THE EFFECTS OF TEMPERATURE, GROWTH-MEDIUM AND DARKNESS ON EXCYSTMENT AND GROWTH OF THE TOXIC DINOFLAGELLATE GYMNODINIUM-CATENATUM FROM NORTHWEST SPAIN

The chain-forming dinoflagellate Gynmodinium catenatum Graham causes r ecurrent outbreaks of paralytic shellfish poisoning (PSP) in the Galic ian Rias Bajas (northwest Spain). A sediment survey in Ria de Vigo in April 1986 indicated that the highest concentrations of cysts of this species were located in the middle sections of the ria, with maximum a bundance of 310 cysts cm-3. The effects of temperature, growth medium composition and irradiance on the germination of laboratory-produced r esting cysts were investigated. Newly formed cysts required very littl e time for maturation, as excystment was possible within 2 weeks of en cystment. Growth media did not affect germination success. In contrast , the excystment rate was retarded significantly in darkness. Germinat ion was also strongly affected by temperature, with approximately 75% excystment success at 22-28-degrees-C and little or no germination bel ow 11-degrees-C after 1 month of incubation. In culture. the optimum g rowth rate of vegetative cells was between 22 and 28-degrees-C. the hi ghest rate being 0.53 divisions day-1 at 24-degrees-C. Growth did not occur at temperatures < 11-degrees-C or >30-degrees-C. These results a re important with respect to the different hypotheses proposed to expl ain the initiation of G.catenatum blooms in the Galician Rias Bajas an d Northern Portugal. The pattern of G.catenatum bloom development alon g this coast has been related to seasonal upwelling in the area, with major blooms occurring during the autumn as warmer offshore surface wa ter is transported towards the coast when upwelling relaxes. The landw ard transport of established offshore populations of G.catenatum with the warm surface layer remains a viable explanation for the observed b looms within the rias, but alternatively. our data suggest that cysts within the rias can provide the inoculum population at times conducive to growth and bloom formation. Even though newly formed G.catenatum c ysts have a very short maturation time and can germinate in darkness a cross a wide temperature range, bloom development will be significant only during the late summer and early autumn, since in other months li ght levels at the sediment surface and temperatures throughout the wat er column are too low for significant germination or growth.