CONTROL OF GERMINATION OF ALEXANDRIUM-TAMARENSE (DINOPHYCEAE) CYSTS FROM THE LOWER ST. LAWRENCE ESTUARY (CANADA)

Cysts of the toxic dinoflagellate Alexandrium tamarense (Lebour) Balec h 1992 from the lower St. Lawrence estuary were used in a test of the following hypotheses: (1) cyst germination is triggered by a change in temperature, and (2) germination rate varies throughout the year and is controlled by a circannual internal biological clock. Results show that cyst germination was not affected significantly by temperature of incubation over the range 1 degrees-16 degrees C, and light showed no significant stimulation of germination. This is supported by the lack of effect of cyst incubation conditions during evaluation of the seas onal changes in germination rate (two temperatures: 4 degrees and 15 d egrees C, and two light conditions: darkness and 150 mu mol photons.m( -2).s(-1)). Thus, direct environmental control through short-term incr eases in temperature and exposure to light has no effect on the germin ation of the cysts tested. The rate of germination, observed monthly o ver a 16-month period showed low germination (<20%) over most of the p eriod tested except for a maximum reaching more than 50% germination i n August to October of the second year of the experiment. This pattern was observed for cysts both from monthly field collections and from l aboratory-stored cysts kept under constant environmental conditions (4 degrees C, in the dark). The peak in germination observed under const ant environmental conditions tin the laboratory), the almost coinciden tal increase in cyst germination observed for the field-collected cyst s, and the absence of effects of temperature and light during incubati on could be explained either by a temperature-controlled cyst maturati on period (the time-temperature hypothesis of Huber and Nipkow 1923 or by the presence of an internal biological clock. However, the large d ecline in the rate of germination 2 months after the maximum provides strong support for the biological clock hypothesis. The ca. 12-month m aturation (dormancy) period observed for the laboratory-stored cysts i s the longest reported for this species to our knowledge; this might b e related to the low storage temperature (4 degrees C), which is close to bottom temperatures generally encountered in this environment (0 d egrees to 6 degrees C). Similar field and laboratory storage temperatu res could explain the coincidental increase in germination rate in the fall of the second year if cyst maturation is controlled by temperatu re. A fraction of the laboratory-stored cysts did not follow a rhythmi c pattern: A rather constant germination rate of about 20% was observe d throughout the year. This continuous germination of likely mature cy sts may supplement the local blooms of this toxic dinoflagellate, as t hese often occur earlier than peak germination observed in late summer . It seems that two cyst germination strategies are present in the St Lawrence: continuous germination after cyst maturation, with temperatu re controlling the length of the maturation period and germination con trolled by a circannual internal rhythm.