Aim for FiveBiology

Analyze a model or visual representation

Energy flow in a lake

  • Unit 8
  • 4 points
  • About 10 minutes

You can use a calculator on this question, just like on exam day.

A short question built around a model or diagram of a biological process. You describe what the model shows, explain how its parts relate, add to or use the model, and connect it to a bigger biological idea. On the exam: Question 5 of 6 in Section II; about 10 minutes suggested.

The question and its sources

Figure 1 is a model of energy flow through a food chain in a lake.

Figure 1 (described)

The model shows four boxes stacked from bottom to top, connected by upward arrows: phytoplankton (producers) at the bottom, then zooplankton, then small fish, then large fish at the top.

The number in each box is the energy stored in new biomass at that level each year: phytoplankton 8,000 kJ/m²/yr (net primary productivity), zooplankton 880 kJ/m²/yr, small fish 90 kJ/m²/yr and large fish 8.5 kJ/m²/yr.

From the side of every box, a wavy arrow labeled 'Heat' points out of the ecosystem. From every box, a straight arrow labeled 'Dead matter and wastes' points to a box labeled 'Decomposers'. From the decomposers box, an arrow labeled 'Nutrients' (such as nitrate and phosphate) points back to the phytoplankton box.

Source: Simplified model; values are hypothetical

Suggested time: 10 minutes

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Part (a)

1 point

Describe what the arrows labeled 'Heat' in Figure 1 represent.

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Part (b)

1 point

Using Figure 1, calculate the percent of the energy in the zooplankton level that is transferred to the small fish level.

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Part (c)

1 point

Using Figure 1, explain why this food chain has only four trophic levels.

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Part (d)

1 point

In Figure 1, nutrients return from the decomposers to the phytoplankton, but no arrow returns energy from the decomposers to the phytoplankton. Explain this difference.

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