Cyanobacteria (blue-green algae) dominate the benthic habitats of Waldo Lake. These photosynthetic microorganisms may be the principal contributors to overall primary production in the lake, although hard data are not yet available. The main biomass of cyanobacteria consists of filamentous, nitrogen-fixing forms, which also produce a yellow-brown, UV-screening pigment (scytonemin) in their extracellular sheaths. This pigment is probably a necessity under these conditions of extreme water clarity in which UV radiation penetrates well. In addition, a conspicuous feature at 1 m to 5 m depths, along much of the shoreline, are living “stromatolites” which resemble shelf-like rock formations, and appear to be formed primarily by the growth of cyanobacteria. Further studies are underway.