Offshore blooms of the red tide dinoflagellate, Alexandrium sp., in the Gulf of Maine

Paralytic shellfish poisoning (PSP) occurs nearly every year in the Gulf of Maine. In a study of dynamics of the causative organism, the toxic dinoflagellate Alexandrium sp., we conducted three surveys of the coastal and offshore waters of Gulf of Maine during the summer of 1998, sampling more than 200 stations during each cruise in June, July and August. Hydrographic data were collected and concentrations of phytoplankton chlorophyll, inorganic nutrients and densities of Alexandrium cells were measured in discrete water samples. The distributions of Alexandrium at the surface and in subsurface waters displayed maximum cell densities in the offshore waters of the Gulf on all three cruises. Highest cell densities in surface waters (ca. 5.5×103 cells l−1) were observed in two broad patches: one in the Bay of Fundy and another in shelf and offshore waters of the central and eastern Gulf of Maine in association with the Eastern Maine Coastal Current. Highest subsurface densities of cells appeared to be associated with the frontal edges beyond the cold surface waters associated with the Eastern Maine Coastal Current. As the summer progressed, the highest surface densities of Alexandrium receded toward the eastern portions of the Gulf and the Bay of Fundy. We suggest that the offshore distributions of relatively high densities of Alexandrium are naturally occurring and can be related to inorganic nutrient fluxes, and to the ambient light field as it varies seasonally and vertically. Locations of high cell densities were described and interpreted using a nondimensional light-nutrient parameter, computed as the ratio of the depth of the 10% surface irradiance to the depth of 4 μM NO3 concentration. Possible mechanisms responsible for periodic development of PSP outbreaks in nearshore shellfish beds are discussed.