Wind-driven river plume dynamics and toxic Alexandrium tamarense blooms in the St. Lawrence estuary (Canada): A modeling study

In the lower St. Lawrence estuary (LSLE, eastern Canada), blooms of the toxic dinoflagellate Alexandrium tamarense are a recurrent phenomenon, resulting in paralytic shellfish poisoning outbreaks every summer. A first coupled physical\â\}€“biological model of A. tamarense blooms was developed for this system in order to explore the interactions between cyst germination, cellular growth and water circulation and to identify the effect of physical processes on bloom development and transport across the estuary. The simulated summer (1998) was characterized by an A. tamarense red tide with concentrations reaching 2.3 \Ã\}— 106 cells L\â\}ˆ’1 along the south shore of the LSLE. The biological model was built with previously observed A. tamarense cyst distribution, cyst germination rate and timing, and A. tamarense growth limitation by temperature and salinity. The coupled model successfully reproduced the timing of the A. tamarense bloom in 1998, its coincidence with the combined plumes from the Manicouagan and Aux-Outardes (M-O) rivers on the north shore of the estuary, and the temporal variations in the north-south gradients in cell concentrations. The simulation results reveal that the interaction between cyst germination and the estuarine circulation generates a preferential inoculation of the surface waters of the M-O river plume with newly germinated cells which could partly explain the coincidence of the blooms with the freshwater plume. Furthermore, the results suggest that the spatio-temporal evolution of the bloom is dominated by alternating periods of retention and advection of the M-O plume: east or north-east winds favor the retention of the plume close to the north shore while west or north-west winds result in its advection toward the south shore. The response of the simulated freshwater plume to fluctuating wind forcing controls the delivery of the A. tamarense bloom from the northern part of the estuary to the south shore. In addition, our results suggest that a long residence time of the M-O plume and associated A. tamarense population in the LSLE during the summer 1998 contributed to the development of the red tide. We thus hypothesize that the wind-driven dynamics of the M-O plume could partly determine the success of A. tamarense blooms in the LSLE by influencing the residence time of the blooms and water column stability, which in turn affects A. tamarense vertical migrations and growth.