The Coral Sea Marine Region

Encompassing the Great Barrier Reef and other globally unique sea environments, the Coral Sea Marine Region is a delicate and precious place

The Coral Sea Marine Region laps 2000km of the Queensland coast from Cape York to Bundaberg. It encompasses a vast area of remote ocean to the edge of Australia’s Exclusive Economic Zone (EEZ) that reaches to 1300km offshore from Mackay. The Coral Sea itself extends much further, to Papua New Guinea and the Solomon Islands in the north and Vanuatu and New Caledonia in the east. 

Most of the Region overlies a continental slope about 1000m deep, descending to over 5000m in some places. Its underwater seascapes include deep sea plains and canyons, plateaus and volcanic seamounts, and the world’s largest coral reef system, the World Heritage-listed Great Barrier Reef. Hundreds of islands, coral cays and sandy islets rise up to 5m above the ocean’s surface and provide terrestrial platforms for unique plants, as well as seabirds and marine turtles that use them for resting, feeding and rookeries. 

MARINE PARKS AND THEIR MANAGEMENT

All this natural splendour is protected by the Commonwealth Great Barrier Reef and Coral Sea Marine Parks, and the Queensland Great Barrier Reef Coast Marine Park, which jointly cover most of the Region. 

The Great Barrier Reef (GBR) is one of the world’s most extraordinary and complex natural wonders. More than 2300km long and up to 250km wide, the GBR system comprises some 3000 coral reefs, 600 islands and 300 sand cays. Not only conserved by the Commonwealth marine park, it lies within a UNESCO World Heritage Area inscribed in 1981.

Inshore of the GBR, the Queensland Great Barrier Reef Coast Marine Park runs the length of the coast from Baffle Creek (north of Bundaberg) to Cape York, enclosing state waters, estuaries, tidal lands and several offshore islands that are declared national parks. Seaward of the GBR, the Commonwealth Coral Sea Marine Park stretches to the outer limit of the EEZ. Spanning just under a million square kilometres, it is one of the world’s largest marine protected areas and protects about 90 separate reefs and cays with a combined area of 15,000 square kilometres.

The Region’s vast size, overlapping State and Federal jurisdictions and complex array of applicable legislation all demand a high degree of cooperation between Australian and Queensland government agencies in the management of these protected areas. This close working relationship is endorsed in the Great Barrier Reef Intergovernmental Agreement 2015.

The Great Barrier Reef Marine Park Authority is the primary Australian Government agency responsible for managing the GBR, supported by the Queensland Parks and Wildlife Service, which also manages state marine parks and national parks. Parks Australia manages the adjacent Coral Sea Marine Park, while the Australian Department of the Environment and Energy coordinates reporting to UNESCO on the state of the World Heritage Area. 

All activities within the marine parks — tourism, fishing, recreation, traditional use, research, defence, shipping and ports — are carefully managed through zoning, management plans, permits and incentives such as eco-tourism certification. Queensland zoning regulations generally match their Commonwealth counterparts, both having conservation as the primary objective.

ANCIENT HISTORY

The Region’s submarine landscape began taking shape about 110 million years ago with the rifting and separation of Australia’s eastern continental crust and spreading of the seafloor. Then, about 60 million years ago, tectonic forces raised Queensland’s continental shelf and caused offshore parts to buckle and subside, forming ridges, plateaus, troughs and the deepest part, the Coral Sea Basin. These forces also generated volcanic eruptions that spawned underwater seamounts and basalt flows that became islands.

Significant geological processes continue to affect the region, with the Australian plate moving northwards by an estimated 7cm each year and the continental crust subsiding in its wake. These plate tectonics produce a lot of seismic activity. From 1866 to 2000, several hundred earthquakes of magnitude between 2 and 6 were recorded along the Queensland coast and in the Coral Sea. In 2007, the Solomon Islands were struck by an earthquake measuring 8.1, followed by scores of aftershocks and a tsunami that killed 52 people and destroyed more than 900 homes.

EVOLUTION OF THE CORAL SEA REEFS

Corals can develop rapidly, expanding by up to 3cm and growing vertically by as much as 25cm in a year. But they only grow above a depth of 150m because they need sunlight. Until about 25 million years ago, much of the Coral Sea was south of the tropics and its temperate waters were too cool to support coral growth. As the Coral Sea Basin edged into tropical waters, some began to grow, only to be stymied by sedimentation and turbidity from land-based erosion.

The build-up of sediment formed a coastal plain along the continental margin that became the foundation for coral communities after being submerged by rising seas. Significant fluctuations in climate, sea level and water temperature saw reefs grow and die, forming hard substrates for later generations of coral when favourable conditions returned. Scientists have found coral ‘skeleton’ deposits dating back 600,000 years. Since then, there have been at least six phases of reef growth when interglacial sea-levels inundated the continental shelf, punctuated by periods of emergence when sea-levels fell during the ice ages. 

The current, living reef structure began growing on top of older limestone platforms, relics of previous growth phases, about 20,000 years ago when the sea level was 120m lower than it is today. As sea levels rose, the corals grew higher on the newly submerged coastal margins and formed atolls on top of sunken seamounts and islands. The modern reefs are estimated to be only 6000 years old, relatively young in geological terms. 

THE MARINE ENVIRONMENT

The Coral Sea basks in a tropical climate, with temperatures typically 18–27 degrees throughout the year and rain between 1000–3000mm, falling mainly between December and March. Prevailing winds vary with the seasons: south-easterly trade winds dominate from April to September but swing to the north-west with the summer monsoons.

One or two cyclones may occur during the summer months and as late as April in latitudes above 10°S. These tend to be less frequent during El Niño events, occuring more often under the influence of La Niña. Since 2014, six tropical cyclones have impacted the Queensland coast, five of them severe (Category 3 or above), affecting 68 per cent of the Region’s reefs. Cyclone-force winds shape islands and coastlines, drive waves and currents, and influence the pathways and distribution of marine life across the Region. Waves generated by cyclones can damage reef structure, destroy mangrove forests and cause erosion on islands, all of which affect fish and coral biomass.

Three major ocean currents dominate the Coral Sea: the South Equatorial Current (SEC), the Hiri Current and the East Australian Current (EAC). The westward-flowing SEC enters the Coral Sea in a series of streams between the Solomon Islands, Vanuatu and New Caledonia and cuts through the centre of the marine park between latitudes 13 and 22 degrees south. 

On meeting the Great Barrier Reef, the SEC splits into the Hiri Current, which branches north to form a slow clockwise gyre in the Gulf of Papua, and the EAC, which flows south in a series of eddies before being pushed east by the Tasman Front off northern New South Wales. Generally, the Hiri and East Australian Currents are of similar strength, but this changes seasonally, as the bifurcation point moves north in winter and south in summer, and between years under the influence of El Niño and La Niña.

The currents significantly influence ecosystem functioning and biodiversity throughout the marine parks. The SEC creates a barrier that reduces the mixing of species, with biological communities in the north being quite distinct from those in the south. Within these separate bioregions, currents form slow eddies around the submarine plateaus, raising nutrient-rich deep-sea water to enhance productivity and dispersing species among the different habitats. At the region’s southern margin, a semi-permanent ocean gyre mixes warm Coral Sea waters with the cooler Tasman Sea, creating an area of high biodiversity off Fraser Island that includes both tropical and temperate species at the limit of their respective ranges.

While Coral Sea waters are mostly very clear, with the visibility of about 30m, inshore water quality is adversely affected by land-based run-off of sediment and pollutants from adjacent catchments. Historically, the surface water temperature is stable in the region’s north at around 27 to 28 degrees throughout the year but varies in the south from 19 to 24 degrees. However, climate change has driven substantial increases in average sea surface temperatures across most of the region since 2012, with present day reef waters almost one degree warmer than when records began. 

Most marine animals are cold-blooded ectotherms, whose body temperatures are determined by the external environment. Changing sea temperatures directly impact their physiology, growth and productivity, as well as influencing their distribution as they move to areas where they feel most comfortable. But many organisms are anchored to the reefs and lack the option of mobility. Due to their narrow thermal tolerance, corals are particularly sensitive to rising sea temperatures and consecutive marine heatwaves since 2016 have produced back-to-back bleaching and mortality over large sections of the Great Barrier Reef.

CONTINENTAL SHELF, SLOPE AND TROUGHS

The continental shelf runs along the Region’s coastal margin for about 2000km from the tip of Cape York to Fraser Island, at depths generally less than 200m. The shelf is narrowest and shallowest in the north, where reefs and Halimeda (calcified macroalgae) banks occupy the entire shelf, and becomes wider and deeper towards the south, where reefs are confined to the mid-outer shelf. At its southern end, the shelf narrows again considerably, with several reef complexes along the shelf break separated from the mainland by Curtis Channel.

Along the seaward edge of the shelf the continental slope begins the descent from shallow water to deep ocean basins more than 1000m below. From the central reef north to the Torres Strait, the shelf becomes steeper and deeper and is scored by more than 100 submarine canyons and numerous landslides. As well as providing habitats for deep-water marine life, the canyons funnel upwellings of nutrients and cooler water onto the reef shelf, supporting phytoplankton blooms that enhance productivity and reducing thermal stress that bleaches coral. 

At its base, the slope is defined by a sequence of deep troughs created by the tectonic separation of the Australian continental plate from the Coral Sea Basin. Running from north to south, the Pandora, Bligh, Queensland, Townsville and Cato Troughs form an almost continuous gutter at depths of between 900m and 3500m, and 10km to 200km wide. At the southern end, the Cato Trough links the Coral Sea to the Tasman Basin in a dynamic interaction of nutrient-rich currents that support significant populations of billfish.

LIMESTONE PLATEAUS, ATOLLS AND REEFS

Offshore from the Great Barrier Reef, the trough margin rises to the Eastern, Queensland, and Marion Plateaus, ancient limestone platforms that now support living reefs. Many of the coral atolls are wholly submerged or dry only at low tide, while most of the islets and cays are composed of sand, rocks and coral rubble that rise no higher than five metres above sea level. The atolls vary widely in size, from a few kilometres in diameter to Lihou Reef which, at 2500sq km, is possibly the second largest atoll in the world by area (including lagoon). 

The reefs provide complex habitats for diverse and abundant marine species, many of which are distinct from those found on the GBR and are more akin to species in the western Pacific. Lagoons within the reefs are nursery sites for sharks and predatory fish, while the sandy cays support critical nesting sites for migratory turtles and 14 species of oceanic seabirds, like the red-footed booby, red-tailed tropicbirds and frigatebirds.

Eastern Plateau is the most northerly, covering 31,000sq km at depths of about 1500m. Surrounded by steep-sided troughs, the plateau’s gently convex surface rises to the Eastern Fields Reef, a modern coral platform 45km wide. Two large atoll reefs, Ashmore and Boot, have formed atop limestone platforms that rise from depths of around 1200m to within 100m of the surface. 

The Queensland Plateau lies offshore from Cairns and is the largest at 165,000 square kilometres. Its sides are dissected by numerous submarine canyons, especially in the north where the plateau drops steeply to 4000m on the abyssal plain of the Coral Sea Basin. Ancient reefs have formed broad limestone terraces over half the plateau’s surface, on which emergent reefs and cays have grown at or just below sea level. The largest of these reefs are the Tregrosse and Lihou complexes, each almost 100km long and up to 50km wide, with the smaller Willis, Diana, and Coringa-Herald Reefs to the north-west. On the plateau’s western edge, steep-sided pinnacles rise from more than 2000m to form coral platforms within 10m of the surface, known as Flinders, Bougainville, Holmes, and Osprey Reefs. The only known spawning aggregation of black marlin in the Pacific Ocean occurs near Osprey Reef.

The Marion Plateau is located off the central Queensland coast between Mackay and Rockhampton. The 77,000sq km plateau is roughly triangular in shape and encompasses limestone terraces that were drowned about four million years ago. These carbonate platforms support three major reef systems: Marion Reef, a large circular atoll comprised of three small sand cays and a number of smaller reefs; Saumarez Reef consisting of three main reefs and sand cays in a crescent shape 39km across; and Frederick Reef, which rises 3000m from deep ocean to project from the surface around a semi-enclosed lagoon (Anchorage Sound) of 30 square kilometres. Other reefs on the Marion Plateau include Wreck Reef, named for HMS Porpoise and HMS Cato which both foundered here in 1803; and Cato Reef, which incorporates Cato Island (6m), once claimed as the capital of the Gay and Lesbian Kingdom of the Coral Sea.

TASMANTID SEAMOUNTS

The Tasmantid Seamount Chain is a sequence of underwater mountains extending 2500km along the 155°E meridian from the Coral Sea into the Tasman Basin. Its 16 seamounts originated as volcanoes that were active between 6–40 million years ago as the Australian Plate moved north over a ‘hotspot’ in the mantle beneath the ocean floor. 

Multibeam sonar bathymetry reveals that the seamounts vary considerably in size, structure and sea depth. They generally have four configurations: large, elongated guyots (flat-topped tablemounts) with multiple terraces; rugged mountains with numerous, cross-cutting rifts; conical peaks with summit vents; and low shields surrounded by fields of volcanic cones. The southern seamounts rise from depths of around 4800m to summits within 150m of sea level, while most of those in the north occur in much shallower water and often break the surface to form islands. 

The Coral Sea seamounts begin on the Kenn Plateau about 500km north-east of Fraser Island in depths of less than 2000m, with living reefs on limestone caps. The most northerly is Kenn Reef, a coral atoll consisting of four reefs spread over 40sq km, with an islet known as Observatory Cay that rises 2m above the high water mark. About 100km to the south lies the Wreck seamount, capped by a narrow 25km chain of reefs and small cays over which the sea always breaks. Cato Reef, 100km further south, is part of the Cato Bank which occupies 200sq km across the top of a large shield volcano. 

South of the Kenn Plateau, the Tasmantid Chain continues with the Fraser, Recorder and Moreton seamounts, which rise from the abyssal plain of the Tasman Basin at depths of around 4600m and do not reach sea level. The tallest of these is Fraser at 4060m above the sea floor, nearly twice the height of Mt Kosciusko.

The Coral Sea seamounts contain a wide range of habitats, from deepwater sponge gardens to shallow coral reef systems, that are hotspots of biodiversity. They support large aggregations of fish that include commercial species, such as grouper, snapper, gemfish and blue-eye trevalla. They are also feeding and reproduction grounds for billfish, turtles, oceanic seabirds and marine mammals.

For more information about the Coral Sea Marine Park visit parksaustralia.gov.au/marine/parks/coral-sea.