Red tide. View of an algal bloom known as a red tide. Red tides consist of vast numbers of red dinoflagellate single-celled algae. They result from a surge in the level of nutrients in coastal waters through the upwelling of cold nutrient-rich water from deep within the ocean. In some cases red tides may be due to untreated sewage or agricultural fertilizers entering the ocean. Certain dinoflagellates produce toxins which become concentrated in animals who feed on them. As a result it is dangerous to eat clams, mussels and certain fish after a red tide. Photographed in the 1990s on the Bountiful Islands, Gulf of Carpentaria in Queensland, Australia.