The Southwest Shelf Transition, also known as the Houtman marine ecoregion, is a biogeographic region of

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...

and coastal waters. It includes the subtropical coastal waters of

Southwest Australia Southwest Australia is a biogeographic region in Western Australia. It includes the Mediterranean-climate area of southwestern Australia, which is home to a diverse and distinctive flora and fauna. The region is also known as the Southwest Aus ...

Geography

The Southwest Shelf Transition includes the coastal waters and continental shelf of the west coast of

Western Australia Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to th ...

, extending from Kalbarri to

Perth Perth is the capital and largest city of the Australian state of Western Australia. It is the fourth most populous city in Australia and Oceania, with a population of 2.1 million (80% of the state) living in Greater Perth in 2020. Perth is ...

, and including the Houtman Abrolhos archipelago. It is a transitional zone between the subtropical Central Western Shelf Province to the north, and the warm temperate

Southwest Shelf Province The Southwest Shelf Province, also known as the Leeuwin marine ecoregion, is a biogeographic region of Australia's continental shelf. It includes the temperate coastal waters of Southwest Australia. Geography The Southwest Shelf Province includes ...

to the south. The warm Leeuwin Current runs southwards along the coast."A Guide to the Integrated Marine and Coastal Regionalisation of Australia Version 4.0". Department of Environment and Heritage, Government of Australia. June 2006. ISBN 0 642 552274. The continental shelf is generally narrow. The coastline is affected by moderate-energy swells from the Indian Ocean. Depth ranges from zero to nearly 300 metres, with a mean depth of 41 metres. Geomorphically the seabed is almost entirely shelf, but there is a small area of scarp at the northern limits. The Houtman Abrolhos Archipelago lies about 60 km from the mainland coast, near the edge of the continental shelf. The archipelago is made up of three groups of low islands. Each group rests on a shallow undersea carbonate platform that includes extensive coral reefs. It is the southernmost coral reef system in the Indian Ocean.

Ecology

Biologically, it is a highly complex transition zone, with many species from other provinces reaching a limit of their range in this bioregion. In particular, it represents a northern limit of a number of warm temperate species, and the southern limit of a number of sub-tropical and tropical species. The Houtman Abrolhos is home to the highest concentration of west coast endemic species.

Meso-scale bioregions

The Integrated Marine and Coastal Regionalisation of Australia (IMCRA) identifies two meso-scale bioregions that make up the transition. * Abrolhos Islands (ABR), consisting of the marine environment immediately surrounding the Houtman Abrolhos *

Central West Coast Central is an adjective usually referring to being in the center of some place or (mathematical) object. Central may also refer to: Directions and generalised locations * Central Africa, a region in the centre of Africa continent, also known as ...

(CWC) which comprises the remaining area.

Delineation

This bioregion was referred to as the South Western Biotone in Integrated Marine and Coastal Regionalisation of Australia (IMCRA) Version 3.3, and as the Southwest IMCRA Transition in the

National Marine Bioregionalisation of Australia National may refer to: Common uses * Nation or country ** Nationality – a ''national'' is a person who is subject to a nation, regardless of whether the person has full rights as a citizen Places in the United States * National, Maryland, ce ...

. Both ''Southwest IMCRA Transition'' and ''Southwest Shelf Transition'' are used in IMCRA Version 4.0.

References

{{reflist Biogeography of Western Australia Ecoregions of Australia Marine ecoregions Southwest Australia IMCRA provincial bioregions Temperate Australasia