Toxic species of the genus Alexandrium are responsible for outbreaks of Paralytic Shellfish Poisoning (PSP), a recurrent and serious problem in the Gulf of Maine. Understanding bloom dynamics of Alexandrium spp. is therefore of a major interest. Earlier studies in the region have demonstrated that the highest Alexandrium cell concentrations are located in offshore waters, away from most coastal shellfish beds, and are delivered to inshore waters by various physical mechanisms. An intriguing question is What restricts Alexandrium from blooming in inshore waters? One hypothesis is that Alexandrium bloom dynamics may be controlled not only by physical and chemical factors but also by biological interactions with other phytoplankton taxa, in particular, diatoms. Initial results from laboratory experiments and recent field measurements suggest that during spring and in inshore waters, high growth rates of diatoms impede the growth of A. fundyense. However, once the Alexandrium bloom becomes established, it may impede the development of a second diatom bloom via allelopathy. The order of this species succession appears to be dictated by the relative concentrations of dissolved inorganic nitrate and silicate.