Red tide in the Gulf of Maine starts in the Eastern Maine Coastal Current, an underwater river beginning in the Bay of Fundy off Nova Scotia and traveling south several miles off Maine’s coast.

Cold water from the deep ocean moves into the bay from the Scotian Shelf and through the Northeast Channel, creating the nutrient-rich yet comparatively shallow environment of the current, where Alexandrium tamarenses, the organism that causes red tide, can thrive.

Every spring, when increasing light and warmth spark Alexandrium reproduction, blooms appear all along the length of the current. In a normal year, that means islands such as Monhegan, located closer to the current, are plagued with red tide. Other parts of Maine’s coast can regularly experience small red tide blooms when eddies break off the current, carrying Alexandrium to shore.

But this May, the rare spring northeasters that kept the region cold and rainy also pushed the current inshore.