Minimum dissolved oxygen for healthy aquatic life For sustaining fish and typical freshwater biota, the dissolved oxygen (DO) concentration in water should not fall below approximately how many ppm (mg/L)?

Introduction / Context:
Dissolved oxygen (DO) is a primary indicator of water body health. Adequate DO supports aerobic metabolism for fish, macroinvertebrates, and beneficial microbes. Wastewater discharges with high BOD can depress DO and trigger fish kills. Environmental standards therefore specify minimum DO thresholds for various water uses.

Given Data / Assumptions:

• Freshwater stream or lake supporting aquatic life.
• Typical temperature conditions; exact DO saturation varies with temperature and salinity.
• Target is a practical minimum to avoid stress and mortality.

Concept / Approach:
Most water quality criteria recommend maintaining DO at or above about 5 mg/L to prevent acute stress for many fish species. Sensitive life stages (eggs/larvae) often require higher levels, but 5 mg/L is a commonly cited general minimum for warm-water fisheries. Values in the hundreds or thousands of mg/L are physically unrealistic; saturation at room temperature is roughly 7–10 mg/L.

Step-by-Step Solution:

Verification / Alternative check:
Field guidelines and regulatory criteria commonly use 5–6 mg/L for warm waters and higher values for cold-water species, confirming the order of magnitude.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing mg/L with percent saturation; always account for temperature and salinity when comparing to saturation values.

Final Answer:
5