How can fertilizer runoff from agriculture lead to eutrophication even if only a portion of the field loses runoff?

• A. Runoff has no effect unless the entire field fails.
• B. Runoff carries nitrates and phosphates into water bodies, triggering algal blooms.
• C. Runoff removes nutrients from water bodies.
• D. Runoff only affects soil, not water.

Answer: (B)

Fertilizers add nutrients such as nitrates and phosphates that plants use to grow. When rainfall or irrigation causes runoff, those nutrients can wash into nearby water bodies. In the water, they act like fertilizer for algae and other aquatic plants, triggering rapid algal blooms. As the algae multiply and then die, bacteria break down the dead material, which uses up a lot of dissolved oxygen. That drop in oxygen creates stressed or dead zones for fish and other aquatic life, and the water becomes less healthy overall. This sequence—nutrient-enriched runoff entering a water body, causing excessive algal growth, followed by oxygen depletion—explains eutrophication. Even a small amount of runoff can start this process because nutrients concentrate in the water and the effects cascade through growth and decay. The other ideas don’t fit: runoff doesn’t remove nutrients from water; it adds nutrients to water bodies; and it does affect water, not just soil.