Which statement best describes eutrophication in freshwater systems?

• A. Decrease in nutrients and clearer water.
• B. Salinization from ocean water intrusion.
• C. Nutrient enrichment (N and P) leading to algal blooms and oxygen depletion; often from fertilizer runoff and sewage.
• D. Increased dissolved oxygen due to cooler water.

Answer: (C)

Eutrophication in freshwater systems happens when a water body receives excess nutrients, especially nitrogen and phosphorus, which fuel rapid growth of algae and aquatic plants. This nutrient boost often comes from fertilizer runoff, sewage, and other waste entering lakes, rivers, or ponds. The resulting algal blooms shade and alter the ecosystem, and when the blooms die, decomposers use up a lot of dissolved oxygen. That depletion of oxygen can create dead zones that stress or kill fish and other aquatic life. So the best description is nutrient enrichment (nitrogen and phosphorus) that leads to algal blooms and oxygen depletion, commonly from fertilizer runoff and sewage. The other ideas don’t fit: eutrophication is not about a drop in nutrients or clearer water, it’s the opposite; salinization from ocean intrusion involves salt entering the system, not nutrient-driven growth; and eutrophication tends to reduce, not increase, dissolved oxygen.