LIFE-HISTORY AND IN-SITU GROWTH-RATES OF ALEXANDRIUM-TAYLORI (DINOPHYCEAE, PYRROPHYTA)

Alexandrium taylori Balech is a phototrophic marine dinoflagellate. It produced recurrent blooms during the summer months (July and August) of 1994 to 1997 in La Fosca beach (NW Mediterranean). In addition to a motile vegetative form, A. taylori had two benthic forms: temporary c ysts and resting cysts. Temporary cysts were a temporally quiescent st age produced from the ecdysis of the vegetative cell in both natural p opulations and laboratory cultures. Temporary cysts may divide to form motile cells. Resting cysts had a thicker wall than the temporary cys ts and had a red accumulation body. Gametes and planozygotes were also observed in laboratory cultures. Alexandrium taylori showed in situ d iurnal vertical migration with an increase of vegetative cells in the water column in the morning through midday, with concentrations peakin g in the afternoon followed by lower levels at night. Most vegetative cells lost their thecae and flagella, and with them their motility, tu rning into temporary cysts that settled in the early evening: The numb er of temporary cysts in the water column rose in the evening and at n ight. The temporary cysts gave rise to motile cells the following morn ing. Synthesis of DNA occurred in vegetative cells at night, and a pre ferential period of cell division occurred at sunrise. The estimated d ivision rate in the;field was 0.4-0.5 vegetative cells.day(-1). Tempor ary cysts had twice the DNA of a G(1) vegetative cell. The minimum in situ division rate of the temporary cysts was 0.14 day(-1). The role o f the resting and temporary cyst population in the annual recurrence a nd maintenance of the A. taylori bloom is discussed.