Studies on the genera Gyrosigma and Pleurosigma (Bacillariophyceae) - Species from the Salton Sea, California, USA, including Pleurosigma ambrosianum, nov sp.

Abundant populations of three sigmoid diatom species were observed in the S alton Sea, a highly saline inland lake in California, U.S.A. The occurrence of Gyrosigma balticum (Ehrenberg) Rabenhorst so far inland is considered u nusual. The ecology of Gyrosigma wormleyi (Sullivant) Boyer, previously typ ified as a freshwater species, must apparently be extended to include salin e habitats. A new species, Pleurosigma ambrosianum nov. sp., is characteriz ed by its very fine oblique striation, often at the limit of resolution of the LM. P. ambrosianum is the dominant member of the plankton diatom assemb lage during the winter months.