What causes a red tide?

• A. Harmful algal bloom
• B. Red tide
• C. Algal crust bloom
• D. Bacteria bloom

Answer: (A)

Red tide happens when a harmful algal bloom occurs. A bloom is an unusually large and dense population of microscopic algae in coastal waters, and in many red-tide events the algae (often pigmented dinoflagellates) color the water red. Some of these blooms release toxins that can harm marine life and pose risks to people who contact or ingest contaminated water or shellfish. While the water’s vivid color is striking, the underlying cause is the rapid growth of algae that makes it a harmful algal bloom. Terms like algal crust bloom or bacteria bloom don’t describe this phenomenon, and red tide is the common name for the visible result of an algal bloom.