Harmful algal blooms [HAB] are sudden influxes of massive amounts of colored single-cell algae that can convert entire areas of an ocean or beach into a blood red color. It is more popularly known as red tide.

The term red tide is being phased out by scientists and researchers for the following reasons; 

• red tides are not necessarily red....many have no discoloration at all
• it is not related to tidal movements
• a wide variety of algal species are known as bloom-formers

So, harmful species are now accurately referred to as harmful algal bloom, and the non-harmful species simply as algal bloom.

Red tide is caused by a high concentration of Karenia brevis, a microscopic marine algae that occurs naturally, but normally in lower densities. While they can be fatal, the constituent phytoplankton in red tides are not harmful in small numbers.

Red tide is also potentially harmful to humans. People have become seriously ill from eating oysters, mussels and other shellfish harvested from red tide contaminated waters.

harmful algal bloom #1