Which outcome is most commonly associated with runoff containing both nitrogen and phosphorus?

• A. Both nitrogen and phosphorus
• B. Nitrogen only
• C. Phosphorus only
• D. Carbon only

Answer: (A)

Nutrient pollution from runoff can trigger eutrophication, where excess nutrients fuel rapid growth of algae in water bodies. When runoff contains both nitrogen and phosphorus, algae and aquatic plants can proliferate more intensely because these nutrients together support high rates of growth. The resulting algal bloom reduces light penetration and, as the organisms die and decompose, bacteria consume a lot of oxygen, leading to hypoxic conditions that harm fish and other aquatic life. While nitrogen alone or phosphorus alone can drive some growth, the combination of both is most strongly associated with the pronounced bloom and oxygen depletion seen in eutrophied waters. Carbon by itself isn’t the limiting factor for these nutrient-driven blooms.