Which statement describes nutrient enrichment leading to dead zones?

• A. Sulfates
• B. Lead
• C. Nitrates and phosphates
• D. Mercury

Answer: (C)

Nutrient enrichment leads to dead zones through eutrophication: nitrates and phosphates from agricultural runoff, wastewater, and detergents fuel rapid algal growth. The resulting algal bloom uses a lot of oxygen when the algae die and sink, and bacteria that decompose the organic matter consume even more dissolved oxygen. The water becomes hypoxic or anoxic, so aquatic life that needs oxygen can’t survive, creating a dead zone. This is the classic sequence that links nutrient input to oxygen depletion in waterways, which is why nitrates and phosphates are the best fit.

Sulfates, lead, and mercury aren’t the nutrients that drive this oxygen-depletion process. Sulfates influence water chemistry in other ways, while lead and mercury are toxic contaminants that cause health and ecological problems rather than eutrophication-driven dead zones.