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Difficulty: MediumEnergy Flow, Food Chains, and Ecological Pyramids

A biological study of a freshwater lake ecosystem recorded the trophic interactions among several species. Arrange the following organisms in sequence from the HIGHEST available energy content to the LOWEST available energy content.

  1. 1Phytoplankton (Microscopic Algae)
  2. 2Zooplankton (Daphnia)
  3. 3Small Fish (Tilapia)
  4. 4Fish Eagle (Apex Predator)

Answer

The correct sequence from highest to lowest available energy is Phytoplankton, followed by Zooplankton, Small Fish (Tilapia), and finally the Fish Eagle.
Primary producers (phytoplankton) capture solar radiation directly to generate biomass, making them the largest energy reservoir in the food web. At each subsequent trophic transfer—from primary consumers (zooplankton) to secondary consumers (small fish) and tertiary consumers (fish eagle)—approximately 90% of the energy is lost to metabolic processes, respiration, and heat. Consequently, total energy availability decreases sequentially from the lowest trophic level to the highest.

Step-by-Step Solution

1
Determine the trophic level for each organism in the lake ecosystem.
Phytoplankton are primary producers (Trophic Level 1), zooplankton are primary consumers (Trophic Level 2), small fish are secondary consumers (Trophic Level 3), and fish eagles are tertiary consumers (Trophic Level 4).
Energy enters an ecosystem at the producer level and flows unidirectionally up consumer levels.
2
Apply the second law of thermodynamics / 10% energy transfer rule across trophic levels.
Only approximately 10% of stored chemical energy is transferred from one trophic level to the next, while about 90% is dissipated as metabolic heat and unconsumed waste.
Energy availability decreases progressively as energy is lost at each metabolic transfer step.
3
Order the organisms from maximum available energy to minimum available energy.
Phytoplankton → Zooplankton → Small Fish (Tilapia) → Fish Eagle.
Lower trophic levels always store significantly more energy than higher trophic levels.

Key Concept

Unidirectional energy flow and thermodynamic energy dissipation across trophic levels
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Topics

Energy Flow, Food Chains, and Ecological Pyramids

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