In the deeper water, nutrient concentrations have been rising all summer, due to remineralization. When the winds pick up due to autumn weather patterns, the water column can suddenly reverse, bringing the warmer water and its higher concentration of nutrients back to the surface. When this occurs, it will initiate a fall bloom, not as strong as the spring bloom, but still significant to annual primary production. One of the largest ecological events known on Earth is the spring bloom in the North Atlantic, which can clearly be seen in regional and seasonal composite CZCS images of the North Atlantic. The spring bloom is a wave of productivity that begins between Cape Hatteras and the Strait of Gibraltar in March, and progresses northward, dominating the central North Atlantic in May and the eastern North Atlantic and North Sea in June.
Blooms are usually beneficial events in the ecological calendar of a given body of water.
At times, though, there can be villains in a bloom's cast of characters. A well-known villain is Gymnodinium breve , one of the plankton species responsible for red tides. A red tide occurs because this species of plankton produces a potent neurotoxin, which can kill fish and make breathing uncomfortable if inhaled. The most serious threat to humans from red tides comes from shellfish, who filter the plankton from the water to consume, which concentrates the toxin in their flesh. Eating shellfish contaminated with red tide neurotoxin can kill.