves inhabit margins of tidal creeks and mud shoals in the tropical semi-arid King Sound area, affected also by 10 m tides. Photo: Vic SemeniukMangroves in Western Australia are often affected by adverse and changing climatic conditions particularly from episodic severe events, like cyclones in the north. Such natural factors pose significant threats to mangrove forests, especially in conjunction with human impacts. Sedimentation downstream is an anticipated consequence of catchment disturbance from losses of vegetation cover and increased instances of heavy rainfall. Heavy impacts of severe wind and waves on the cyclone-prone north-west coast of Pilbara and Kimberley regions. These, and the strong seasonal flows observed in the Kimberleys, leave these areas highly vulnerable to estuarine sedimentation from catchment vegetation clearing and land disturbance. Increased levels of rainfall levels over coming decades are expected to result in an expansion of mangroves across saltpan areas within tidal wetlands.
Mangrove habitat dissected by a major road causeway near Port Hedland, with dead and dying areas on the partly impounded upland side (to the right). This is indicative of disrupted hydrological flows. Western Australia is the most urbanised Australian state with 80% between Geraldton and Esperance in the south-west corner. Elsewhere, there are less than 30 people per km2, leaving northern tidal wetland ecosystems largely pristine except for the localised effects of coastal mining, extractive industries and salt farms. Mangroves are affected both directly and indirectly by human activities. Direct destruction of mangroves in the Pilbara and Kimberley regions has occurred where new port harbours and facilities were constructed for extraction and export of commodities from mineral rich regions, the development of gas/petroleum industries, and the establishment of solar salt ponds. Associated expansion of human settlements, such as Broome, Derby, Cossack, Bunbury, amongst others, have resulted in the reclamation of mangrove vegetated tidal flats, construction of roads and causeways across these flats (too often without adequate drainage engineering), and construction of small boat harbours. Indirect human impacts on mangroves have resulted from altered tidal flat hydrological patterns by roads and causeways, changes in run-off dynamics, increases in nutrients, effects of stock/cattle grazing, plus the impacts of oil spills and other pollutants. Examples of indirect impacts include: sedimentation of estuaries leading to possible mangrove expansion in Kimberleys, and localised oil spills around Karratha and Port Hedland which have resulted in patches of mangrove dieback.
Community conservation & education
Public interest in the marine environment in the State is shown by the large number of people who enjoy fishing and relaxing by the sea. This interest extends to mangroves. Bunburys’ Mangrove Cove is a 200m public boardwalk with stated objective ‘to provide visitors with a unique insight into the maritime history of Bunbury and the magnificent ecosystem of the famous white mangrove colony’. Restoration and management of this curiously isolated southern enclave of mangroves is undertaken by the Water and Rivers Commission, Leschenault Inlet Management Authority, Leschenault Community Nursery, with support from Coastwest and Coastcare. Further north, at Ningaloo Marine Park, visitors are encouraged to comment on management plans and regulations where around 84% of visitors cite fishing as their highest priority. Appropriate use of marine protected areas is controlled by park rangers and Fisheries Department officers who educate and enforce protective legislation as well as monitor visitors and infringements. Research on mangroves and their habitat is notably lacking in Western Australia considering the many unique areas in the north. The acknowledged chief mangrove researcher in the state is Vic Semeniuk with his group of dedicated collaborators. As noted for other parts of the country, there are no Statefunded programs addressing key and fundamental gaps in our knowledge and understanding of the composition, structure and function of local mangrove ecosystems.
No specific laws protect mangroves and tidal wetlands in Western Australia, only guidelines. Protection, conservation and management of mangrove habitat in Western Australia falls under the: Wildlife Conservation Act 1950 to protect flora and fauna (max. penalty of $4000); Fisheries Act 1905 to manage fisheries and habitat including mangroves; Environmental Protection Act 1986 to generally control impacts of mining, fishing, shipping, industrial pollution or tourism. The Conservation and Land Management Act 1984 (CALM Act) enabled establishment of conservation reserves under their own managing agency (CALM). CALM also recognises ecological communities at risk. Two mangrove communities are listed as priority one, including: the inland site near Anna Plains Station (Great Sandy Desert); and the Bunbury site. Under the CALM Act, Marine Nature Reserves protect marine habitats using multiple-use zones. A mangrove area in the Mid West has been protected under this act since 1987 within Ningaloo Marine Park. There are plans to apply this Act to species-rich mangrove habitats of the Pilbara and Kimberleys. In addition, the Waterways Commission was established to ensure waterways are managed to maintain or enhance environmental quality and public amenity. Responsible regulatory agencies include: Department of Planning and Urban Development; Department of Agriculture; WAEPA (WA Environmental Protection Authority); Department of Marine and Harbours; CALM; Department of Local Government; Fisheries Department; and Waterways Commission, plus local shire and city governments. Coordination is often undertaken by the Coastal Management Co-ordinating Committee which includes broad representation from several agencies. To compliment state protective legislation, Commonwealth Acts, like the National Parks and Wildlife Conservation Act 1975, have been applied on occasion, most notably for Ningaloo Marine Park.