How does runoff from agricultural activities typically affect surface water quality?

Runoff from agricultural activities can significantly impact surface water quality, primarily by introducing excessive nutrients and pesticides into the water bodies. When it rains, water that flows over agricultural fields can pick up fertilizers high in nitrogen and phosphorus, as well as chemicals like pesticides. These substances enter nearby rivers, lakes, or streams, leading to nutrient pollution.

Excess nutrients can cause algal blooms, which can deplete oxygen levels in the water as the algae die and decompose, potentially leading to hypoxic conditions that harm aquatic life. Pesticides can be toxic to various organisms and disrupt the entire ecosystem, contributing to a decline in biodiversity.

The other options do not accurately represent the primary effects of agricultural runoff on water quality. While runoff might have impacts on temperature or clarity, those are not the direct consequences as fundamentally as nutrient and pesticide pollution, making the introduction of these substances the most critical issue regarding surface water quality in relation to agricultural runoff.