Northern Territory Monsoonal, Hot Wet Summers
Mangroves in the Northern Territory represent 42% of Australia’s mangrove communities. They vary from species rich in the highly seasonal rainfall areas of the Tiwi Islands, north-east Arnhem Land and Kakadu, to species poor in the seasonally arid mangrove areas in the Gulf of Carpentaria and Joseph Bonaparte Gulf. Species numbers decrease with increasing latitude to the south. However, a notable pattern in Northern Territory mangroves is the increase in species numbers in areas of higher rainfall. Coastal areas are generally characterised by monsoonal climates that have uniformly high temperatures and highly seasonal rainfall. The dry season from April to November is the dominant feature of the climate, with 95% of river flow occurring in the wet from December to March. Tidal ranges vary from over eight metres in Darwin Harbour and the west coast, to around two metres in parts of the Gulf of Carpentaria. A feature of many coastal areas is the large macrotidal rivers that drain low relief coastal landforms. Mangroves also penetrate up to 100 km inland because of the greater influences of tide and salinity from larger tidal amplitude assisted by the low relief landforms and seasonal rainfall. Most of the coastline is affected by low wave energy, supporting mangrove grow to reportedly occupy 42% of the 10,953 km coastline.
Northern Territory mangroves are generally the least impacted in Australia, and possibly the world. They are affected by minor harvesting by traditional owners, public boat ramps and access. More recently, however, sites have been impacted by urban and port expansion around Darwin.
Aboriginal use and knowledge
Mangroves are an important resource for Aboriginal coastal groups who have an extensive and detailed knowledge of their traditional use. The Northern Territory coastline is 70% Aboriginal land and permits are required to access most areas.
Low human population numbers
Coastal areas have relatively low population numbers apart from the urban and rural populations near Darwin, Palmerston, the Litchfield rural area, and the mining towns of Gove on Nhulunbuy Peninsula and Alyangula on Groote Eylandt.
- Acanthus ebracteatus subsp. ebarbatus - BO,TE,WG
- Acanthus ilicifolius - BO,TE,WG
- Acrostichum speciosum (TE,WG)
- Aegialitis annulata - BO,TE,WG
- Aegiceras corniculatum - BO,TE,WG
- Avicennia integra -TE
- Avicennia marina var. eucalyptifolia - BO,TE,WG
- Bruguiera exaristata - BO,TE,WG
- Bruguiera gymnorhiza -TE,WG
- Bruguiera parviflora - BO,TE,WG
- Bruguiera sexangula -TE
- Camptostemon schultzii - BO,TE,WG
- Ceriops australis - BO,TE,WG
- Ceriops decandra -TE
- Ceriops tagal -TE
- Cynometra iripa -TE,WG
- Diospyros littorea - BO,TE,WG
- Excoecaria agallocha var. agallocha -TE,WG var. ovalis - BO,TE,WG
- Lumnitzera littorea -TE,WG
- Lumnitzera racemosa - BO,TE,WG
- Nypa fruticans -TE
- Osbornia octodonta - BO,TE,WG
- Pemphis acidula -WG
- Rhizophora apiculata -TE
- Rhizophora X lamarckii -TE
- Rhizophora stylosa - BO,TE,WG
- Scyphiphora hydrophylacea -TE,WG
- Sonneratia alba - BO,TE
- Sonneratia lanceolata -TE
- Sonneratia X urama -TE
- Xylocarpus granatum -TE
- Xylocarpus moluccensis - BO,TE,WG
- Avicennia integra
Protection of coastal wetlands needs to be maintained.
A growing urgency for the preservation of traditional Aboriginal knowledge as senior elders with specific languages and experiences pass away. The knowledge about mangroves is ancient and detailed.
Mangroves occur naturally in most coastal areas of the Northern Territory (Ozestuaries 2006) largely affected by monsoonal wet hot summers and low waves. Three regions are based on the types of mangrove habitat determined by varying rainfall and tidal ranges, plus their geological and physiographic settings.
Bonaparte (BO) - Macrotidal West
Keep and Victoria Rivers to the Daly River – 5 local catchment areas, spanning semi arid areas affected by macro tides. Along the west coast to the Western Australian border there are macrotidal coastal waters and relatively species poor tidal wetlands. This region has 17 species of mangroves, with a total mangrove area of ~1,059 km2..
Top End (TE) - Monsoonal Wet Coast
Beagle Gulf and the Finniss River, to the Durabudboi River and Arnhem Bay – 12 local catchment areas, spanning humid areas affected by meso to macro tides. The top end coast is species rich, with large areas of tidal wetlands. Darwin Harbour contains some of the largest and most diverse mangroves in the territory. The most notable mangrove species is the Northern Territory and Australian endemic, Avicennia integra. The region has 31 mangrove taxa, with a total mangrove area of ~2,725 km2.
Western Gulf of Carpentaria (WG) - Arid Tropical Flats
Nhulunbuy and Koolatong River, to Vanderlin Island and Robinson River – 9 local catchment areas, spanning semi arid areas affected by micro tides. The western coast of the Gulf to the Queensland border has micro- and mesotidal coastal waters with relatively arid and species poor tidal wetlands. This region has 21 mangrove taxa, with a total mangrove area of ~832 km2.
Extensive defoliation by locusts has severely affected Sonneratia lanceolata stands throughout the South Alligator River estuary.Northern Territory mangroves are occasionally affected by adverse natural climatic conditions that are manifest seasonally and annually in the first instance, and episodically with severe events like cyclones. Various natural factors pose significant threats to mangrove forests. Changing rainfall conditions result in expansion or contraction of mangrove areas depending on rainfall fluctuations in particular locations. There is physical damage to foliage and stems caused by severe wind storms. Sea level rise results in a shift of the intertidal zone replacing upland vegetation. Insect infestations, like locust plagues result in significant defoliation inhibiting mangrove growth and survival in times of stress. Erosion destabilizes taller trees, and redeposition of fine sediments buries breathing roots to suffocate trees.
Severe alteration to tidal streams in built-up areas around Darwin has lead to their reduced biodiversity and ecological function.The Northern Territory has relatively minimal human pressures on natural mangrove habitat because of its relatively low population. However, the situation is changing. Large areas of mangroves, and there ubiquitous occurrence along the low energy coastline, means that any further human expansion along the coastal strip will influence and impact on mangrove condition. For instance, landfill of mangrove areas for housing development and industrial estates has occurred around Darwin Harbour. In the Litchfield Shire and Daly basin, nutrient enrichment and increased sediment load from agricultural activities affect mangrove areas. Increased pressures also result from coastal residential developments like Dundee Beach, Mandorah and Channel Point. Expanding coastal developments, such as mining, residential, tourist and agricultural development are likely to also have indirect affects on mangrove communities with increased runoff of chemicals, fine sediments, boat traffic, effluent and waste discharge. Broad-scale land-clearing is associated with increases in agriculture and forestry. Coastal development and alterations are associated with large-scale extractive mineral operations in north-east Arnhem Land, Groote Eylandt, and the Gulf of Carpentaria.
Community conservation & education
Public boardwalks with educational signage are located in Darwin at East Point, and at the George Brown Darwin Botanic Gardens. There are no formal activities for community mangrove monitoring, though a number of community groups are indirectly involved in mangrove awareness, understanding and promotion. In 2001 and 2002, community members participated in a mangrove monitoring program. Community groups that focus at times on mangroves include: Waterwatch, Coastcare, Top End Native Plant Society, Greening Australia - Northern Territory, Northern Territory Naturalists, Junior Ranger DNRETA (Department of Natural Resources, Environment and the Arts), Environment Centre NT, WWF Darwin. Chief research and education facilities are located at CDU (Charles Darwin University), DNRETA, DPFIM (Department of Primary Industry, Fisheries and Mines), and ATRF (Arafura Timor Research Facility).
There is no formal territory-wide co-ordination of the management of mangroves and tidal wetlands, although the Northern Territory Government (NTG) has primary responsibility for relevant coastal planning and management. Whilst there are relatively low pressures compared with other States, a number of key management initiatives have been implemented. Legislation relevant to the Northern Territory coastal zone is outlined in coastal management policy, and includes over 35 Acts. Key policy and legislation to protect mangroves includes: the Northern Territory Coastal Management Policy Implementation Strategy 2001 (Lands, Planning and Environment, NTG); and the Mangrove Management in the Northern Territory 2002 (Department of Infrastructure, Planning and Environment, NTG). In 2003, 465 km2 of mangroves and associated habitats around Darwin Harbour were zoned conservation areas under an amendment to the Northern Territory Planning Scheme. This includes 95.6% of mangroves in the harbour. Key agencies include: DNRETA; EPA (Environmental Protection Agency); DPI (Department of Planning and Infrastructure); DPIFM; Northern Land Council; Tiwi Land Council; Anindilyakwa Land Council. A number of international conventions and treaties apply to coastal areas, as do some local government activities and legislation.