Final answer:
Fish die-offs in eutrophic lakes are directly caused by the severe depletion of dissolved oxygen due to excessive algae growth, which is fueled by an overabundance of nutrients like nitrogen and phosphorus.
Step-by-step explanation:
The direct cause of fish die-offs in highly eutrophic lakes is the severe depletion of dissolved oxygen in the water. This occurs when there is an overabundance of limiting nutrients, such as nitrogen and phosphorus, often from sources like fertilizer runoff and sewage.
These nutrients lead to excessive algae growth, known as algal blooms, which can cover the water's surface and reduce light penetration. When the algae eventually die and decompose, this process uses up a significant amount of the water's oxygen, leaving insufficient oxygen for fish and other aquatic organisms, leading to their death.