How does agriculture typically impact surface water quality?

Agriculture typically impacts surface water quality primarily through the introduction of fertilizers and pesticides into the environment. These substances, while beneficial for crop production, can run off into nearby water bodies during rainfall or irrigation events. This runoff can lead to nutrient pollution, primarily from nitrogen and phosphorus found in fertilizers. An excess of these nutrients in surface waters can cause eutrophication, which leads to harmful algal blooms, oxygen depletion, and overall degradation of aquatic ecosystems. Pesticides can also contaminate water sources and harm aquatic life, further compromising water quality.

The other options do not adequately address the primary mechanisms through which agricultural practices can affect surface water quality. Providing shade to water bodies, reducing water usage in farming, or improving irrigation systems might have localized benefits or contribute to water conservation but do not have direct, widespread negative impacts like the use of fertilizers and pesticides.