Final answer:
Eutrophication is the process whereby oxygen is depleted by the growth of microorganisms due to excess nutrients in aquatic systems. This results in dead zones and the death of ecosystem fauna.
Step-by-step explanation:
The process whereby oxygen is depleted by the growth of microorganisms due to excess nutrients in aquatic systems is called eutrophication. Excessive nutrients, such as phosphorus and nitrogen from fertilizer runoff and sewage, lead to the rapid growth of microorganisms, particularly algae. This growth depletes the dissolved oxygen levels in the water, causing the death of many ecosystem fauna and resulting in dead zones in lakes and at the mouths of major rivers.