Lying about 400 m below sea level in a rift valley along the Israel-Jordan border, the surface of the Dead Sea is the lowest of any lake, and it is one of the saltiest with a current salinity of around 340gl~\ Diversions of the Jordan River, the main inflow, resulted in a 20 m decline in lake level over the last century and an increase in salinity. One consequence of the evaporative concentration of the upper waters was the termination of meromixis that had persisted for several hundred years. With the exception of a few years, the lake now mixes completely each year.

At the time of the pioneering microbiological studies by Benjamin Elazari-Volcani in the 1930s and 1940s, the lake's salinity was about 260 g l_1. Using enrichments and microscopy he was able to describe a variety of halophilic and halotolerant microbes as well as the phytoflagellate, Dunleilla viridis, several cyanobacteria, diatoms, green algae, and a ciliate. Subsequent application of modern molecular techniques has considerably expanded the number of microbes, but the higher salinities have eliminated some organisms noted earlier.

During times when the whole lake reaches salinities of around 340 g l_ , bacterial densities are low and algae are absent. However, in response to periods with large amounts of rainfall and runoff, the upper waters can be diluted to as low as 250 g l_ , and blooms of Dunaliella and red Archaea develop. The abrupt decline of the bacterial blooms cannot be attributed to protozoan grazing, since these organisms no longer occur, and may be caused by bacteriophages, as viruses have been identified in the lake.

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