THE COMPOSITION AND TOXICITY OF DINOFLAGELLATE BLOOMS IN THE SALTON SEA, CALIFORNIA

Phytoplankton blooms have been implicated in mortality events of diverse groups of organisms including fish, birds and humans. About 300 species have been reported to form "red tides," or surface discolorations due to high densities, but only 60–80 of these species produce harmful blooms. In marine systems, dinoflagellates account for 75% of all harmful algal bloom species. The Salton Sea is a large saline lake located in southeastern California, USA. The lake is eutrophic largely because it is in a closed basin and receives most of its input from agricultural and municipal wastewaters. Dinoflagellates comprise a significant portion of the phytoplankton biomass, particularly in winter, often resulting in "red" or "brown" tides. To date, 16 species of dinoflagellates have been identified from the Salton Sea, and many other unidentified forms have also been documented. In 1992, 150,000 eared grebes were found dead over a period of several months at the Salton Sea. This mortality event was among the largest of any bird species. The principal cause remains unknown, but algal toxins were suspected. A survey of the composition and toxicity of algal blooms was undertaken in 1999, and we report results from blooms where dinoflagellates dominated. Dominant species included Gonyaulax grindleyi , Gymnodinium spp., Gyrodinium uncatenum , Heterocapsa niei , and an unidentified scrippsielloid. Although most samples showed activity in a brine shrimp lethality assay, all were negative in a mouse bioassay. This evidence suggests that toxins from dinoflagellate blooms in the Salton Sea are not responsible for eared grebe mortality events.