Biostrome (stromatolite), Gilbert Bay, Great Salt Lake

Biostromes are rocks formed by the growth of organisms. In the Great Salt Lake biostromes (sometimes called stromatolites) cover vast areas in water less than 2 m (6 feet) deep. The photosynthetic cyanobacteria growing on the surface changes the pH of the water and causes limestone to precipitate. In some ways these biostromes are similar to coral reefs.

In the lake, biostromes are virtually the only solid substrate where the cyanobacteria can grow. Larvae of brine flies graze on the cyanobacteria, and pupate there before they emerge as adults. The laval and adult brine flies are important as foods for millions of migratory birds that utilize the lake.

Photo: March 2009 of a biostrome kept in a aquarium at Utah State University.