Red tide can occur in coastal ocean water around the world. It's called red tide because it can look like a reddish brown sheen on the surface of the waves. Although it might look like pollution, it's a natural phenomenon, caused by a sudden increase in the population of tiny, single-celled algae. This microscopic population explosion at the bottom of the food chain can be harmful to those at the top, because many of the algae responsible for red tide produce potent chemical toxins in their microscopic bodies.
In some regions, people can experience irritation of the eyes, nose, or throat when they are at the shore near a concentration of red tide, because wind, crashing surf, and even boat propellers can mix the toxic algae into the ocean spray.
The effects on marine life can be more serious, however. Oysters, clams, and shellfish eat by filtering algae out of the seawater, so they can concentrate the toxin in their bodies, making some of these animals unsafe for humans to eat. The toxins can also kill fish, which absorb these chemicals through their gills, and kill other marine animals that eat fish, such as dolphins.
Researchers are still working to understand this serious natural phenomenon.