The term for ocean-water discoloration caused by an algal bloom is red tide.
A red tide occurs when microscopic marine organisms multiply in unusually high concentrations, sometimes tinting seawater red, brown, or reddish-orange. Many of the organisms involved are dinoflagellates, although different species can produce different colors. The term is widely used for conspicuous coastal blooms.
The name can be misleading in two ways. The water is not always red, and the event is unrelated to ocean tides. Also, not every red tide is toxic. Some blooms are harmless, while others are harmful algal blooms that produce toxins, deplete oxygen, damage fish gills, or contaminate shellfish.
One well-known example is Florida’s red tide in the Gulf of Mexico, commonly caused by the dinoflagellate Karenia brevis. It produces brevetoxins that can harm marine animals and cause illness through contaminated seafood or airborne coastal aerosols. Nutrient-rich runoff, coastal conditions, winds, temperature, and ocean currents can all influence bloom development.
Scientists increasingly use the more precise terms “algal bloom” or “harmful algal bloom” when color or toxicity is not certain.