Which essential nutrients can lead to undesirable aquatic life when discharged in excessive amounts?

The essential nutrients that can lead to undesirable aquatic life when discharged in excessive amounts are nitrogen and phosphorus. These two nutrients play a significant role in the growth of aquatic plants and algae. When they are present in excessive concentrations, often due to runoff from agriculture, sewage discharge, and urban stormwater, they can stimulate algal blooms.

These blooms can be detrimental to aquatic ecosystems for several reasons. First, they can lead to eutrophication, a process that depletes oxygen in the water as the algae die and decompose. This oxygen depletion can create "dead zones" where aquatic life, such as fish and invertebrates, cannot survive. Additionally, some algal blooms can produce toxins that are harmful to aquatic organisms and even to humans, leading to further complications in the water body’s health and its usability for recreational and drinking purposes.

While other options such as pesticides and herbicides can also negatively impact aquatic life, nitrogen and phosphorus are specifically recognized for their role in promoting excessive growth of algae and plants, making them key nutrients to monitor in water quality management.