Aquatic ecology — The heavily polluted, highly organic waste–rich zone of a water body is properly termed which saprobic zone?

Introduction / Context:
The saprobic system classifies stretches of rivers and reservoirs according to organic pollution load and biological indicators. Correct terminology distinguishes heavily polluted reaches from moderately and slightly polluted waters. This is essential for biomonitoring, wastewater impact assessment, and ecological restoration planning.

Given Data / Assumptions:

• “Heavily polluted” implies high biochemical oxygen demand and low dissolved oxygen.
• Indicator taxa shift from sensitive to tolerant species as pollution increases.
• Saprobic zones progress from polysaprobic → mesosaprobic → oligosaprobic as water quality improves downstream.

Concept / Approach:
Polysaprobic waters contain abundant easily degradable organic matter. Microbial respiration is intense, causing hypoxia and favoring tolerant organisms. Mesosaprobic zones indicate moderate loads with partial recovery. Oligosaprobic denotes slight pollution or near-clean conditions with higher oxygen and sensitive biota.

Step-by-Step Solution:

Verification / Alternative check:
Classic limnology texts and bioindicator protocols use these exact zone names and associate taxa to each class.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing similar-sounding terms (polysaprobic vs mesosaprobic) or mixing saprobity with other water quality classifications like salinity or trophic state.

Final Answer:
Polysaprobic zone (heavily polluted, low dissolved oxygen).