A red tide is a “bloom” of a group of phytoplankton called
dinoflagellates. A “bloom” occurs when a particular species of
dinoflagellate begins reproducing rapidly, resulting in millions of cells in
each gallon of water. Not all phytoplankton species produce visible blooms. The
phytoplankton cells that cause a red tide contain pigments for capturing
sunlight needed for cell nourishment, growth, and reproduction. These pigments
can give off a reddish color in the water when enough are present. If you would
like to read more information about red tides, take a look at the Red Tide leaflet (PDF) on the U.C. Davis Seafood Network
Information Center web site.
The photo below is of a visible bloom, or "red tide", of the dinoflagellate
Lingulodinium polyedrum along
the coast of La Jolla, San Diego County (photo courtesy of Kai Schumman).