When environmental pollutants severely impact a river ecosystem, the ultimate mechanism leading to a “dead” river most directly results from: (Identify the proximal cause that collapses aerobic aquatic life.)

Introduction:
River “death” refers to the collapse of aquatic life due to severe pollution. This question probes the immediate physiological and ecological cause of fish kills and biodiversity loss commonly observed during eutrophication and organic pollution events.

Given Data / Assumptions:

• Pollutants may include nutrients (N, P), organic load (high BOD), and toxicants.
• Microbial decomposition consumes dissolved oxygen (DO).
• Fish and many invertebrates require minimum DO levels to survive.

Concept / Approach:
While algal blooms and sedimentation are contributing processes, the proximal trigger of mass mortality is often hypoxia or anoxia—dissolved oxygen depletion—caused by microbial respiration (oxidation of organic matter) and algal die-offs. Low DO disrupts respiration in aquatic organisms, leading to asphyxiation.

Step-by-Step Solution:
Connect pollution inputs to BOD and algal blooms. Recognize that decomposition of blooms elevates oxygen demand. Identify hypoxia/anoxia as the immediate limiting factor for aerobic life. Select the option that names oxygen depletion explicitly.

Verification / Alternative check:
Field measurements during fish kills show DO dropping below critical thresholds (<2–3 mg/L), aligning with the mechanism of mortality.

Why Other Options Are Wrong:

• Overpopulation of algae: Contributes upstream but mortality is due to subsequent DO crash.
• Toxic proteins: Specific toxins may occur, but generally DO depletion explains widespread kills.
• Buildup of sediment: Can alter habitat but is not the immediate cause of acute die-offs.

Common Pitfalls:
Confusing drivers (nutrients/algae) with the proximate cause (oxygen depletion) that actually kills fish.

Final Answer:
the depletion of oxygen is the ultimate mechanism leading to river “death.”