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. The sea has also re-flooded some coastal birridas, creating inland bays of exceptional beauty which contribute towards World Heritage status. Two flooded birridas notable for their beauty and ecological significance are Big Lagoon and Little Lagoon.
- More than 100 birridas can be found on the Peron peninsula.
- 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.
Big lagoon - a series of inundated birridas
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.
- 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.
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). 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.
Have a look at this 360 degree panorama of a beautiful birrida!