Final answer:
Eutrophication depletes oxygen levels in water, causing the death of aquatic animals by creating hypoxic conditions or 'dead zones' due to excessive algae growth from nutrient runoff.
Step-by-step explanation:
Eutrophication is a dangerous process for aquatic animals because it depletes the ecosystem of oxygen. This process, often resulting from excessive nutrient runoff from fertilizers and sewage, leads to the overgrowth of microorganisms, specifically algae, which in turn consume a large amount of dissolved oxygen through decay and respiration, creating hypoxic conditions. When the oxygen level in the water drops below a critical level, it can result in anoxic environments, or 'dead zones,' where aquatic life such as fish, shellfish, and finfish cannot survive. Over time, this can lead to severe reductions in water quality and significant impacts on the entire aquatic ecosystem.