Why is fertilizer runoff a major APES issue?

• A. It decreases water temperature.
• B. It reduces sedimentation.
• C. It increases water clarity.
• D. It adds nitrates and phosphates to water, causing eutrophication and dead zones.

Answer: (D)

Fertilizer runoff is about nutrient pollution. Fertilizers deliver high levels of nitrogen (nitrates) and phosphorus (phosphates) to water bodies. These nutrients are the main drivers of algal and aquatic plant growth. When they enter streams, lakes, or coastal waters, they trigger rapid, often dense algal blooms. The problem isn’t just the algae themselves; as these blooms die off, bacteria decompose the dead matter and consume a lot of the dissolved oxygen in the water. That oxygen depletion creates hypoxic or even anoxic conditions, which can kill fish and other aquatic life and produce “dead zones.” So the key idea is that extra nitrates and phosphates fuel excessive growth, which leads to lower oxygen levels and harmed ecosystems.

Other options don’t capture this chain. Fertilizer runoff doesn’t generally lower water temperature or reduce sedimentation, and nutrient pollution often makes water murkier rather than clearer due to algal blooms reducing light penetration. The emphasis here is on how added nutrients drive eutrophication and oxygen depletion, not on temperature or clarity changes alone.