What negative consequence can nutrient runoff into waterways cause?

Nutrient runoff into waterways is a significant environmental concern, primarily because it leads to the proliferation of algal blooms, which can drastically degrade water quality. When excess nutrients, particularly nitrogen and phosphorus from fertilizers, enter aquatic systems, they provide a rich food source for algae. This rapid growth can result in harmful algal blooms that deplete oxygen in the water, block sunlight, and produce toxins harmful to aquatic life and human health. Such blooms can lead to hypoxic conditions, where oxygen levels drop significantly, leading to fish kills and loss of biodiversity in aquatic ecosystems.

In contrast, options like increased soil fertility, enhanced biodiversity, and reduction in pest populations do not accurately reflect the negative impacts of nutrient runoff. While nutrient inputs can enhance soil fertility, the context of the question pertains to water quality and the environmental effects of runoff, which is detrimental. Enhanced biodiversity usually signifies a healthy ecosystem, while runoff generally depletes the diversity of aquatic life rather than enhancing it. Similarly, nutrient runoff does not contribute to the reduction of pest populations, making it crucial to connect nutrient management practices with their broader environmental consequences.