Most Algal Blooms in the Ocean Are Not Harmful

News stories about a red tide make any bloom of algae sound like a disaster. NOAA’s National Ocean Service gives a more balanced picture. A red tide is a common term for a harmful algal bloom, which occurs when colonies of algae grow out of control while producing toxic or harmful effects on people, fish, shellfish, marine mammals and birds. As the name suggests, the bloom often turns the water red. But not all algal blooms are harmful. Most blooms, in fact, are beneficial, NOAA says, because the tiny plants are food for animals in the ocean. A small percentage of algae, however, produce powerful toxins that can kill fish, shellfish, mammals and birds, and may cause illness in people. Scientists prefer the term harmful algal bloom to red tide.