Algae, like all organisms, normally grow in natural balance with their ecosystems. But sometimes, certain species of algae reproduce so rapidly that they cause damage. In the ocean, microscopic forms of algae, known as dinoflagellates, can "bloom" into dense patches near the surface, often referred to as "red tides." Some of these harmful algal blooms (HABs) are dangerous, producing toxins that can kill marine organisms, taint shellfish, cause skin irritations, and even foul the air.

HABs occur worldwide. They seem to be increasing in size, intensity, and persistence—possibly due to nutrient-rich runoff from land or a warming climate. In the U.S., HABs have devastated Long Island's scallop fishery and caused seasonal closures of other shellfish beds along the Atlantic, Gulf of Mexico, and Pacific coasts. Scientists suspect the blooms may also have contributed to the deaths of hundreds of manatees, sea lions, and other marine mammals. Airborne toxins from the blooms have made vacationers and coastal residents sick, and damage from HABs costs coastal economies tens of millions of dollars each year.

Learn more about what scientists are doing to predict and prevent harmful algal blooms.