77. A small lake has three trophic levels- phytoplankton(autotrophs),Zooplankton (herbivore)
    and planktivorous fish (primary carnivore). Into this lake, a population of piscivorous fish (secondary carnivore) was introduced to study the ‘top-down’ effects”. What is the expected long-term consequence of such an introduction to phytoplankton and zooplankton trophic levels? 
    

(1) Zooplankton biomass will increase and phytoplankton biomass will decrease.
(2) Zooplankton biomass will decrease and phytoplankton biomass will increase.
(3) The biomasses of both zooplankton and phytoplankton will increase.
(4) The biomasses of both zooplankton and phytoplankton will decrease.

Introduction

Aquatic ecosystems are shaped by complex interactions between organisms at different trophic levels. When a new predator is introduced, especially at the top of the food chain, it can trigger a cascade of ecological changes. This phenomenon, known as top-down control, is well-documented in lake ecosystems. In this article, we examine what happens to zooplankton and phytoplankton biomasses when piscivorous fish are introduced into a lake with an existing food web of phytoplankton, zooplankton, and planktivorous fish.

The Lake Food Web: Before and After

Original Structure

• Phytoplankton (autotrophs): Base of the food web, primary producers.

• Zooplankton (herbivores): Feed on phytoplankton.

• Planktivorous fish (primary carnivores): Feed on zooplankton.

After Introduction

• Piscivorous fish (secondary carnivores): Feed on planktivorous fish, adding a new top predator.

The Top-Down Cascade: What Science Shows

When piscivorous fish are introduced:

• Planktivorous fish populations decline due to predation from piscivores.

• With fewer planktivorous fish, zooplankton experience less predation and their populations increase.

• The increase in zooplankton leads to greater grazing pressure on phytoplankton, resulting in a decrease in phytoplankton biomass.

This series of changes is a classic example of a trophic cascade, where effects initiated at the top of the food web ripple down to impact lower trophic levels.

Evidence from Research

• Studies in lakes worldwide confirm that introducing piscivorous fish reduces planktivorous fish, increases zooplankton biomass, and decreases phytoplankton biomass, improving water clarity and altering ecosystem structure.

• Field experiments and long-term monitoring provide robust support for this top-down control model, highlighting its importance in lake management and restoration.

Expected Long-Term Consequence

Zooplankton biomass will increase and phytoplankton biomass will decrease.

This outcome is due to reduced predation on zooplankton (by planktivorous fish), which allows zooplankton populations to grow. The larger zooplankton populations then consume more phytoplankton, leading to lower phytoplankton biomass.

Why This Matters

• Water Quality: Lower phytoplankton biomass often means clearer water and fewer algal blooms.

• Biodiversity: Changes in predator-prey dynamics can alter community composition and ecosystem function.

• Lake Management: Understanding these cascades helps managers use biomanipulation (like stocking piscivores) to control algal blooms and improve water quality.

Conclusion

Introducing piscivorous fish into a lake ecosystem triggers a powerful top-down cascade: zooplankton biomass increases and phytoplankton biomass decreases. This well-established ecological principle is central to understanding and managing aquatic food webs.

Correct Answer:
(1) Zooplankton biomass will increase and phytoplankton biomass will decrease.