Environmental Engineering – Minimum dissolved oxygen for aquatic life in rivers What is the minimum DO (ppm) generally required to sustain aquatic life?

Introduction / Context:
Dissolved oxygen (DO) is a primary indicator of river health. Fish and aerobic microorganisms require oxygen; when DO falls too low, stress and mortality occur, and septic conditions may develop. This question targets the commonly cited minimum DO for sustaining aquatic life in flowing waters.

Given Data / Assumptions:

• Riverine environment with typical temperature ranges.
• Focus on minimum threshold for survival, not optimal levels.
• DO expressed as mg/L (ppm).

Concept / Approach:

While desirable DO levels for healthy ecosystems are higher (often 5–8 mg/L or more), a widely used minimum survival threshold is about 4 mg/L. Below this, many fish species experience stress; prolonged exposure risks fish kills and anaerobic conditions.

Step-by-Step Solution:

Verification / Alternative check:

Field monitoring programs and fisheries guidance often flag 4 mg/L as a critical minimum, with advisories to maintain 5–6 mg/L where feasible.

Why Other Options Are Wrong:

0 mg/L is anoxic and fatal; 2 mg/L is hypoxic for most fish; 8 mg/L is a healthy target but not the minimum survival threshold.

Common Pitfalls:

Ignoring diurnal DO swings due to photosynthesis/respiration, or not accounting for higher oxygen demand downstream of wastewater discharges.

Final Answer:

4