Why do fish die during eutrophication?

• A. Dissolved oxygen drops too low for them to survive.
• B. Water temperature increases significantly.
• C. Salinity becomes too high.
• D. pH becomes highly acidic.

Answer: (A)

When nutrients like nitrogen and phosphorus from runoff feed large algal blooms, the water becomes crowded with algae. After the blooms die, bacteria break down all that organic matter, and their activity uses up a lot of dissolved oxygen in the water. With less oxygen available, fish can’t breathe properly and may die. This loss of dissolved oxygen, especially in deeper or stratified parts of a lake, is the main reason fish die during eutrophication.

Other options aren’t the primary cause. Temperature can influence how much oxygen water can hold, but eutrophication’s immediate danger is the oxygen depletion from decomposition. Salinity changes aren’t a defining feature of freshwater eutrophication, and while pH can shift due to various factors, it’s not the typical driver of fish kills in this scenario.