Fresh water enters the Gulf of Maine from several large rivers in southern Maine, and the freshened coastal waters flow south in a manner that is influenced by the amount of rainfall and snowmelt, the local wind stress, and the underlying circulation of the Gulf of Maine. Toxic Alexandrium populations are closely associated with this buoyant water mass. The hydrographic mechanisms underlying PSP blooms in down-east Maine are more poorly understood than those described for the region to the southwest. The relaxation events or downwelling, brought about by a change in wind speed or direction, carry established Alexandrium populations toward shore, resulting in rapid increases in toxicity in nearshore shellfish. There is currently no evidence that this also occurs in Washington or Alaska.
Some areas are well-studied, and others are virtually unknown.
Alexandrium blooms generally do not involve large cell accumulations that discolor the water and may be below the water surface where they are not visible. Low density populations can cause severe problems due to the high potency of the toxins produced by these species. Furthermore, Alexandrium species can grow in relatively pristine waters, and it is difficult to argue that anthropogenic nutrient inputs are stimulating the blooms. These characteristics are important when considering mitigation and control strategies. The economic impact of these outbreaks is significant, though difficult to estimate in total.