Red tides, which often contain harmful algal blooms (HABs), are caused by chemical reactions that occur between algae and other substances. Red by day, blue by night, this colorful ocean phenomenon is a relatively rare natural occurrence that has spawned a number of imitations in movies and literature, the most recent example being a rather striking scene in the visually-driven movie Life of Pi.
Although naturally occurring, red algae blooms can be hazardous to swimmers, animals and marine life. Depending on the algae species, the red tide can contain high levels of ammonia that cause rashes and eye irritation. Poisonous algal blooms rob organisms of sunlight and oxygen, and may emit poisonous toxins.