The Shark Bay Environment

In Shark Bay's hot, dry climate, evaporation rates are very high.
This causes the waters in its shallow bays to become concentrated with salt.

Seagrass banks play a very important role in maintaining high salt concentrations by restricting tidal flows into and out of Shark Bay. This ensures the open ocean does not dilute Shark Bay's salt-concentrated waters.

Different parts of Shark Bay have different salt concentrations, Shark Bay's waters are saltiest in Hamelin Pool and Lharidon Bight.
Water in these areas is hypersaline, or twice as salty as normal seawater.
The waters below Eagle Bluff are metahaline, meaning they are up to 1.5 times as salty as the open ocean.

Most marine life cannot survive in hypersaline waters, so there are few predators or competitors in Hamelin Pool and Lharidon Bight.
Because of this, salt-tolerant life-forms, such as the cyanobacteria which build stromatolites, and the heart cockles which make-up Shell Beach, can flourish in great numbers.

The marine life living in Shark Bay's metahaline waters have also had to adapt to high salt concentrations.
Some of these animals, including Pink Snapper and clams, are genetically very different from their cousins outside of Shark Bay, and are growing more dissimilar as time passes.

Shark Bay has three populations (stocks) of Pink Snapper.
One consists of wide-ranging oceanic snapper, found in waters north of the peninsulas, and along much of the west coast.
The other two snapper populations are salt-adapted, and live exclusively in Shark Bay's salty inner gulfs, one in the eastern gulf, the other in the western gulf.
These populations do not interbreed, and are genetically different from one another.

Given a few more thousand years, Shark Bay's salt-adapted marine life may actually evolve into new species.
In fact some species, including the Shark Bay seasnake and the jellyfish that lives among the stromatolites in Hamelin Pool, already have!

Seagrasses are different from seaweed.

Seaweeds are marine algae that have no roots or flowers.

Seagrasses have evolved from land plants that have adapted to life underwater.
They have roots and produce underwater flowers.
Pollination occurs as ocean currents carry pollen from flower to flower.

After the ice age, rising seas began to flood into Shark Bay (about 7'000 years ago).
As the water rose, seagrasses started to grow in the Bay's expanding shallow waters.
Seagrasses can only grow in shallow waters because like most plants, they need lots of light for photosynthesis.

Because seagrass banks provide food and a sheltered environment, they attract a wide abundance of marine life.
Seagrass banks slowly grow in height and size as they accumulate the shells and skeletons of dead marine life.
These accumulations of shells, corals and skeletons are called carbonate sediments.

Ironically, seagrasses' amazing ability to accumulate sediment it also their downfall.
Eventually the seagrass banks grow too high, and the seagrasses die, unable to survive exposure to the air and sun during low tides, leaving a bare sandy or shelly flat behind.

Today, Shark Bay is home to the largest seagrass banks in the world, occupying some 400'000 ha (about the size of the entire Perth metropolitan area).
These are the largest seagrass banks in the world, and still growing.



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Last updated: Thursday, 20.03.2008 12:36 PM