The South-east Marine Region
In part five of our coverage of Australia’s marine parks, Chris Whitelaw shines a light on the South-east Marine Region.
The South-east Marine Region spans almost two million square kilometres of Commonwealth waters from Bermagui on the far south coast of New South Wales to Cape Jervis near Kangaroo Island in South Australia. It includes Bass Strait and the Southern Ocean around Tasmania and Macquarie Island. The Region extends seaward 200 nautical miles (370km) from the outer edge of state waters to the limit of Australia’s Exclusive Economic Zone (EEZ), and beyond areas of ‘extended continental shelf’ claimed by Australia under the United Nations Convention on the Law of the Sea (UNCLOS).
In 2007, the Region was the first to be proclaimed in the Australian Network and contains 14 marine parks in itself, covering more than 388,000sqkm — Apollo, Beagle, Boags, East Gippsland, Flinders, Franklin, Freycinet, Huon, Macquarie Island, Murray, Nelson, South Tasman Rise, Tasman Fracture, and Zeehan.
THE PHYSICAL ENVIRONMENT
The Region’s physical landscape was shaped by tumultuous geological events that occurred over hundreds of millions of years, including the break-up of the great supercontinent Gondwana and the movement of oceanic and continental tectonic plates. A dominant feature of the south-east shelf is the Bass Strait, formed by a shallow depression about 250km wide linking the Australian mainland and Tasmania, with islands at its eastern and western margins.
Outside the Strait, the continental shelf is about 200km wide across the Lacepede Shelf offshore from the mouth of the Murray River, but elsewhere is generally narrow — especially around the coast of Tasmania and southern New South Wales. Multibeam sonar surveys by Geoscience Australia have revealed the Region’s complex seafloor terrain in spectacular detail, including rocky reefs, deep canyons, and steep escarpments dropping to abyssal plains interrupted by undersea mountains and towering ridges. Three major ocean currents — the East Australian, Zeehan and Antarctic Circumpolar Currents — integrate subtropical and subpolar waters that transport nutrients for food production and determine the distribution of species.
BIODIVERSITY
The significant variation in seafloor features, water depth (40–6000m), salinity, and temperature all contribute to a diversity of marine life that is remarkable by global standards. Marine mammals are a notable feature of the Region’s fauna, which includes 24 whale species, seven dolphin and seal species, and sea lions. More than 20 species of migratory seabirds reproduce and forage here. The Region is also renowned for a large number of endemic species found nowhere else in the world; more than 60 per cent of the Region’s marine plants are found only in these waters. Its fish fauna includes around 600 species of which 85 per cent are endemic. Among other groups of animals, up to 95 per cent of molluscs and 90 per cent of echinoderms are also considered endemic to the waters of southern temperate Australia.
Eight ecological features of the South-east marine environment have been identified as crucial in promoting this biodiversity.
Seamounts off South-east Tasmania
In deep water extending from east of Flinders Island to the South Tasman Rise, 550km south-east of Hobart, the abyssal plain is dotted with clusters of extinct volcanic peaks called seamounts.Each group lies within a marine park specifically designed to protect it.
Flinders Marine Park (27,000sqkm) stretches from the outer shelf off Banks Strait to the edge of Australia’s EEZ on the Tasman Plain, where the seabed lies at a depth of 6000m. Freycinet Marine Park (58,000sqkm) begins offshore from Bicheno and reaches directly east to similar depths far into the Tasman Sea. Huon Marine Park (10,000sqkm), only 170km south of Hobart, spans the outer continental shelf and slopes down to 3000m, embracing the largest cluster (120) of underwater peaks in Australia. The South Tasman Rise Marine Park lies far offshore in the Southern Ocean and covers the sunken remnant of the landbridge that connected Tasmania to Antarctica until, about 100 million years ago, the two land masses were torn apart as the Australian continent moved north.
The spectacular topography of the seafloor off Tasmania’s east coast was revealed for the first time in 2015 when scientists from the Australian National University carried out detailed mapping during a 25-day research voyage on the CSIRO research vessel Investigator. Three-dimensional bathymetry shows numerous undersea mountains, in rounded clusters or elongated chains, rising from the abyssal plain to heights of between 2000–4000 metres. The seamounts vary in size and shape, some with sharp peaks, others wide flat plateaus dotted with small conical hills, though all were formed by volcanic activity during the last 40 million years.
Acting as obstacles to deep ocean currents, the seamounts intensify their flow and generate turbulence that sweeps the summits and slopes clear of sediment, exposing rocks for colonisation by stony corals and bottom-dwelling filter-feeders. Sustained by nutrient-rich waters that swirl up from the seabed, more than 850 species of marine life inhabit the seamounts. About a third of these are new to science and are thought to live only in these deep sea environments. These submarine mountains are also important habitats for sharks and deep sea fish, providing foraging waypoints and navigational aids for migratory whales.
West Tasmanian Canyons
The edge of the narrow continental shelf from eastern South Australia along the west coast of Tasmania to Macquarie Harbour is scored by numerous submarine canyons covering over 35,000sqkm. Beginning at the shelf at a depth of around 300m, the canyons carve steep-sided valleys for about 60km as they descend to 3500m near the base of the continental rise.
Acting as sinks for organic sediments and debris that support spectacular communities of diverse seabed organisms such as bryozoans, nudibranchs, brittle stars, sponges, lace corals, and giant crabs, the canyons are also foraging and nursery grounds for variety of commercial fish species.
The Murray Canyon off the South Australian coast is considered one of the most spectacular geological formations on Australia’s continental margin. Stretching more than 150km and plummeting to 4600m, the submarine gorge is so enormous it could accommodate America’s Grand Canyon. It is encompassed by the Murray Marine Park (25,800sqkm), which itself extends from SA state waters to the limit of the EEZ.
The Zeehan Marine Park (20,000sqkm)straddles the continental shelf to the abyssal plain at 3000m. Featuring a series of four submarine canyons interacting with the south-flowing Zeehan Current, it creates upwellings of nutrients that support foraging grounds for many seabirds and great white sharks.
The Tasman Fracture Marine Park lies off the south-west corner of Tasmania, complementing the Tasmanian Port Davey Marine Reserve. At 42,500sqkm, the Commonwealth reserve is the second largest in the South-east Region and encloses a complex array of geological features including deep canyons, steep escarpments, seamounts, and part of a plateau that is more than 400km long and rises 3000m above the seafloor.
Big Horseshoe Canyon
The Gippsland Basin occupies a large section of the continental shelf south of Victoria’s Gippsland Coast. At its eastern end, the Basin descends rapidly into a complex system of submarine valleys — arguably the largest and most spectacular of its kind on Australia’s cool temperate margin. Dominated by the massive Bass Canyon, a southeast-trending funnel-shaped chasm 80km long bounded by sheer rock walls up to 1000m high, the canyon floor slopes from 1500–4000m and debouches through a 15km-wide mouth onto the abyssal plain beneath the Tasman Sea.
The edge of the continental slope surrounding the head of the chasm is deeply incised by numerous smaller valleys and tributary canyons, the most easterly of which is the Big Horseshoe Canyon. Enclosing a steep, rocky area of 319sqkm at depths ranging from 120–1500m, it protects habitats for a wide array of bottom-dwellers. The Canyon’s hard seabed provides an ideal habitat for dense beds of sponges, soft corals and sea fans that are inhabited by starfish, brittle stars, sea urchins, and sea cucumbers. They also provide refuges for commercially important fish like pink ling.
Bonney Coast Upwelling
The ‘Bonney Coast’, from Portland (Victoria) to Robe (South Australia), is the location of one of the largest, most predictable upwellings of deep ocean water in southern Australia. Driven by strong south-easterly winds between November and April, a vast plume of cool nutrient-rich water is funnelled by a series of large submarine canyons from the abyssal plain onto the narrow continental shelf.
This sustained phenomenon introduces large amounts of plankton into the sunlit nearshore waters to feed dense swarms of krill, which in turn attract schools of small pelagic fish and a succession of predators ranging from southern bluefin tuna, penguins, and seabirds to fur seals and sharks. Fifty species of fish are targeted by commercial fisheries that operate along the coast.
The Bonney Upwelling also sustains an internationally significant feeding area for large aggregations of migratory blue whales. It is the largest foraging ground for this species in south-east Australia and one of only twelve areas in the world where they congregate in high numbers.
Hunted to near extinction during the last century, blue whales are listed as an endangered species under Australian law and are protected by international treaties.
Bass Cascade and Upwelling East of Eden
Currents within Bass Strait flow mainly eastward, increasing in winter with the strengthening Zeehan Current. Wind blowing over the surface of the Strait cools the water, which becomes heavier and sinks towards the shallow bottom. At the eastern end of the Strait, this stream of dense, cold water forms the ‘Bass Cascade’, an underwater waterfall that plunges off the edge of the continental shelf to depths approaching 5000m and sweeps northeast under the warmer Tasman Sea.
During its northward progression, the current mixes with nutrient-rich sub-Antarctic waters, which are concentrated and displaced upwards. As the currents pass the New South Wales-Victoria border at Cape Howe, they enter the East Gippsland Marine Park (4167sqkm) which straddles deep waters adjacent to a shelf break sculpted by submarine canyons, escarpments, and a knoll that juts out from the base of the continental slope. Here the south-flowing East Australian Current delivers warm subtropical water, and the interaction of all the currents with the rugged shelf terrain produces dynamic eddies and huge upwellings of algae and plankton towards the sunlit surface.
These surface blooms attract masses of krill and pelagic fish that drive the food chain for higher order species, such as gemfish, yellowfin tuna, marine mammals, and sharks. The area east of Eden is a known feeding ground for migratory blue whales and humpbacks and is an important foraging habitat for many oceanic seabirds.
Subtropical Convergence Zone
Oceanography in the South-east Marine Region is strongly influenced by the Southern Ocean, in which the Antarctic Circumpolar Current (ACC) — the strongest ocean current in the world — flows clockwise from west to east around Antarctica. The ACC is divided into several branches separated by ‘fronts’, and the area where the most northerly front crosses the Tasman Sea east of Tasmania is called the Subtropical Convergence Zone.
Within this zone, warm subtropical waters brought south by the East Australian Current (EAC) mix with colder nutrient-rich sub-polar waters driven by westerly winds. Biological productivity in the zone depends on the concentration of chlorophyll and nutrients in the mixture, which varies with the seasons. In summer, the EAC is strongest, pushing the zone south below the east coast of Tasmania, reducing productivity. As the EAC wanes during winter, the zone moves north with a corresponding increase in productivity in spring and, to a lesser extent, in autumn.
At their peak in the higher latitudes, eddies of nutrient-rich water fuel blooms of light-sensitive phytoplankton that feed swarms of krill, the diet of small pelagic fish and their predators up the food chain: mackerel, tuna, barracuda, seals, orcas and toothed whales. Flinders Marine Park, north-east of St Helens, and Freycinet Marine Park, west of Bicheno — both areas of high productivity — are important foraging areas for oceanic seabirds, killer whales, and southern right whales.
Rocky Reef and Hard Substrates
Rocky reefs and hard seabed structures are scattered over the continental shelf, Bass Strait and around the Tasmanian coast. In Commonwealth waters, they occur in depths between 50 and 220m, often only a few metres high, and sometimes corresponding with the ancient coastline. The submerged reefs, caves, crevices, and overhangs present a range of habitats for spectacular marine life, richer in species than similar habitats elsewhere in the world. Of the 4500 species of red algae known to science, at least 800 occur in the South-east Region, and as many as 90 per cent of the Region’s invertebrate groups are found nowhere else. The hard substrates are attached by kelp, sponges, bryozoans, and soft corals creating underwater gardens for the benefit of crabs, rock lobster, abalone, sea stars, and octopuses.
MACQUARIE ISLAND WORLD HERITAGE AREA
Macquarie Island lies deep in the sub-polar waters of the Southern Ocean, about 1500km south-east of Tasmania, geographically closer to Antarctica and New Zealand than it is to Australia. Straddling latitudes known as the ‘Furious Fifties’ and exposed to howling winds and stormy seas, this remote outpost comprises one of the most inhospitable and unpredictable oceanic environments in the world. In 1997, the island was inscribed on the UNESCO World Heritage List for its unique geological and natural features.
34km long and 5.5km wide, the Island comprises steep-sided plateaus at its north and south ends, joined by a low, narrow isthmus spanning about 128sqkm. Its highest points are Mount Elder (385m) and Mount Hamilton (430m).
These terrestrial formations are the exposed crest of the undersea Macquarie Ridge, running 1600km north along the junction of the Pacific and Australian oceanic plates towards New Zealand. On both sides of the ridge are trenches — the deepest of which is the Hjort Trench, plunging to around 6700 metres. The ridge is the product of plate tectonics and geological processes that began 10 million years ago and still continue, gradually squeezing Macquarie Island above the surface of the ocean. The island is unique for being composed entirely of uplifted oceanic crust and the only location in the world where this has occurred in a major oceanic basin. It is also one of the few geological features on the planet that presents a major barrier to the Circumpolar Current as it sweeps around Antarctica.
Macquarie Island, adjacent islets, and all surrounding waters out to three nautical miles form Tasmanian Nature Reserve. Most of the waters out to 200 nautical miles east of the reserve are within the Commonwealth Macquarie Island Marine Park, managed by the Australian Government in cooperation with the Tasmanian Parks and Wildlife Service.
Extending in a quadrant east and south of the island, the Commonwealth reserve covers 162,000sqkm to depths greater than 6000m and is the largest marine protected area in the Region. It contains a Sanctuary Zone that provides the highest level of protection for birds and other marine life, where scientific research is the only human activity. Either side of this are Habitat Protection Zones that preserve critical feeding grounds for penguins, fur seals, and migratory seabirds. The only commercial activity conducted in the habitat zones is a trawl fishery for Patagonian toothfish, operated by a single boat subject to stringent controls and catch limits set by the Australian Fisheries Management Authority.
Macquarie Island’s World Heritage listing is international recognition of its status as one of the largest breeding habitats for sub-Antarctic marine life in the world, critical to the survival of many species. Despite harbouring fewer species than other parts of the South-east Region, the abundance of animals that congregate here is staggering.
Each year the island supports around 3.5 million seabirds of 38 species. The majority of these are penguins — gentoos, rockhoppers, kings, and endemic royals — that gather in vast colonies. The breeding population of royal penguins alone is estimated at over 850,000 pairs during their annual nesting season. Four of the world's albatross species nest on rugged cliffs and forage in the surrounding waters. Macquarie Island is an important habitat for five species of seals, including elephant seals that form impressive colonies numbering up to 80,000 on beaches during the breeding season. Other marine mammals include right, sperm, and pilot whales, with orcas commonly seen preying on young seals.
Since 1948, the Australian Antarctic Division (AAD) has maintained the Macquarie Island Station (known as ‘Macca’), housing 20 to 40 scientists and support personnel engaged in research activities focussed on biology, geology, auroral physics, and meteorology. Shipping in the region is minimal and consists of vessels resupplying the station and tourism. Entry to Macquarie Island is by permit only.
For more information about the marine parks in the South-east Marine Region, visit: parksaustralia.gov.au/marine/parks/south-east.