What are harmful algal blooms (HABs)?

• A. A reduction in algae growth due to nutrient scarcity.
• B. A process of nutrient removal by algae that improves water quality.
• C. Excessive growth of algae, often triggered by nutrient enrichment.
• D. A decline in algae due to nutrient limitation.

Answer: (C)

Harmful algal blooms are when algae grow excessively in water bodies, usually because nutrients from fertilizers, manure, or wastewater run off into the water provide more food for them. This rapid growth can be harmful because some blooms produce toxins that affect people, pets, and aquatic life, and when the blooms die, their decomposition can drain oxygen from the water, harming fish and other organisms.

The best description says there is excessive algae growth, often triggered by nutrient enrichment. That matches what happens in HABs: nutrient pollution fuels the rapid, large growth, and the result can be toxic or otherwise damaging to water quality and ecosystems.

Why the other ideas don’t fit: they describe decreases in algae or processes that improve water quality, which is the opposite of a bloom; HABs are about too much algae, not too little.