The coastal waters of Australia are generally low in nutrients all year round and are not highly productive (exceptions to this are the shallow waters of the tropics and the shallow shelf and gulf systems, such as Torres Strait). This means that the diverse species and ecosystems of these waters are very sensitive to the addition of even small amounts of land-derived or ocean-derived nutrients, and disturbances of the seabed that resuspend nutrients.
The low nutrient status of our waters is a result of the limited penetration of nutrient-rich deep ocean currents into shallow coastal waters where there is enough sunlight to drive primary production (where organisms such as phytoplankton use solar energy to convert carbon dioxide and nutrients into new organic materials). As a result, Australia's oceans do not support a large biomass of fish or dependent predators, as occurs in waters off South Africa and South America. However, in the high density of canyons along the edge of the continental shelf, there are areas that experience periodic small intrusions of deep, cold ocean waters. These canyons and shelf-edge features are therefore small hot spots that are rich in diversity and biomass of invertebrates, fish, and their prey and predators. For example, the line of shelf-edge canyons on the west coast of Australia and their associated production systems are thought to support the ‘whale highway’—the annual migration pathway of humpback whales from the Antarctic to their calving grounds in the warm tropical waters of the Kimberley region. A cold-water upwelling system (where sea water rises from the depths to the surface, typically bringing nutrients to the surface) also regularly occurs along the west coast of Victoria and near South Australia's Kangaroo Island, extending to the Eyre Peninsula.8
Elsewhere, the remoteness, diversity of habitat types and low-nutrient waters have created highly diverse flora and fauna. Many areas have locally endemic (unique to the region) species, and assemblages of low species diversity that are unique and highly ecologically valued. For example, the tropical, subtropical and temperate reefs, shelves, bays and gulfs around the Australian coast are home to a rich diversity of species and ecosystems. At different times of the year, depending on river inputs and coastal run-off, these coastal features can be dominated by turbid and productive waters or, alternatively, by clear waters with low nutrient status. The flora and fauna of these areas are specialised and resilient to such variable coastal conditions. The Australian waters have a high number of endemic species, particularly in the southern regions that are most isolated from other land masses in the Pacific and Indian oceans.9-10
While our knowledge of the distribution and taxonomy of Australia’s marine biodiversity (particularly the invertebrates) remains patchy, the recently conducted Census of Marine Life10 summarised our knowledge of animals from the major marine biodiversity databases. The census found approximately 33 000 marine species that were confirmed to occur in Australian waters; of these, 130 species are introduced, 58 are listed as threatened, and an unknown (but likely to be large) number of species are endemic. Levels of endemism are low in the tropics because many species also occur across the broader Indo–West Pacific region but, in the temperate waters of southern Australia, endemism is likely to be high (possibly up to 90%). Although the taxonomy of marine plant species is reasonably well known, particularly for the 75 species of seagrasses and mangroves, they are not well represented in the national biodiversity database systems that describe their distributions.
The marine animal species confirmed to occur in Australian waters are dominated by molluscs (8525 species), crustaceans (6365) and fish (5184)—a pattern that is consistent across all the world’s oceans.11 A further estimated 17 000 species are likely (reported but not confirmed) to occur in our waters, including the many soft-bodied pelagic and benthic (sea-floor) invertebrate species (such as worms) that play important ecological roles. Crude estimates based on the rate of biological exploration and discovery suggest that the total number of marine species (those known to occur, likely to occur and yet to be discovered) in Australian waters is around 250 000 macroscopic species, and many more if microscopic species are included.