Plankton Dynamics Gypsum Blooms And Sulfide Events At The Salton Sea California

Restoration of the Salton Sea, a large salt lake in the desert area of southeastern California, is presently under consideration by state and federal agencies. It experiences wildly fluctuating fish populations, algal blooms and occasions of anoxia with toxic levels of hydrogen sulfide. Overturn events, caused by windstorms, occur from early summer to early fall leading to upwelling. These are associated with patches of bright green water with low oxygen and high sulfide levels in surface waters, called "green tides", and are visible by satellite. Orders of magnitude drops in both phytoplankton and zooplankton populations occur in these green waters. Phytoplankton was studied over the three years 1997-1999 as well as in fall 2005 during a "green tide" event. Major groups were dinoflagellates, diatoms, cryptomonads and a raphidophyte. A euglenoid was often abundant in green waters. Biovolume densities and chlorophyll a concentrations each year were highest in spring and lowest within green waters in August or September. An increase in abundance of large cells and colonial algae over the three years is attributed to a release in grazing pressure due to the decline in the planktivorous fish, tilapia. Ciliates were the major protozooplankters but usually accounted for less than half of the zooplankton in 1997-1999. Large ciliates increased in abundance over the three-year period, likely due to decreasing fish populations. An anaerobic ciliate, Sonderia sp., became abundant during the anoxic green water events. Anecdotal reports of "green tides" date back decades. Usually attributed to algal blooms, these are due to suspended microscopic gypsum crystals. Formed when sulfide-rich bottom waters mix into oxygen-rich surface waters, the crystals give a bright signal in the green waveband, and serve as an indicator of these sulfide irruptions. Gypsum blooms have been observed by satellite every year during 1979-1985 and 1998-2006, generally in summer or fall and in mid-lake, lasting days to a week or more. Their occurrences has been also been accompanied by mass mortalities of fish and have increased in intensity and duration since 2002 due to a decrease in fish populations and an increase in decomposition of organic matter at depth