In simplest terms, a red tide is a “bloom” of phytoplankton. Phytoplankton are microscopic, single-celled plants that occur naturally in our coastal waters. A “bloom”
occurs when a particular species of phytoplankton begins reproducing rapidly and results in millions of cells in each gallon of water. Not all phytoplankton species produce visible blooms. Red tides are caused by a particular group of phytoplankton
called dinoflagellates which seem to prefer warmer and calmer waters.