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

A persistent and unusual bloom of Dinophysis acuta Ehrenberg in the Rias Bajas (Northwest Spain) from early July to mid-November 1989, was associated with a diarrhetic shellfish poisoning (DSP) outbreak that prevented mussel extraction in some areas from early August to mid-December. Integrated samples (0-5, 5-10 and 10-15 m) were taken once or twice 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 in the thermocline. Cell numbers were minimal when upwelling caused a breakdown 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 October caused a reversal of the circulation. Thus, the two cell maxima occurred during two distinct hydrographic regimes, but in both cases at temperatures of 15-17 degrees C. Numerous cells with one typical D. acuta valve, but the other valve reduced in size, were seen during the bloom, as well as Dinophysis dens Pavillard cells that might be a life cycle stage of D. acuta. Other observations include double-walled, unwinged forms that are perhaps some kind of cyst, and a hypothetical 'fertilizing tube' emerging from the larger cell of a D. acuta/D. dens couplet. It is suggested that the maxima during stratification and during downwelling episodes could correspond to periods with a predominance of in situ growth and physical accumulation, respectively, and that the exceptional meteorological conditions during 1989 provided the optimum scenario for the unusual occurrence of D. acuta.