74. Two lakes (l and ll) with a similar trophic structure ofphytoplankton-zooplankton, planktivorous fish foodchain were chosen. To understand the ‘top-down’ effects, some piscivorous fish (those that feed on planktivorous fish) were introduced into Lake l, making
    it a system with four trophic levels. Lake ll was enriched by adding large quantities of nitrates and phosphates to study the ‘bottom-up’ effects over a period of time. Changes in the biomasses of each trophic level were measured.
    The expected major changes in the two lakes are
    (1) In Lake l zooplankton biomass increases, phytoplankton biomass decreases. In Lake ll both
    phytoplankton and planktivorous fish biomasses increase.
    (2) In Lake l zooplankton biomass decreases, phytoplankton biomass increases. In Lake ll both
    phytoplankton and planktivorousfish biomasses increase.
    (3) In Lake l planktivorous fish biomass and phytoplankton biomass decrease. In Lake ll
    phytoplankton biomass increases, planktivorous fish biomass decreases.
    (4) In Lake l planktivorous fish and zooplankton biomasses increase. In Lake ll both
    phytoplankton and planktivorus fish biomasses increase.

Introduction

Aquatic ecosystems are shaped by both top-down and bottom-up forces. Understanding how changes at one trophic level ripple through the food web is crucial for ecology, fisheries management, and conservation. In this scenario, two lakes with similar food chains were manipulated to study these effects:

• Lake I: Introduction of piscivorous fish (top-down effect)

• Lake II: Enrichment with nitrates and phosphates (bottom-up effect)

Let’s analyze how these interventions are expected to change the biomasses of phytoplankton, zooplankton, and planktivorous fish in each lake.

Lake I: Top-Down Effects (Addition of Piscivorous Fish)

Food Chain Before:
Phytoplankton → Zooplankton → Planktivorous Fish

After Adding Piscivorous Fish:
Piscivorous Fish → Planktivorous Fish → Zooplankton → Phytoplankton

What Happens?

• Piscivorous fish eat planktivorous fish, reducing their numbers.

• With fewer planktivorous fish, zooplankton face less predation and their biomass increases.

• More zooplankton feed on phytoplankton, causing phytoplankton biomass to decrease.

Summary:

• Zooplankton biomass increases

• Phytoplankton biomass decreases

Lake II: Bottom-Up Effects (Nutrient Enrichment)

Food Chain:
Phytoplankton → Zooplankton → Planktivorous Fish

What Happens?

• Adding nitrates and phosphates boosts phytoplankton growth (primary producers).

• More phytoplankton supports more zooplankton and, in turn, more planktivorous fish.

• All upper trophic levels benefit from increased energy at the base.

Summary:

• Phytoplankton biomass increases

• Planktivorous fish biomass increases

Comparing the Options

Option (1) is correct.

Why These Changes Occur

Top-Down Control (Lake I)

• Trophic cascade: Removing or reducing a predator (or adding a higher-level predator) causes a chain reaction down the food web.

• Result: Each lower trophic level alternates in response (increase, decrease, increase, etc.).

Bottom-Up Control (Lake II)

• Resource-driven: Increasing resources at the base of the food web benefits all higher levels.

• Result: Biomass increases at all trophic levels.

Conclusion

In Lake I, zooplankton biomass increases and phytoplankton biomass decreases due to top-down control by piscivorous fish. In Lake II, both phytoplankton and planktivorous fish biomasses increase due to nutrient enrichment.
These outcomes demonstrate the classic effects of top-down and bottom-up forces in aquatic ecosystems, highlighting the interconnectedness of food webs and the importance of both predation and resource availability in shaping community structure.

Correct Answer:
(1) In Lake I zooplankton biomass increases, phytoplankton biomass decreases. In Lake II both phytoplankton and planktivorous fish biomasses increase.