76. In a lake, reducing the population of a fish which feeds on plankton was followed by a declinein the rate of primary productivity. This is consistent with which one of the following hypotheses regarding the regulation of primary productivity?
    (1) Bottom-up control (2) Eutrophication
    (3) Top-down control (4) Trophic pyramid

Introduction

Primary productivity—the rate at which phytoplankton and other producers convert sunlight and nutrients into biomass—is a key indicator of ecosystem health in lakes. Understanding what regulates this productivity is essential for managing fisheries, preventing algal blooms, and maintaining ecological balance. Two major hypotheses explain this regulation: bottom-up control, where nutrients and physical factors drive productivity, and top-down control, where consumers at higher trophic levels influence producers through food web interactions.

The Scenario: Fish, Plankton, and Productivity

In the given scenario, a lake experienced a reduction in the population of a fish species that feeds on plankton. This was followed by a decline in the rate of primary productivity. To interpret this, we need to consider how trophic interactions can cascade through the food web.

Food Web Structure

• Phytoplankton: Primary producers, responsible for photosynthesis and primary productivity.

• Zooplankton: Feed on phytoplankton.

• Planktivorous Fish: Feed on zooplankton.

Top-Down Control: The Trophic Cascade Explained

When planktivorous fish are reduced, their main prey—zooplankton—are released from predation and their populations increase. The increased zooplankton then graze more heavily on phytoplankton, reducing phytoplankton biomass and thus lowering primary productivity.

This is a classic example of a top-down trophic cascade:

• Less planktivorous fish → More zooplankton → Fewer phytoplankton → Lower primary productivity

Supporting Scientific Evidence

• Studies have shown that top-down control by consumers like zooplankton and fish can significantly influence phytoplankton biomass and primary production in lakes.

• Experimental manipulations in lakes and ponds consistently demonstrate that changing the abundance of higher trophic levels (like fish) can cascade down to affect primary producers.

Why Not Bottom-Up Control or Other Options?

• Bottom-up control (Option 1) refers to regulation by nutrients or physical factors, not by changes in consumer populations. In this scenario, the effect is initiated by a change at a higher trophic level, not by altering nutrient supply.

• Eutrophication (Option 2) is the enrichment of water bodies with nutrients, typically increasing primary productivity, not decreasing it.

• Trophic pyramid (Option 4) describes the structure of energy flow, not a regulatory mechanism.

Conclusion

The observed decline in primary productivity following the reduction of planktivorous fish is consistent with the top-down control hypothesis. This demonstrates how changes at higher trophic levels can cascade through the food web, ultimately impacting the abundance and productivity of primary producers.

Correct answer: (3) Top-down control