Great Barrier Reef ecosystem

Summary: The Great Barrier Reef is the largest coral reef system in the world, extending more than 1,200 miles (2,000 kilometers) along the northeastern coast of Australia.

Category: Marine and Oceanic Biomes

Geographic Location: Australia

Australia, an island continent with an extensive tropical coastline and 17 percent of the world's total area of reefs, has the world's largest total area of coral reefs after Indonesia. Conditions for reef development vary considerably along the island's coastline. The Great Barrier Reef runs along the northeastern coastline of Australia, extending out to the margins of the continental shelf, from the Warrior Reefs in the northern Torres Strait for well over 1,200 miles (2,000 kilometers) to the Capricorn-Bunker Group of reefs and islands in the south.

This reef system, extending to Papua New Guinea, actually consists of more than 2,500 individual reefs, coral shoals, and high island fringing reefs, which can range in size from scarcely visible to over 39 square miles (101 square kilometers). The shape and structure of the individual reefs are also of great variety. Two main categories have been defined: lying platform or patch reefs, resulting from radial growth; and wall reefs, resulting from elongated growth, often in areas of strong water currents. There are approximately 300 coral cays, including those with vegetation or not, and over 40 low-wooded islands. There are also 618 continental islands that were once part of the mainland.

Continental drift brought the northern coastline of Australia into tropical latitudes about two million years ago, when the Great Barrier Reef originated with some minor development, but widespread reef development is thought to have begun about 500,000 years ago.

Climate and Currents

Complex patterns of currents in Australian waters are key driving forces and contribute to the different ecological areas within the region. The Great Barrier Reef, bounded by the Coral Sea to the east and the Torres Strait to the north, is characterized by the counterclockwise gyre of the South Equatorial current. Cyclones are seasonally common here in this tropical marine climate. Air temperatures vary between an average maximum of approximately 86 degrees F (30 degrees C) in January and 73 degrees F (23 degrees C) in July, and an average minimum of approximately 74 degrees F (24 degrees C) in January and 64 degrees F (18 degrees C) in July. Mean water surface temperature is at a maximum during February and at a minimum during July.

Current patterns across the continental shelf induce localized upwelling and are predominantly driven by prevailing southeast trade winds, northwest monsoons, and patterns of tidal flow.

The continental shelf of Australia's northern shores connects across the relatively shallow waters of the Torres Strait to Papua New Guinea, which loads considerable freshwater and sediments into the strait. The westernmost areas have the shallowest and most turbid waters, and soft mud dominates the shallow surface of the reefs in the area. The easternmost areas of the strait are characterized by fringing corals. There are very extensive platform reefs around Darnley Island, stretching out toward the edge of the continental shelf and a nearly continuous line of reefs. This particular section of reefs marks the northern edge of the outer Great Barrier Reef. The northern section of the Great Barrier Reef is unique, with well-developed ribbon-type barrier reefs on the outer edge and wide areas of shoals, banks, and fringing reefs.

The southernmost reefs of the Great Barrier Reef are known as the Capricorn-Bunker Group and are a little more than 30 miles (50 kilometers) offshore south of the Swain Reefs Complex. At this relatively small reef complex, the continental shelf narrows rapidly, with steeply sloping reef edges, deep waters, shallow lagoons, and well-developed coral cays. The best-known reefs of the entire Great Barrier Reef system are at One Tree Island at the Tropic of Capricorn, whose cooler waters are largely responsible for the lower coral diversities in these islands.

Biodiversity

The Great Barrier Reef has very high levels of biodiversity, with some 1,500 to 2,000 species of fish, more than 4,000 species of mollusks, and 400 coral species. The high number of coral species decreases toward higher latitudes, with the number decreasing to about 240 species in the southern sections. The differences in the higher levels of nutrient loads, sediments, and amounts of freshwater present near the mainland, and the low inputs on the outer reefs, have led to considerable variation in species assemblages in different locations. Fish populations tend to be relatively small, in that the waters of the Great Barrier Reef are generally nutrient-poor. Some nutrients are supplied by Coral Sea surface water and upwellings, and also by terrestrial runoff. Tidal mixing is a major contributor to the nutrient recycling dynamics of this ecoregion.

The Great Barrier Reef serves as a nesting ground for green, hawksbill, loggerhead, and flatback sea turtles. Raine Island and Pandora Cay are the largest breeding grounds for green turtles, whereas islands in the northern section of the area are nesting sites for hawksbill turtles. About twenty-six species of cetaceans are found within the region, with frequent reports of humpback, minke, and killer whales, and of bottlenose, spinner, and Irrawaddy river dolphins. Humpbacks breed along the east coast of Australia in winter, between July and October.

There are some twenty-three species of breeding birds on more than fifty-five major nesting islands in the northern and southern sections of the Great Barrier Reef, with most bird communities reported in the Capricorn-Bunker Group. Raine Island is a major seabird rookery. Saltwater crocodiles are found in mangrove swamps and river estuaries along the region's coastal fringe, but individuals are seen around offshore coral reefs. Giant clams are also common in the Great Barrier Reef.

Seagrass and mangrove communities are notably important in the Great Barrier Reef, as they are breeding and nursing grounds for many reef and marine species. Seagrass beds are important for marine turtles and provide breeding grounds for significant numbers of dugongs. Seagrass beds both shallow and deep—as much as 49 feet (15 meters) below the surface—are widespread, with an estimate of at least 1,930 square miles (5,000 square kilometers) of cover. The largest known populations of dugongs occur in the northern and southern sections of the Great Barrier Reef region, but the southern groups are declining due to an increase in the number of boat collisions and entanglement in gill nets and shark nets placed near swimming beaches. Mangroves, for the most part, are located away from coral reef communities; some mangroves have been recorded on a few fringing reef systems, with the highest levels of diversity north of Cairns.

The Great Barrier Reef is the site of a mass spawning event that takes place once a year for a few nights after a particular full moon in the late austral spring, when the majority of stony corals, sponges, sea cucumbers, marine worms, and giant clams reproduce, releasing eggs and sperm in synchronicity. Occurring globally, mass spawning on coral reefs allows cross-fertilization between colonies and increases the chances of survival of individual larvae.

Human Interaction

The types of fishing activities that are popular in the Great Barrier Reef are recreational fishing of groupers, emperors, lobsters, fish for the aquarium trade, sea cucumbers, and trochus; and trawling of fish, prawns, scallops, and crustaceans. Repetitive trawling and the size of bycatch have become major concerns associated with trawling activities, as 50 percent to 90 percent of hauls typically include unusable species of benthic (deepwater) organisms as well as fish, sea snakes, and turtles. Illegal trawling often occurs in some of the protected reef areas and seagrass communities. Other destructive fishing practices have threatened the health of the Great Barrier Reef, including cyanide fishing, blast fishing, and muro-ami, which involves banging on reefs with sticks or blocks of cement to scare fish from their hiding places into nets.

Reef health varies, depending on the type and location of reefs, local conditions, and the effect of anthropogenic activities. For a long time, the Great Barrier Reef was not heavily affected by human activities, as the majority of reefs are far offshore, but the effects of human activity can be seen on reefs closest to the mainland. Increases in human activities such as deforestation, poor agricultural practices, high concentrations of agricultural chemicals, nutrients in terrestrial runoff, land-based sediments, overgrazing, oil spills, intensive recreational use, and commercial fisheries are of major and immediate concern.

Cyclones, bleaching events, and coral predators such as the crown-of-thorns starfish have devastating effects on reef structure and development. Bleaching—which can be caused by environmental stress and seawater temperature rise—occurs when corals expel zooxanthellae, a unicellular flagellate (that is photosynthetic), with which the coral have a symbiotic relationship. This expulsion causes a bleached or whitened appearance, and can eventually lead to the decline in the health of the coral. In 2016, the Great Barrier Reef region experienced a widespread bleaching event due to substantially warmer ocean temperatures than usual, in large part due to the effects of climate change. Factors such as changes in salinity, decreases in zooplankton (caused by fishing), changes in water temperatures, bacterial infections, and climate change due to global warming are all associated with human activity. Both bleaching events and outbreaks of the crown-of-thorns starfish may be human-induced. The starfish predation of coral is of greatest concern in Australia because of the potential economic effect. The first outbreak of the crown-of-thorns starfish was observed on Green Island, off Cairns, in 1962, but most outbreaks have been recorded in the central sections of the Great Barrier Reef. Evidence suggests that increased soil runoff may aggravate outbreaks and in some instances may cause them. Ocean acidification due to climate change and the increased uptake of carbon dioxide by the world's oceans will increasingly restrict coral growth and survival. Because of higher-than-average temperatures, a survey of the reef conducted in 2022 found that 91 percent of the reef was affected by coral bleaching.

The great majority of Australia's reefs fall within protected areas. Most of the lagoons, and all offshore reefs from the Capricorn-Bunker Group to the northern section of Cape York Peninsula receive legal protection as the Great Barrier Reef Marine Park, managed since 1975 by the federal Great Barrier Reef Marine Park Authority (GBRMPA) and the Queensland Department of Environment and Heritage. The remaining coastal waters and offshore islands fall within other protected areas.

The marine park has a detailed zoning plan of its 133,205 square miles (345,000 square kilometers), providing areas of strict protection alongside much larger areas of multiple use. About 80 percent of its total area is open for general use, including permitted commercial fishing and trawling, and a further 16 percent is open for general use but no trawling. Only about 5 percent is closed to fishing activities, but this area includes more than 120 reefs, and there are 13 fisheries habitat reserves. The reefs of the Torres Strait are outside the jurisdiction of the Great Barrier Reef and thus do not fall under its legal protection, but a fisheries management agreement with Papua New Guinea offers these reefs some kind of loose protection. The Great Barrier Reef has been a World Heritage Site since 1981. In 2021, UNESCO proposed listing the Great Barrier Reef as being "in danger" due to severe coral bleaching caused by unusually high temperatures in 2016, 2017, 2020, and 2022. However, the Australian government was mostly against the listing because it might decrease revenues generated from tourism. Because of the coral bleaching, the reef was also in danger of losing its status as a World Heritage Site. The United Nations planned to study the extent of the damage in 2022, but a meeting to discuss the issue was canceled when Russia invaded Ukraine.

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