The link between precipitation, river runoff, and blooms of the toxic dinoflagellate Alexandrium tamarense in the St. Lawrence

Abstract

Blooms of the toxic dinoflagellate Alexandrium tamarense, which is responsible for paralytic shellfish poisoning, are annually recurrent events in the Estuary and Gulf of St. Lawrence, Québec, Canada. The analysis of abundance data for this algal species between 1989 and 1998 at Sept-Îles, a presumed initiation site in the north western Gulf of St. Lawrence, revealed yearly fluctuations in the onset, duration, and magnitude of toxic A. tamarense blooms. Hydrological and meteorological data for the region indicate that rainfall, Moisie River runoff, and wind are highly related to the pattern of bloom development each year. Results from the 10-year data set reveal that in this system: (i) high Moisie River runoff from a prolonged spring freshet or from heavy rainfall events in the summer and fall can initiate A. tamarense blooms; (ii) high Moisie River runoff combined with prolonged periods of weak winds (<4 m·s–1) favour the continued development of blooms; and (iii) winds >8 m·s–1 disrupt blooms. Salinity, which reflects the general state of the water column in terms of freshwater input and stability, had a strong negative correlation with the probability of observing A. tamarense cells at this station and could thus be used as a predictive tool for the presence of cells in this system.