Question

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

Answer 1

Eutrophication refers to an excess of nutrients in a body of water such a lake. Excess nutrients (N and/or P) can lead to algal blooms, which prevent sunlight from effectively penetrating the water.

Explanation:

In order for a body of water to be healthy, a balance must be reached when it comes to nutrients. Too little nutrients and you'll end up with nearly sterile lake, but too many nutrients and you end up with algal blooms that cover the water's surface, which ends up blocking sunlight from penetrating the water, thereby hindering the photosynthetic process in aquatic plants.

Also, algal blooms can cause hypoxic or low oxygen conditions. As you can imagine, this can cause a multitude of problems with aquatic life reliant on regular oxygen concentrations. If an algal bloom gets really out of control, what can end up happening is nearly all of the organisms either directly or indirectly reliant on normal concentrations of oxygen being present or just reliant on the aquatic plants reliant on sunlight being blocked by algal blooms. The can ultimately result in ecologically dead zones.

Also a bit unrelated but algal blooms also cause significant economic losses because they hinder watersports such as boating, and other recreational and commercial aspects such as fishing. Plus it just looks terrible!