75. In a lake ecosystem, bottom-up effects (B) refers to control of a lower trophic level by the higher trophic levels and top down effects (T) refer to the opposite. In a lake with three trophic levels – Phytoplankton (P), Zooplankton (Z) and Carnivore (C),
    (1) P and C are controlled by B, and Z is controlled by T
    (2) P, Z and C are all controlled by T
    (3) P is controlled by B, Z is controlled by T and C is controlled by B
    (4) P is controlled by T, Z is controlled by B and C is controlled by T

Introduction

Lake ecosystems are dynamic environments where the abundance of organisms at one trophic level can be influenced by both those above and below in the food chain. These influences are known as top-down and bottom-up effects. Understanding who controls whom is essential for predicting ecosystem responses to change, managing fisheries, and conserving biodiversity.

What Are Top-Down and Bottom-Up Effects?

• Top-down effects (T):
  Control exerted by higher trophic levels on those below. For example, carnivorous fish (C) regulate zooplankton (Z) populations by predation, and zooplankton, in turn, control phytoplankton (P) by grazing.

• Bottom-up effects (B):
  Control exerted by lower trophic levels on those above. For instance, the abundance of phytoplankton (P) determines how much food is available for zooplankton (Z), which then affects carnivore (C) populations.

The Three-Trophic-Level Lake Food Chain

• Phytoplankton (P): Primary producers, base of the food web.

• Zooplankton (Z): Primary consumers, feed on phytoplankton.

• Carnivores (C): Secondary consumers, feed on zooplankton.

Who Controls Whom? Analyzing the Options

Let’s break down each option:

Option (1)

• P and C are controlled by B, and Z is controlled by T

  • Implies both the base and the top are regulated by resources below, while the middle is regulated by predation from above.

  • Not accurate for a classic three-level food chain.

Option (2)

• P, Z, and C are all controlled by T

  • Implies all levels are regulated by predation from above.

  • Not accurate, as primary producers are typically resource-limited, not predator-limited.

Option (3)

• P is controlled by B, Z is controlled by T, and C is controlled by B

  • Correct!

  • Phytoplankton (P): Controlled by nutrient availability (bottom-up).

  • Zooplankton (Z): Controlled by predation from carnivores (top-down).

  • Carnivores (C): Ultimately limited by the abundance of food (bottom-up).

Option (4)

• P is controlled by T, Z is controlled by B, and C is controlled by T

  • Suggests phytoplankton are predator-limited, which is not typical in lakes.

Why Option (3) Is Correct

• Phytoplankton (P): Their population size is set by the availability of nutrients and light (bottom-up).

• Zooplankton (Z): Their numbers are limited by predation from carnivores (top-down).

• Carnivores (C): Their population is limited by the availability of zooplankton (bottom-up).

This pattern is supported by classic ecological studies and is fundamental to understanding energy flow and population regulation in aquatic food webs.

Real-World Application

• Lake management: Knowing which trophic levels are controlled by top-down or bottom-up effects helps in designing interventions to reduce algal blooms or boost fish stocks.

• Conservation: Understanding these dynamics informs strategies for maintaining balanced ecosystems.

Conclusion

In a three-level lake food chain—phytoplankton, zooplankton, and carnivores—the correct pattern of control is:

• Phytoplankton (P): Controlled by bottom-up effects (nutrient availability)

• Zooplankton (Z): Controlled by top-down effects (predation)

• Carnivores (C): Controlled by bottom-up effects (food availability)

Correct answer:
(3) P is controlled by B, Z is controlled by T, and C is controlled by B