The North Marine Region
In our continuing marine parks series, we dive into the North Marine Region stretching from Cape York to the NT-WA border
The North Marine Region comprises all Commonwealth waters from the western side of Cape York to the Northern Territory-Western Australian border, and shares international borders with Indonesia, Timor Leste and Papua New Guinea.
The Region includes eight marine parks — Joseph Bonaparte Gulf, Oceanic Shoals, Arafura, Arnhem, Wessel, Limmen, Gulf of Carpentaria, and West Cape York — collectively spanning 157,480 square kilometres. Each park contains at least one of the many unique tropical ecosystems and associated marine life found in the Region. The parks also support tourism, commercial fishing, mining and shipping activities vital to the economies and welfare of remote coastal towns and Indigenous communities.
ANCIENT HISTORY
The physical structure of the North Marine Region is the product of geological changes that began about 10–15 million years ago when the north-moving Australian continent collided with the Pacific Plate. This impact lifted the earth’s crust and generated volcanic activity that led to the creation of New Guinea. Much of the Region’s present seabed topography was shaped over 150,000 years when it was above sea level and exposed to the erosive elements of wind, rain and the passage of rivers across its gently-sloping terrain.
Australia’s continental shelf formed a continuous land bridge with New Guinea enabling the movement of plants and animals between them. As the last Ice Age began to thaw, about 18,000 years ago, rising seas gradually drowned the shelf, connecting the Indian and Pacific Oceans.
The Region’s seafloor now contains a variety of physical features that support distinctive and important ecological communities — a wide, relatively shallow continental shelf with two large basins (Bonaparte Basin and the Gulf of Carpentaria basin), interspersed with submerged coral reefs, carbonate banks and terraces, limestone pinnacles, plateaux, saddles and canyons.
THE GULF OF CARPENTARIA BASIN
The Gulf of Carpentaria is a vast expanse of continental shelf dominated by a relatively featureless basin that varies in depth from 45m to 80m. The soft, sandy-mud sediments of the basin floor are abundantly populated by invertebrates that include echinoderms (notably brittle stars and heart urchins), sponges, corals, molluscs, crustaceans and sea cucumbers. The Gulf’s waters support many pelagic fish species in a food chain ranging from plankton-eating and schooling fish to predators such as sharks, tuna and mackerel.
The Gulf’s semi-enclosed waters mix little with the Arafura and Coral Seas, and strong tidal currents from the Coral Sea are largely blocked by the islands and submerged reefs of Torres Strait. Despite the absence of external influence, the Gulf experiences unusual tidal patterns of between 2m and 4m. In some locations two high and two low tides occur daily, while elsewhere there is little or no tidal variation at all.
Seasonal differences in prevailing winds drive a complex pattern of currents, producing a clockwise gyre north-west of the basin centre.
SUBMERGED CORAL REEFS IN THE GULF
Twenty per cent of the reefs in the North Marine Region are situated within the Gulf of Carpentaria. Most of this reef growth occurred 8,000–10,000 years ago, when sea levels were around 30m lower than at present and were gradually left behind as rapidly rising seas outpaced coral growth.
Today, submerged reefs form a broken margin around the perimeter of the Gulf at depths of 30–50m. They are closest to the coastline along the western rim near Groote Eylandt and furthest offshore in the south-east Gulf. Relict reef structures support a thin, patchy cover of slow growing (1m per 1000 years) live corals at depths of 20–30m. These sustain a host of tropical invertebrates such as sponges, ascidians (sea squirts), gorgonians and anemones, and provide habitats for many fish species, sea snakes, sharks and turtles.
GULF OF CARPENTARIA COASTAL ZONE
The ‘coastal zone’ stretches in an arc from west Cape York Peninsula to Limmen Bight in the south-west Gulf, extending from the shore to a depth contour of 20m. The inshore waters (within 3nm from the coast) and terrestrial margin lie within the jurisdictions of Queensland and the Northern Territory.
The West Cape York Marine Park covers 16,012 square kilometres adjacent to the northern end of Cape York Peninsula, 40km north-west of Weipa. It protects large seagrass meadows that provide year-round grazing for dugongs and marine turtles.
From there to Aurukun, the coastline is a wave-exposed environment of sandy beaches, extensive tidal sandbanks and offshore reefs. The beaches support large breeding colonies of flatback turtles and rookeries of olive ridley turtles. Estuaries and rivers contain sawfish and juvenile sharks that move offshore as they mature, while fringing mangrove forests provide the most significant crocodile breeding habitat in Queensland. Offshore fisheries target mackerel that feed on juvenile prawns emerging from the estuaries at the end of the wet season.
The coast between Aurukun and the Staaten River is characterised by sandy beaches interspersed with small estuaries and mangrove communities bordered by salt marshes and saltpans, where crocodiles and turtles breed in significant numbers.
The region supports commercial harvesting of mud crabs and barramundi, a viable banana prawn fishery and river-based gillnet fisheries. Several large rivers drain into the south-eastern Gulf, depositing large amounts of sediment and delivering nutrients that create a hotspot for banana prawns and pelagic fish.
The estuarine wetlands and mangrove mudflats are habitats for large populations of migratory wading birds, and offshore islands provide important rookeries for seabirds and breeding grounds for green turtles and dugongs.
The large, shallow bay in the south-west Gulf between Groote Eylandt and the Sir Edward Pellew Group is known as the Limmen Bight and incorporates wide, tide-dominated deltas for the Roper, McArthur and Robinson Rivers. Seagrass meadows over the muddy sands inshore of the islands provide rich foraging grounds for dugongs and turtles, and are important nurseries for crabs, prawns and barramundi. The Limmen Marine Park, 315km south-west of Nhulunbuy (NT), contains the Labyrinthian Shoals, a group of sand banks lying less than 2m deep, around which tidal eddies of nutrients attract marine life that is among the most abundant in the North region. The Commonwealth Marine Park lies adjacent to the Northern Territory and Limmen Bight Marine Park, both of which form part of the traditional ‘saltwater country’ of the Marra, Yanyuwa, Alawa and Wandarang Indigenous people.
PLATEAUX AND SADDLE OF THE WELLESLEY ISLANDS
The Wellesley Islands lie in the south-east corner of the Gulf, 90km from Karumba. Extending to their north-west at depths of 15m–30m are two limestone plateaux connected by a narrow saddle. This submarine feature is the only one of its kind in the North Marine Region and forms a unique ecosystem in the Gulf. The broad, flat-topped plateaux have isolated patches of living corals that host reef fish species unique to the region. Large numbers of snapper, cod, emperor and mackerel provide a reliable food source for frigate birds, boobies and shearwaters.
The Gulf of Carpentaria Marine Park covers 23,771 square kilometres adjacent to the Wellesley Islands and is noted for its network of submerged reefs of large plate corals, abundant soft corals and dense sponge gardens. The Lardil, Yangkaal, Kaiadlit and Gangalidda Indigenous people are the traditional owners of the Wellesley Islands and surrounding sea, and the Thuwathu-Bujimulla Indigenous Protected Area contains 160 sites of cultural significance, including the largest collection of stone fish traps in the southern hemisphere.
WESSEL ISLANDS MARINE PARK
The Wessel Islands extend in a straight line from the north-east corner of Arnhem Land and mark the edge of the Arafura Sill, a seafloor barrier that forms a transition zone for marine life, with different species of sponges, corals and fishes on either side. The region’s complex coastline embraces rocky headlands and reefs interspersed with sandy bays that grade rapidly into muddy, offshore sediments. Strong currents swirl among the islands, delivering nutrients that sustain one of the most diverse and species-rich environments in the North Marine Region — the biodiversity of coral reefs here is comparable to that of the Great Barrier Reef.
The Wessel Marine Park covers 5908 square kilometres between the northern tip of the Wessel Islands and Cape Arnhem, 22km north-east of Nhulunbuy. The park protects waypoints for migratory seabirds, rookeries for waterbirds and nesting sites for marine turtles. Inshore seagrass meadows provide nurseries that support the largest tiger prawn fishery in the Gulf. The Yolngu Aboriginal people have strong cultural and physical associations with the islands. Local Indigenous rangers are actively involved in the management of ‘ghost nets’, discarded or abandoned fishing nets that drift at sea to entangle turtles and other marine life and present a serious navigational hazard for vessels.
ARNHEM MARINE PARK
The Arnhem Marine Park lies off the coast of West Arnhem Land, about 100km south-east of Croker Island, and covers 7125 square kilometres at depths ranging between 15m–70m.
Its gently sloping shelf is topped with pinnacles, around which currents and eddies of nutrient-rich tropical waters sustain diverse communities of marine life and attract large numbers of foraging seabirds. The park lies on migration routes of marine turtles and is adjacent to coastal wetlands that are important breeding habitats for seabirds and waders.
THE ARAFURA SYSTEM
Within the North Marine Region, the Arafura System is a vast expanse bordered in the west by the Van Diemen Rise and in the east by the Arafura Sill; the northern boundary is the outer limit of the Exclusive Economic Zone (EEZ). It embraces part of the Arafura Sea, which spans 650,000 square kilometres between Australia and New Guinea. The Arafura Shelf is that part of Australia’s continental shelf that underlies the sea. It is a gently sloping, relatively featureless plain topped by areas of banks, shoals and submerged reefs. The western edge of the shelf descends by a series of terraces to a depth of 3,000m in the Timor Trough, which runs parallel to the island of Timor.
In the north, the shelf slopes down to the Arafura Depression, which is extensively dissected by ‘tributary canyons’ — a network of ridges and shallow valleys that remain from a drowned river system. The largest canyon extends some 400km from the Depression back to Cape Wessel. The base of the Depression is connected to the Timor Trough through the Arafura Fan, at depths of 200–300m. The Fan’s hummocky surface reflects its fluvial origins as sediments deposited at the outflow of the canyons about 2 million years ago.
The widely varied seafloor topography creates many different ecosystems, in which biodiversity relies on deep water upwellings of nutrients driven by strong currents through the canyons, and surface water circulation by seasonal monsoon winds. Benthic (seafloor) surveys have identified at least 245 species of invertebrates, such as sponges, corals, sea anemones, tunicates, worms, crustaceans, brittle stars and feather stars, with a possible 500 further species yet to be identified from samples collected from the area.
Pelagic species that dominate the open sea include red snapper, Spanish mackerel, sawfish, large predatory fish as well as sea snakes. Migratory dugongs, turtles and whale sharks also transit the area.
The Arafura Marine Park has been established over 22,924 square kilometres of the region, enclosing a large section of the shelf and many tributary canyons, ranging in depth from 15m to 500m. Located 250km north-east of Darwin, Arafura is Australia’s most northerly marine park. Its southern reaches lie near the Northern Territory Garig Gunak Barlu Marine Park around the Cobourg Peninsula, whose deeply incised bays and estuaries feature live coral reefs, seagrass, reef and pelagic fish populations, marine turtles and dugongs. Areas of both marine parks are within the traditional sea country of the Yuwurrumu Indigenous people.
OCEANIC SHOALS MARINE PARK
Far out in the Timor Sea, 155km north-west of Darwin, the Oceanic Shoals Marine Park extends to the limit of Australia’s EEZ, covering the edge of the continental shelf adjacent to the maritime boundary with Timor-Leste.
At 71,743 square kilometres, it is the largest marine park in the North Marine Region. Within its boundaries are several unique seafloor features of ecological significance — the pinnacles of the Bonaparte Basin, and the carbonate banks and terraces of the Sahul Shelf and Van Diemen Rise.
The Bonaparte Basin emerges from the Joseph Bonaparte Gulf at the NT-WA border and slopes north to the Timor Trough. In the mid-outer part of the basin, 110 limestone pinnacles 50m high and more than 100km long cover an area of more than 520 square kilometres — the largest concentration of such structures along the Australian margin. They are thought to be the eroded remnants of underlying strata formed about 350 million years ago. Deep currents produce upwellings of nutrient-rich water around the pinnacles that sustain benthic communities of invertebrates, and aggregations of snapper, emperor and grouper. Marine turtles also forage among the pinnacles.
The Sahul Shelf is a shallow north-westerly extension of Australia’s continental shelf, underlying the Timor Sea as far as the Timor Trough. Along the shelf’s eastern margin is a broad limestone terrace known as the Van Diemen Rise, characterised by a complex system of carbonate banks and shoals, each less than 10 square kilometres in area with flat tops and steep sides. They rise from depths of 100m to within 20m of the ocean surface. The banks are separated by sinuous channels (flooded paleo-rivers) up to 150km long and 240m deep, which direct strong currents bearing plankton and other nutrients to food webs in higher waters. The banks and shoals within the marine park make up 80 per cent of such features in the North Marine Region.
Considerable differences in water depth and seafloor bathymetry contribute to a wide range of unique ecosystems and a rich diversity of marine life across the park. All the banks and channels support gardens of sponges and octocorals (sea fans, sea whips and soft corals), with 350 species recorded so far.
The deeper channels provide refuge for a number of fish species, while sharks, mackerel, red snapper and a distinct gene pool of gold band snapper are found in the open ocean. Sea snakes are common. It's also an important foraging ground for threatened marine turtles and the Australian snubfin dolphin, described as a species new to science in 2005.
JOSEPH BONAPARTE GULF MARINE PARK
At the southern margin of the Bonaparte Basin, the Joseph Bonaparte Gulf Marine Park straddles 8597 square kilometres of nearshore waters adjacent to the WA-NT border, 90km from Wyndham. It lies within the traditional sea country of the Miriuwung, Gajerrong, Doolboong, Wardenybeng and Gija and Balangarra Indigenous people.
No deeper than 100m, it is one of the shallowest marine parks in northern Australia. Its seabed is a gently sloping expanse of soft sediment with gravelly shoals, sand banks and localised rocky reefs that rise to within 2m to 3m of the sea surface. The shelf is also incised by valleys and channels up to 40m deep that extend many kilometres offshore, defining the ancestral pathways of large rivers that still drain into the Joseph Bonaparte Gulf. The park’s physical environment is strongly influenced by these rivers which enter the gulf via two major estuaries, one from Western Australia and the other from the Northern Territory. Collectively, they discharge large quantities of freshwater and almost 200 million tonnes of sediment into a well-defined coastal zone at the head of the gulf. A combination of high tidal range (up to 7m), strong currents, monsoonal winds and regular cyclones mix and circulate sediment and nutrients in a turbid and highly productive inshore margin within 10km of the shoreline.
The muddy seafloor and sediments along the edge of channels support communities of crustaceans and echinoderms (sea fans, sea squirts and lace corals) that filter food particles from the murky water, and demersal (bottom-feeding) species such as sawfish, stingray and catfish that hunt for food using their highly sensitive snouts. A very productive banana prawn fishery operates in clearer shelf waters outside the coastal zone, and whales migrate through offshore areas during winter months on their way to calving grounds in the Kimberley.
The nearshore areas and estuaries are the feeding and birthing grounds of several species of dolphin. The park is a Biological Important Area (BIA) for foraging by threatened and migratory marine turtles and supports the highest concentration of nesting flatback turtles in the world. Shorebird colonies are distributed around the coastal wetlands and are considered internationally significant for migratory species in the East Asian Australasian Flyway; the western boundary of the marine park is a BIA for breeding of the lesser crested tern.
The western half of the Joseph Bonaparte Gulf lies within a bioregion known as the North-west Shelf Transition, which forms part of the North-West Marine Region and abuts the adjacent Western Australian North Kimberley Marine Park.
For more information about the marine parks in the North Marine Region visit parksaustralia.gov.au/marine/parks/north.