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 Â 10 6 cells . L À 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 À 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.

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

“…Consequently, during the incubation experiments, phosphate availability appeared to control the A. tamarense growth rate, while nitrate availability controlled the duration of the growth phase and, thus, the biomass yield. This result gives additional support to the hypothesis proposed by Fauchot et al (2005), which stipulates that A. tamarense growth may be limited by phosphate in the St. Lawrence estuary during red tide events. Combining the 2 data sets (1998 and 2000, see Fauchot et al 2005), we found a K s (phosphate concentration at which the A. tamarense growth rate is one half the maximum rate) of 0.26 µM.…”

“…The difference in net growth rate between the phosphate and nitrate + phosphate addition experiments may result from the necessary adaptation of cellular metabolism to the sudden and simultaneous availability of nitrate and phosphate in the light. As previously hypothesized by Fauchot et al (2005), our results suggest that the growth rate of A. tamarense in the St. Lawrence estuary is controlled by phosphate availability, while the biomass reached during a bloom depends on nitrate availability and on the cells ability to perform vertical migrations.…”

“…The K s of 0.26 µM calculated from Alexandrium tamarense growth rate data from this study and another study in 1998 (Fauchot et al 2005) is lower than the K s values of 1 and 1.68 µM reported for 2 other A. tamarense strains (Frangópulos et al 2004). This apparent high affinity for phosphate exhibited by A. tamarense in the St. Lawrence estuary could represent an adaptation to the low-phosphate concentrations encountered in surface waters where the cells develop and to the ability of A. tamarense to use deep nitrogen pools.…”

“…The results from the nutrient addition experiment show that the Alexandrium tamarense growth rate in the St. Lawrence estuary may be limited by phosphate as hypothesized by Fauchot et al (2005). The low NO 3 -:PO 4 3 -ratio in surface waters, the observations of vertical migrations to deep layers during the night and the high affinity of A. tamarense for phosphate confirm the hypothesis that the ability of A. tamarense to reach deep nitrate pools pushes this species towards phosphate limitation in the St. Lawrence estuary.…”

“…First, it would give A. tamarense an advantage over other phytoplankton species in the nitrogen-poor surface waters of the estuary during the summer. Second, as hypothesized by Fauchot et al (2005), the ability of A. tamarense to perform nocturnal vertical migrations in situ and acquire nitrate at depth may push this species towards phosphate limitation in surface waters. Finally, the interactions between vertical migrations and the estuarine circulation prevailing in the St. Lawrence estuary are likely to affect the distribution of the blooms and, thus, the geographical extent of their impacts.…”