In the classic stream self-purification profile below a wastewater outfall, in which zone do algae typically die out due to lack of light and dissolved oxygen?

Introduction / Context:
After sewage is discharged to a river, the water quality changes longitudinally through characteristic zones: degradation, active decomposition, recovery, and clean water. Understanding biota responses (like algal presence) helps interpret DO sag curves and biological indices.

Given Data / Assumptions:

• Typical warm-season conditions with significant organic load.
• Limited reaeration immediately downstream of outfall.
• Standard zone nomenclature from stream sanitation theory.

Concept / Approach:

In the active decomposition zone, conditions are often anaerobic or severely oxygen-depleted. Black color, odours (H2S), and high reducing conditions prevail. Photosynthetic organisms like algae need light and dissolved oxygen; they typically die out or are greatly suppressed in this zone. They reappear downstream as reaeration restores DO in the recovery zone.

Step-by-Step Solution:

Verification / Alternative check:

DO sag models (Streeter–Phelps) show minimum DO within or near active decomposition; field observations align with low algal biomass there, increasing toward recovery.

Why Other Options Are Wrong:

(a) Degradation zone still has some DO and turbulence; algae need not “die out.” (c) Recovery zone sees improving DO and algal resurgence. (d) Cleaner water zone supports normal aquatic life.

Common Pitfalls:

Confusing “degradation” with “decomposition,” and assuming the lowest DO is immediately at the outfall rather than further downstream where BOD exertion peaks.

Final Answer:

Zone of active decomposition