Good-quality habitat is essential to support species populations and to allow natural ecological processes to operate. Habitat quality is defined using structural and functional intactness, relative to the conditions at the time of European settlement of Australia.
This section reports at the national level on our best understanding of the status and trends of marine habitat quality in 21 types of habitats that occur broadly and in more than one region, and 60 habitat types that occur principally in only one region.
The habitats of the south-west region are overall in good condition. There are, however, a number of localised coastal areas of historical heavy impact where the effects remain—these include pollution and dredging of seagrass beds in Cockburn Sound, Perth; pollution-induced losses of seagrasses in Gulf St Vincent, Adelaide; and pollution of Albany harbours in Western Australia. Away from areas of coastal development or river run-off, many habitats remain in good condition. Water conditions overall are very good, particularly away from the shoreline. Conditions of habitats of the estuaries and lagoons of this region are considered overall to be very poor.
Seagrass beds are a dominant habitat in the south-west of Australia, occurring in many intertidal and subtidal areas of coastal waters and estuaries, and in offshore locations down to 50 metres depth. Seagrasses provide important habitat for many fish and invertebrate species, and they host important parts of the lifecycle of a number of fished species. Although two species of seagrass (Posidonia sinuosa and P. australis) are considered threatened or near-threatened with extinction,29 in most places in this region seagrasses are in very good condition.
The habitats of the north-west region are overall in very good condition. Much of this region is very remote (particularly the north) and, as a result, many habitats are considered to be very good and in nearly pristine condition. These include the large gulfs and bays, fringing coral reefs, and seagrass and algal bed systems of the Kimberley, and most of the offshore shoals and islands, canyons and shelf-break ecosystems of the region. Some of the world’s most extensive undisturbed tropical and subtropical habitats occur in the shallow waters of the Kimberley, Ningaloo Reef, Roebuck Bay and Shark Bay. Nonetheless, there are localised areas where the habitats are in very poor condition, such as near Dampier, Port Hedland and Onslow, where ports and shipping activities have heavily impacted coral and mangrove habitats. Offshore habitats are generally in good condition, although the deepwater corals and sponges of the North West Shelf are still considered to be heavily degraded and only slowly recovering from the extensive impacts of historical trawling, and some offshore islands have been heavily impacted by foreign fishing.
In the Kimberley, there are 343 islands with more than 20 hectares of land above mean high water, and many more smaller islands.30 Almost all the islands have fringing reef systems of complex hard coral and algal (rhodolith) habitats. Most of these are remote from human influences and in very good and near-pristine condition.
Like the north-west, the habitats of the north region are also remote and pristine tropical habitats, and most are considered to be in very good condition. These include the nearshore shallow-water marine systems, the extensive shoreline wetlands, and the bays and gulfs of the region. However, the pressures of coastal development are evident in some areas, such as Darwin Harbour and Melville Bay (Nhulunbuy, Northern Territory), where a localised, biologically dead area has been created by mining wastes. Most of the rivers are substantially unmodified. Exceptions are the Ord River, which is heavily modified by the Ord River Dam, resulting in substantial impacts on the estuarine habitats of the delta in Cambridge Gulf; and the Macarthur River, which is modified by mining.
The east region includes the Great Barrier Reef, Torres Strait, the Coral Sea plateau and islands, Fraser Island, Sydney Harbour, Jervis Bay, and the many smaller islands, bays and estuaries of the New South Wales coast. Habitats of the northern part of the region are considered to be in good condition overall, despite considerable pressure from land-based sources of pollution. The Great Barrier Reef region has been considered in detail, and a condition assessment is presented in the Great Barrier Reef outlook report 2009.31 However, the habitats of the central and southern part of the region are more degraded, and many are considered to be poor. This is mainly the result of population pressures in coastal areas (such as in south-east Queensland and northern New South Wales), beach modifications, loss of major areas of seagrass and corals, historical effects of heavy trawling on the continental shelf, and major modification of rivers, some of which (such as the Tweed River) have significantly modified catchments for agriculture and altered freshwater flow regimes feeding to the estuaries and bays. Herbicides have been found in all water sampling sites in the inshore waters of the Great Barrier Reef and, in some places, are approaching levels that may have significant impacts on coral and other marine life.32 In New South Wales, the seagrass Posidonia australis is proposed to be declared as an endangered species in six areas where it formerly occurred widely, because of various impacts (such as dredging and pollution) over the past decades.33
The overall quality of habitat in the south-east region is poor; the pressures of population, shipping, fishing and development in many places have degraded habitats of inshore waters, bays and estuaries. This is the only region where a habitat type has been made functionally extinct by human activities—the oyster reef beds that formerly dominated a number of the estuaries and small bays were exterminated by mining and fishing practices by the end of the 1800s. Seven of the 11 formerly existing oyster reefs assessed are functionally extinct, while the remaining 4 were assessed as having more than 90% of their area lost.34 This has had a significant impact on ecological systems, reducing habitat for many other species and probably greatly affecting the overall water filtering (purification) capacity of these affected areas and their capacity to assimilate nutrient inputs.