What consequence arises from excess fertilizer runoff increasing nitrates into water?

• A. Algal blooms and oxygen depletion in aquatic systems
• B. Increased water clarity and oxygen saturation
• C. Reduced nutrient cycling efficiency in soils
• D. Decreased plant growth in terrestrial ecosystems

Answer: (A)

Nitrates from excess fertilizer runoff fuel rapid growth of algae and other aquatic plants, causing algal blooms. This huge bloom blocks light and, when it dies, bacteria decompose the dying matter, using up dissolved oxygen in the water. The resulting low oxygen levels harm aquatic life and can create dead zones. This sequence—nutrient-driven algal blooms leading to oxygen depletion—is eutrophication.

Other options don’t fit because they describe outcomes opposite or irrelevant to water bodies: increased water clarity is unlikely with big algal blooms, and the question focuses on aquatic consequences rather than soil nutrient cycling or terrestrial plant growth.