THE DEAD-SEA - ALIVE AGAIN

In the thirteen years of quantitative studies on the microbiology of t he Dead Sea from 1980 onwards three distinct periods can be discerned. Mass development of the green unicellular alga Dunaliella parva (up t o 8,800 cells/ml) and red archaeobacteria (2 x 10(7) cells/ml) was obs erved in 1980, following a dilution of the upper water layers by rain floods. This bloom disappeared at the end of 1982 as a result of a com plete mixing of the water column. During the period 1983-1991 the lake was holomictic, and no Dunaliella cells were observed. Viable bacteri a were present during this period in very low numbers. Heavy rain floo ds during the winter of 1991-1992 caused a new stratification as the u pper five meters of the water column became diluted to 70% of their no rmal salinity. In this upper water layer Dunaliella reappeared (up to 3 x 10(4) cells/ml at the beginning of May, rapidly declining to less than 40 cells/ml at the end of July), and a bloom of red archaeobacter ia (3 x 10(7) cells/ml) once more imparted a red coloration to the lak e.