Diel vertical distributions of the red tide dinoflagellate Alexandrium fundyense in the Gulf of Maine

Two 24-h experiments, designed to test whether the toxic dinoflagellate Alexandrium fundyense exhibited significant changes in vertical distribution, were performed in offshore waters of the Gulf of Maine in June 2000. Standard hydrocasts with a CTD/carousel water sampler package were made hourly on-station while following a drogue set at 25 m depth. Continuous measurements of T, S, and chlorophyll fluorescence were made on each down cast, and discrete water samples were collected at 5-m intervals from 55 m depth to the surface on the up cast, for analyses of extracted phytoplankton chlorophyll, inorganic nutrients and cell densities of Alexandrium In the first experiment we observed a bimodal vertical distribution of cells, with relatively high cell densities near the surface (<15 m depth) and a second peak of relatively high cell densities at depths between 25 and 40 m, coincident with the depth of the pycnocline and nitricline. Internal waves of 10–15 m amplitude appeared to exert control over the depth distribution of the deep population. Approximately 12 h into the first experiment, a relatively warm surface water mass with low Alexandrium cell densities intruded over the drogue station, leaving only the deep population. In the second experiment overall cell densities of Alexandrium were much lower, but again we observed initially a bimodal depth distribution of cells. As in the first experiment, the surface population effectively disappeared after a few hours, leaving only the deep population; in this case, however, there was some evidence of an initial downward movement of the surface population prior to its complete disappearance. Evidence for intrusion of a surface-water layer was not as clear in the second experiment as in the first. In addition to higher-frequency internal waves, as was observed in the first experiment, we also observed a low-frequency internal tidal wave of greater than 20 m amplitude that controlled the vertical distribution of the deep population of cells. We were unable to discern evidence of diel vertical migratory by the Alexandrium population in either of the two experiments.