ENVIRONMENTAL FACTORS CONTROLLING ALEXANDRIUM TAMARENSE (DINOPHYCEAE) GROWTH RATE DURING A RED TIDE EVENT IN THE ST. LAWRENCE ESTUARY (CANADA)1

Abstract

The dinoflagellate Alexandrium tamarense (Lebour) Balech 1985 is responsible for recurrent outbreaks of paralytic shellfish poisoning in the St. Lawrence Estuary. In July 1998, an A. tamarense red tide developed in the estuary with maximum cell concentrations reaching 2.3 × 106 cells·L−1 in brackish surface waters. To estimate the growth rate of these cells, surface water samples from different locations and days during the bloom were incubated for 5 to 9 days under in situ temperature and light conditions. Growth rates varied both spatially and temporally between 0 and 0.55 day−1, reaching the maximum growth rate reported for this species in culture. High growth rates were measured even during the peak of the red tide, suggesting that the extremely high cell concentrations observed did not solely result from aggregation or physical concentration but also involved active cellular growth. Alexandrium tamarense cells were found over a large range of salinity (20.8–29.5 psu), but high densities and significant growth were only measured when salinity was lower than 24.5 psu. Under these conditions, the number of divisions achieved by A. tamarense was proportional to the amount of nitrate available at the beginning of the incubations, whereas variations in growth rate were apparently controlled by the availability of phosphate. We hypothesize that the ability of A. tamarense to perform vertical migrations and acquire nitrate at night pushes this species toward phosphate limitation in the St. Lawrence Estuary.