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).