An algal bloom where the algae produce powerful toxins that can kill fish, birds, and mammals, and ultimately cause illness in humans. The harmful bloom can also cause oxygen depletion in the water due to the death and decomposition of non-toxic algae species.

A colloquial term for a harmful algal bloom in which DINOFLAGELLATES accumulate rapidly forming dense patches and often discolor the water (but not always red). Some red tides are associated with the build up of natural toxins and some with the depletion of dissolved oxygen. They are unrelated to EARTH TIDES.