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A population of the toxic dinoflagellate Alexandrium tamarense was followed in a 48 h survey of the St. Lawrence estuary in order to determine its ability to perform vertical migrations in nature. A. tamarense performed daytime and nighttime migrations during our study. Cells were gen- erally found close to the surface during daytime, started their descent down to the nitracline before sunset, and returned toward the surface after sunrise (with an estimated migration speed of 2 m h -1 ). Our results suggest that A. tamarense cells were able to fulfill their nitrogen requirement while migrating to the deep layers during the night. During the day, A. tamarense cells adjusted to daily fluctuations in light intensity using their motile capacity and aggregated at a depth where light inten- sity did not exceed 300 µmol photons m -2 s -1 . The nutritional status of the A. tamarense population near the surface was determined at the onset of the 48 h study during a set of onboard nutrient addi- tion experiments. Results from these experiments indicated that the net growth rate of A. tamarense was controlled by phosphate availability, while the biomass was dependent on nitrate availability. These results suggest that the ability of A. tamarense to reach the deep nitrate reservoir at night allows this species to reach elevated biomass as observed during red-tide events, and could push this species toward phosphate limitation in the St. Lawrence estuary.

Daytime and nighttime vertical migrations of Alexandrium tamarense in the St. Lawrence estuary (Canada)