Birridas of Shark Bay

The most westerly point of the Australian continent, next to the Indian Ocean is called Shark Bay, an area of exceptional natural features and a World Heritage site. Scattered around Shark Bay, especially within Peron Peninsula inside the Francois Peron National Park, are several saline lakes of gypsum, known locally as birridas.

Thousands of years ago, when sea levels were much higher than they are today, birridas were landlocked saline lakes between sand dunes. The water was rich in sulphate of lime that was deposited onto the lake floor. When the sea level dropped, the lakes dried up creating 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 central section corresponds to the level of the water table during the late Pleistocene Period, about 10 000 years ago. Today, during very high winter tides or after heavy rains, when the groundwater level is raised, these moats fill with water to a shallow level. Most birridas retain water for several months following rain. At these times, dormant eggs hatch and the birridas teem with brine shrimp, horse-shoe crabs and other invertebrates. They provide a feast for wading birds such as red-necked stints (Calidris ruficollis) and bar-tailed godwits (Limosa lapponica) that have migrated to Shark Bay from as far away as Siberia.

Some birridas are connected to the sea by channels and receive seawater, where they form shallow bays. These bays are important fish breeding and nursery areas, however, most birridas at Shark Bay are isolated.

Birridas are common in Francois Peron National Park where there are more than 100 on the east coast of the Peron Peninsula. You will see birridas when driving around the park, however to appreciate the shapes and sheer number of birridas it is best to take a flight.


Algae Bloom on Lake Erie

Lake Erie is the fourth largest of the five Great Lakes in North America, and the eleventh largest lake in the world, by surface area. Lake Erie, aside from providing drinking water to the neighboring population, is a source for many waterborne commerce, navigation, and manufacturing. Outflow from Lake Erie spins the huge turbines at Niagara Falls providing hydroelectric power to Canada and the U.S. The intensive industrial development along the shores of the lake has been damaging the lake’s environment for decades with issues such as overfishing, pollution and more recently rapid algae blooms.

During the summer months, Lake Erie along with the rest of the five Great Lakes smothers under huge swaths of green algae, often thousands of square kilometers in size. The algae proliferates by feeding on excess nutrients in the form of phosphorous in the water. The phosphorus comes from sewage treatment plants and fertilizer used in farms that runoff along with rain water and enter into streams and rivers eventually winding up in Lake Erie. Blue-green algae also thrives on light. Lake Erie, being the shallowest of the Great Lakes, especially at its west end, is more susceptible to algae than its deeper cousins, which do not have the same penetration of sunlight.

The algae float on the surface and multiply rapidly, and when they die, they sink to the bottom of the lake, where they decay and absorb the oxygen in the water creating dead zones where most aquatic animals cannot survive. Hundreds of thousands of dead fish washed up on Erie’s shores during 2011 when the lake saw the biggest algae bloom in recorded history. The blue-green algae invaded Lake Erie covering as much as one-sixth of the surface, extending from Toledo, Ohio to beyond Cleveland and along the Ontario shore. It extended over 20 kilometers from the shores, and in the central basin it was observed at a depth of at least 60 feet.

Not all types of algae are destructive, but the bloom is primarily microcystis aeruginosa, an algae that is toxic to mammals. Microcystis aeruginosa produces a liver toxin, microcystin, that commonly kills dogs swimming in infected water and causes skin irritation, respiratory difficulty and gastrointestinal distress in humans.

Algae blooms were common in the lake’s shallow western basin in the 1950s and 60s. Phosphorus from farms, sewage, and industry fertilized the waters so that huge algae blooms developed year after year. The blooms subsided a bit starting in the 1970s, when regulations and improvements in agriculture and sewage treatment limited the amount of phosphorus that reached the lake. But the problem has resurfaced in recent years.

Satellite image of Lake Erie taken in 2003.