A BLOOM OF DUNALIELLA-PARVA IN THE DEAD-SEA IN 1992 - BIOLOGICAL AND BIOGEOCHEMICAL ASPECTS

A bloom of the unicellular green alga Dunaliella parva (up to 15 000 c ells ml(-1)) developed in the upper 5 m of the water column of the Dea d Sea in May-June 1992. This was the first mass development of Dunalie lla observed in the lake since 1980, when another bloom was reported ( up to 8800 cells ml(-1)). For a bloom of Dunaliella to develop in the Dead Sea, two conditions must be fulfilled: the salinity of the upper water layers must become sufficiently low as a result of dilution with rain floods, and phosphate must be available. During the period 1983- 1991 the lake was holomictic, hardly any dilution with rainwater occur red, and no Dunaliella cells were observed. Heavy rain floods in the w inter of 1991-1992 caused a new stratification, in which the upper 5 m of the water column became diluted to about 70% of their former salin ity. Measurements of the isotopic composition of inorganic carbon in t he upper water layer during the bloom (delta(13)C = 5.1 parts per thou sand) indicate a strong fractionation when compared with the estimated -3.4 parts per thousand prior to the bloom. The particulate organic c arbon formed was highly enriched in light carbon isotopes (delta(13) C = -13.5 parts per thousand). The algal bloom rapidly declined during the months June-July, probably as a result of the formation of resting stages, which sank to the bottom. A smaller secondary bloom (up to 18 50 cells ml(-1)) developed between 6 and 10 m depth at the end of the summer. Salinity values at this deep chlorophyll maximum were much bey ond those conductive for the growth of Dunaliella, and the factors res ponsible for the development of this bloom are still unclear.