The seabed of Australia's marine jurisdiction is diverse and complex, reflecting the large area it covers and its span from the tropics to the Antarctic, with many coastal and offshore islands and their fringing geomorphic structures. The continental mainland has a coastline of around 36 000 kilometres, and spans more than 5000 kilometres from the tropics (9°S) to temperate latitudes (47°S).
Recent mapping of nearly 50 million square kilometres of the seabed in our region has identified 21 types of geomorphic features. These include major features such as the continental shelf, slope, plateaus and abyssal plain (bottom of the deep ocean at a depth of more than 2000 metres); and smaller features such as basins, terraces, reefs and seamounts (Figure 6.3). The geomorphology at the margin of the continent is the most complex aspect of the region and includes marginal plateaus, terraces, trenches, troughs and submarine canyons. The plateaus along the Australian margin cover around 1.5 million square kilometres—20% of the total world area of marginal ocean plateaus.5 The great diversity of geomorphic structures provides an equivalent diversity of habitat types for animals and plants that live on, or are closely associated with, the seabed.
Broadscale mapping of seabed environments into a series of 'seascapes' has uncovered great diversity in Australia's marine jurisdiction, including aspects of the seabed that may be important for conservation and biodiversity management. These seascapes combine aspects of water depth, sediment type, geomorphology, sea-floor temperature, disturbance by tides and waves, slope of the seabed and amount of primary production. These parameters are related in complex ways to the distribution of biodiversity, so areas where there are many seascapes might also be areas with a high diversity of species or ecological communities.6
In coastal waters, recent studies have revealed the complex interplay between sediment types, local geomorphology and ocean conditions, producing a classification of sediment compartments that describe the structure of coastal seabed systems. The compartments broadly reflect the vulnerability of coastal seabed and beach systems to ocean-driven change, and therefore contribute to regional development planning and conservation activities.