A "red tide" is a discoloration of the ocean that results from unusually dense accumulations of phytoplankton, the single-celled "plants" of the sea.
Most red tides are caused by dinoflagellates microscopic, single-celled organisms propelled by tiny whips. In high concentrations (2 million cells per liter), they color the water red, rusty brown, green or yellow, depending on the species (see photo above).
Are "red tides" toxic?Some dinoflagellate species produce one or more strong toxins. These toxins are concentrated up the food chain during dense blooms, and can cause illness in humans who consume tainted seafood. The species usually responsible for the red tide off San Diego's coast is not known to be toxic and should pose no health hazard. Locally, the last recorded toxic bloom was in 1902.
What triggers the sudden bloom of these organisms?
Scientists don't know precisely, but increased nutrients from runoff or pollution, high salinity, temperature, light, upwelling, and/or sediment disturbance may play a role. Red tides are one of many important subjects of study at Scripps Institution of Oceanography.
Look for a light show in the waves
Like tiny fireflies, dinoflagellates can create their own light through an internal chemical reaction. Flashing is stimulated by mechanical agitation. After dark, you may observe a bluish glow along the crest of breaking waves. You may also notice your footprints glowing in the wet sand as you walk along the beach.
See it for yourself
For a fun activity, fill a glass jar with water from the beach. Take it home, and put it in a cool, dark place (like a cupboard). A few hours after sunset, take the jar to a dark room, let your eyes adjust, then give the jar a swirl. You should see a beautiful sparkling light show!