Design an investigation

Acid deposition and two mountain lakes

Units 1 and 7
10 points
About 23 minutes

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

You read about a real-world environmental situation with a described figure or model, then pick out the parts of a scientific investigation (hypothesis, variables, controls), explain the science behind it, and propose solutions to the problems it raises. On the exam: Question 1 of 3 in the 70-minute free-response section (Section II is 40% of the exam score).

The question and its sources

Two lakes in the same mountain range lie about 60 km downwind of several coal-fired power plants. Biologists noticed that one lake has far fewer mayflies, whose aquatic young (nymphs) are an important food for trout.

Figure 1: Cross-section of the two lakes

The diagram shows a west-to-east cross-section. On the far left, smokestacks release gases, and arrows show prevailing winds carrying them east toward the mountains. Rain clouds form over the mountains, and rain falls on both lakes and their watersheds.

North Lake sits in a basin of granite bedrock with thin, sandy soil. Its pH is 5.0.

South Lake sits in a basin of limestone bedrock (calcium carbonate). Its pH is 7.4.

Rain falling on both watersheds has a pH of about 4.6.

Source: Hypothetical diagram

Planned laboratory investigation

Students will collect mayfly nymphs from a stream with pH 7.0 and place 20 nymphs in each of five aquariums. The water in the aquariums will be adjusted to pH 7.0, 6.0, 5.5, 5.0 and 4.5. After 10 days, the students will count the nymphs that are still alive in each aquarium.

Source: Hypothetical study design

Suggested time: 23 minutes

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Part (a)

1 point

Identify one primary air pollutant released by coal-fired power plants that contributes to acid deposition.

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Part (b)

1 point

Using Figure 1, explain why South Lake has a much higher pH than North Lake even though both receive the same acid rain.

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Part (c)

1 point

Identify the independent variable in the students' investigation.

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Part (d)

1 point

Identify the dependent variable in the students' investigation.

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Part (e)

1 point

Identify the control group in the students' investigation.

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Part (f)

1 point

Identify one variable that the students should keep the same in all five aquariums.

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Part (g)

1 point

State a testable hypothesis for the students' investigation.

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Part (h)

1 point

Explain how the low pH of North Lake could affect the lake's trout population, other than by directly harming the trout.

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Part (i)

1 point

Propose one realistic solution that the power plants could use to reduce acid deposition.

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Part (j)

1 point

Explain how the solution you proposed in part (i) would reduce acid deposition.

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