vrijdag 14 juni 2013

Australia, Shark Bay, Western Australia

At the most westerly point of the Australian continent, Shark Bay, with its islands and the land surrounding it, has three exceptional natural features: its vast sea-grass beds, which are the largest (4,800 km2) and richest in the world; its dugong (‘sea cow’) population; and its stromatolites (colonies of algae which form hard, dome-shaped deposits and are among the oldest forms of life on earth). Shark Bay is also home to five species of endangered mammals.


At the most westerly point of the Australian continent, Shark Bay, with its remarkable coastal scenery and islands, has three exceptional natural features: its vast seagrass beds, which are the largest (4,800 km2) and most species-rich in the world; its dugong population (estimated at 11,000); and its stromatolites (colonies of algae that form hard, dome-shaped deposits and are among the oldest forms of life on Earth).
The inland terrestrial landscape of Shark Bay is predominantly one of low rolling hills interspersed with birridas inland saltpans. Shark Bay itself is a large shallow embayment, approximately 13,000 km2 in area with an average depth of 9m, enclosed by a series of islands. Influx of oceanic water is through channels: Naturaliste Channel in the north and South Passage in the south.
The outstanding feature of the bay is the steep gradient in salinities. It ranges from oceanic in the northern and western parts of the bay through metahaline to hypersaline. The salinity gradient has created three biotic zones that have a marked influence on the distribution of marine organisms within the bay.
For almost 3,000 million years (i.e. 85% of the history of life) only microbes populated the Earth. The only macroscopic evidence of their activities is preserved by stromatolites, which reached their greatest diversity 850 million years ago. The stromatolites encrypt evidence of the biology of the microbial communities that created them and the nature of the environments in which they grew. They dominated the shallow seas and formed extensive reef tracts rivaling those of modern coral reefs.
Although microbes have not declined in importance, their activity in building organo-sedimentary structures has, it being more efficient to occupy niches in reefs constructed by faster growing organisms, or indeed to occupy positions within the organisms themselves. Consequently stromatolites and other microbialites have declined in importance over this period, although they have remained locally significant in environments such as Hamelin Pool in Shark bay , where biotic diversity has been limited for one reason or another. The stromatolites and microbial mats of Hamelin Pool were the first modern, living examples to be recognized as comparable to those that inhabited the early seas.
Modern day analogues such as occur in great diversity and abundance in Hamelin Pool
greatly assist in the understanding of the nature and evolution of the Earth's biosphere until the early Cambrian. The Hamelin Pool stromatolites are considered to be a 'classic site' for the study and classification of stromatolitic microbiolites, as the morphology and biology of diverse living types can be studied through a range of environments.
The Shark Bay region is an area of major zoological importance, primarily due to the isolation habitats on peninsulas and islands being isolated from the disturbance that has occurred elsewhere. Of the 26 species of endangered Australian mammals, five are found on Bernier and Dorre Islands. These are the boodie (burrowing bettong), rufous hare-wallaby, banded hare-wallaby, the Shark Bay mouse and the western barred bandicoot. The Shark Bay region has a rich avifauna with over 230 species, or 35%, of Australia's bird species having been recorded. The site is renowned for its marine fauna, the population of about 11,000 dugong, for example, is one of the largest in the world. Humpback and southern right whales use the bay as a migratory staging post. Bottlenose dolphin occur in the bay, and green turtle and loggerhead turtle nest on the beaches. Large numbers of sharks including bay whaler, tiger shark and hammerhead are readily observed. There is also an abundant population of rays, including the manta ray.
The record of aboriginal occupation of Shark Bay extends to 22,000 years BP. At that time most of the area was dry land, rising sea levels flooding Shark Bay between 8000 BP and 6000 BP. A considerable number of aboriginal midden sites have been found, especially on Peron Peninsula and Dirk Hartog Island which provide evidence of some of the foods gathered from the waters and nearby land areas. Shark Bay was named by the English buccaneer William Dampier in the late 17th century. It is the site of the first recorded European landing in Western Australia, with the visit of Dirk Hartog in 1616, followed by William Dampier in 1699.

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