Shark Bay’s Birridas

An unusual feature of Shark Bay is its gypsum claypans, known locally as birridas. Originally land-locked saline lakes, these shallow depressions support specially-adapted plant and animal species. But the sea has also re-flooded some coastal birridas, creating inland bays of exceptional beauty which contribute towards World Heritage status.

Have a look at this 360 degree panorama of a beautiful birrida!

Inland lakes

Shark Bay’s spectacular coastal scenery contributed to its World Heritage status. The region’s contrasting red sandstone and white limestone are part of a geology that is simultaneously prehistoric and modern.

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Ancient origins

Thousands of years ago, when sea levels were much higher, most birridas were inland saline lakes. The water deposited sulphate of lime (calcium sulphate dihydroxide) onto the lake floor. Eventually the lakes dried and became salty hollows, and the sulphate of lime evaporated and became loose, powdery gypsum.

• Birridas are circular or oval in shape and range from 100 m to 1 km wide.
• They commonly consist of a central, raised platform ringed by a moat-like depression.
• The height of the platform is believed to correspond to water levels in the Late Pleistocene epoch, more than 10,000 years ago.
• The moat was formed when fresh groundwater seeped from the surrounding dunes and dissolved the gypsum around the edge of the platform.

Salty habitats Birridas may fill with water after heavy rain, or when very high winter tides raise the groundwater level. At these times, dormant eggs hatch and the birridas teem with brine shrimp and other invertebrates. They provide a feast for migratory wading birds such as red-necked stints (Calidris ruficollis) and bar-tailed godwits (Limosa lapponica). Find out more about Shark Bay birdwatching here. Birridas also feature salt-adapted plants, such as frankenia, white pigface (Carpobrotus candidus), silver saltbush (Atriplex bunburyana) and samphire (Halosarcia sp.).

Only specially salt-adapted plants can survive the harsh environment of a birrida.

Blue lagoons Most birridas remain land-locked, but some have been re-filled by the sea. Two flooded birridas are notable for their beauty and ecological significance. Big Lagoon is a thriving ecosystem featuring seagrass, sand flats and mangroves. It is a safe haven for dolphins, dugongs and young fish and crustaceans. Green turtles enjoy basking on the warm surface waters. Little Lagoon is connected to the sea via a channel. Located on the outskirts of Denham, its shallow protected waters are an important fish breeding and nursery area.

Big lagoon - a series of inundated birridas