Red Tide - Definition

Definition

Red tide is a colloquial term used to refer to one of a variety of natural phenomena known as a harmful algal blooms or HABs. The term red tide specifically refers to blooms of a species of dinoflagellate known as Karenia brevis. It is sometimes used to refer more broadly to other types of algal blooms as well.

The term red tide is being phased out among researchers for the following reasons:

1. Red tides are not necessarily red and many have no discoloration at all.
2. They are unrelated to movements of the tides.
3. The term is imprecisely used to refer to a wide variety of algal species that are known as bloom-formers.

As a technical term it is being replaced in favour of more precise terminology including the generic term harmful algal bloom for harmful species, and algal bloom for non-harmful species.

The term red tide is most often used in the United States of America to refer to Karenia brevis blooms in the eastern Gulf of Mexico, also called the Florida red tide. These blooms occur almost annually along Florida waters. The density of these organisms during a bloom can exceed tens of millions of cells per litre of seawater, and often discolor the water a deep reddish-brown hue.

The term red tide is also sometimes used to describe harmful algal blooms on the northern east coast of the United States, particularly in the Gulf of Maine. This type of bloom is caused by another species of dinoflagellate known as Alexandrium fundyense. These blooms of organisms cause severe disruptions in fisheries of these waters as the toxins in these organism cause filter-feeding shellfish in affected waters to become poisonous for human consumption due to saxitoxin. The related Alexandrium monilatum is found in subtropical or tropical shallow seas and estuaries in the western Atlantic Ocean, the Caribbean Sea, the Gulf of Mexico and the eastern Pacific Ocean.