What is eutrophication? Why would allowing nitrogen or phosphorous fertilizers to run into a body of water can negatively affect life in it?

An aquatic body needs a certain amount of nutrients in order to be healthy; too little and the lake will become almost completely sterile, but too many nutrients will cause algal blooms that cover the surface of the water, preventing sunlight from penetrating the water and impeding the photosynthetic process in aquatic plants.

Additionally, algal blooms can result in hypoxic or low oxygen conditions, which can cause a variety of issues for aquatic life dependent on regular oxygen concentrations. If an algal bloom gets out of control, almost all of the organisms that are either directly or indirectly dependent on the presence of normal oxygen concentrations or simply dependent on the aquatic plants that depend on sunlight being blocked by algal blooms may end up in ecological dead zones.

A little off topic, but algal blooms also seriously impair boating and other water sports, as well as other recreational and commercial activities like fishing, and they look awful! Algal blooms also result in large financial losses.

Eutrophication is the process where excessive nutrients, particularly nitrogen and phosphorous from fertilizers, enter a body of water, leading to increased plant growth. This excess vegetation consumes oxygen during decomposition, depleting oxygen levels in the water. Consequently, aquatic life is negatively affected as oxygen depletion hampers the survival of fish and other organisms.