A red tide is an ocean-coloration effect caused by a population explosion of certain dinoflagellates which release may also release an alkaloid,
poisonous to fish, into the water. A more general name for this type of
phenomenon is algal bloom. Red tides are typically seen
in areas near the coast. The red coloration is due to photosynthetic
pigments present in the organisms, which become visible on a large scale
in high concentrations.