The seeding of two red tide blooms by the germination of benthic Gonyaulax tamarensis hypnocysts

Abstract

Complementary laboratory and field data are presented that demonstrate the seeding of a spring and a fall bloom of the toxic dinoflagellate Gonyaulax tamarensis by the temperature-induced germination of benthic hypnocysts. Nutrient, salinity, temperature and rainfall data collected before, during, and after a spring bloom in a Cape Cod salt pond indicate that germination of the overwintering hypnocysts was initiated by a temperature increase. Laboratory experiments with hypnocysts exposed to a gradual temperature increase after storage at 5 °C for four months verified this result. The fall bloom was apparently seeded by hypnocyst germination as well, but with excystment initiated by a temperature decrease from the summer level of 20–22 °C. This result was also confirmed in the laboratory using hypnocysts stored at 22 °C and subjected to a gradual decrease in temperature. The reasons for the eventual decline of both blooms are not known, but it is proposed that the motile populations were transformed into new hypnocysts that settled to the sediments. Immediately after both bloom peaks, there was a sharp decrease in the percentage of hypnocysts that germinated during laboratory incubation after isolation from fresh, unstored sediment samples. The duration of these periods of limited excystment after the declines suggests that cold storage (or overwintering) is not a required conditioning mechanism prior to germination but that a prolonged period of conditioning at higher temperatures will suffice. Finally, data are presented that suggest that the extreme localization of the blooms within the salt pond was due to limited tidal advection of the motile population and the favourable trace metal chemistry of the estuarine environment compared to the adjacent coastal waters.