What effect does excessive runoff have on local water bodies?

Excessive runoff has significant implications for local water bodies, primarily by transporting nutrients such as nitrogen and phosphorus from land to water systems. When there is heavy rainfall or rapid snowmelt, water can carry these nutrients from agricultural fields, urban areas, and other sources into lakes, rivers, and streams. This influx of nutrients can lead to nutrient pollution, which disrupts the natural balance of aquatic ecosystems.

As nutrient levels rise, they can stimulate rapid growth of algae, a phenomenon known as algal blooms. These blooms can deplete oxygen levels in the water, making it difficult for fish and other aquatic organisms to survive. Additionally, some types of algae can produce toxins, posing risks to both human health and wildlife. The consequences of algal blooms can be severe, affecting water quality, recreational usage, and overall ecosystem health.

In contrast, enhancing aquatic biodiversity, increasing water clarity, and benefiting aquatic plant growth are generally associated with a balanced environment, where nutrient levels are within natural limits. Excessive runoff disrupts that balance, leading to the degradation of water body health. Therefore, the correct choice highlights how excessive runoff can lead to adverse ecological effects, particularly through nutrient pollution and subsequent algal blooms.