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Naturally occurring microscopic algae, called phytoplankton, are common in coastal waters. Though phytoplankton are small, they can grow explosively, creating something called a "bloom". In many blooms, these organisms can be so numerous and concentrated that they color the water red, hence the term "red tides". An intense bloom can produce harmful impacts on marine ecosystems. For example, when masses of algae die and decompose, they can deplete oxygen in the water, causing the water to become so low in oxygen that animals either leave the area or die. A small percentage of algae produce powerful toxins that can kill fish, shellfish, marine mammals and birds, and may directly or indirectly cause illness in people. While both types have harmful impacts, the term, “harmful algal bloom”, is usually associated with blooms that kill with a toxin. “Red tide” is a popular term for harmful algal blooms, however HABs is preferred by scientists as they can be different colors. In some cases, the algae may not produce a discoloration of the water, yet still be present in sufficient numbers to affect shellfish or other marine life.

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