AUTOECOLOGY AND SOME LIFE-HISTORY STAGES OF DINOPHYSIS-ACUTA EHRENBERG

A persistent and unusual bloom of Dinophysis acuta Ehrenberg in the Ri as Bajas (Northwest Spain) from early July to mid-November 1989, was a ssociated with a diarrhetic shellfish poisoning (DSP) outbreak that pr evented mussel extraction in some areas from early August to mid-Decem ber. Integrated samples (0-5, 5-10 and 10-15 m) were taken once or twi ce a week at six stations. Maximum numbers of D. acuta (up to 2.3 x 10 (4) cells l(-1) in the integrated samples) and percentage of observed dividing cells (up to 31%), appeared in August, and formed a maximum i n the thermocline. Cell numbers were minimal when upwelling caused a b reakdown of stratification. High numbers of D. acuta (up to 1.8 x 10(4 ) cells l(-1)) appeared again when persistent south winds in late Octo ber caused a reversal of the circulation. Thus, the two cell maxima oc curred during two distinct hydrographic regimes, but in both cases at temperatures of 15-17 degrees C. Numerous cells with one typical D. ac uta valve, but the other valve reduced in size, were seen during the b loom, as well as Dinophysis dens Pavillard cells that might be a life cycle stage of D. acuta. Other observations include double-walled, unw inged forms that are perhaps some kind of cyst, and a hypothetical 'fe rtilizing tube' emerging from the larger cell of a D. acuta/D. dens co uplet. It is suggested that the maxima during stratification and durin g downwelling episodes could correspond to periods with a predominance of in situ growth and physical accumulation, respectively, and that t he exceptional meteorological conditions during 1989 provided the opti mum scenario for the unusual occurrence of D. acuta.