Analyze an environmental problem doing calculations

Mercury in a lake's fish

Units 1, 7 and 8
10 points
About 24 minutes

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

You work through an environmental scenario using numbers: set up and solve calculations (showing your work), then explain the science and propose solutions. Several of the 10 points come from the calculations, usually one for the setup and one for the answer. On the exam: Question 3 of 3 in the 70-minute free-response section (Section II is 40% of the exam score).

The question and its sources

A lake lies downwind of a coal-fired power plant. Biologists measured the mercury concentration in organisms at different trophic levels in the lake. A health agency sets a safe daily intake (reference dose) for mercury of 0.0001 mg per kg of body mass per day.

Table 1: Mercury concentrations in the lake's food chain

Organism	Trophic level	Mean mercury concentration (mg per kg of body mass)
Phytoplankton	Producer	0.004
Zooplankton	Primary consumer	0.020
Yellow perch	Secondary consumer	0.10
Walleye	Tertiary consumer	0.60

Source: Hypothetical data

Suggested time: 24 minutes

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Part (a)

2 points

Calculate how many times greater the mercury concentration is in walleye than in zooplankton. Show your work.

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Part (b)

2 points

A 60 kg adult eats 0.21 kg of walleye from the lake each week. Calculate the adult's average mercury intake in mg per kg of body mass per day. Show your work.

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Part (c)

2 points

Calculate the greatest mass of walleye, in grams, that the 60 kg adult could eat each week without going over the reference dose. Show your work.

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Part (d)

1 point

Explain why the mercury concentration increases at each higher trophic level in the lake.

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Part (e)

1 point

Describe how mercury released from the power plant reaches the lake.

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Part (f)

1 point

Describe one effect of mercury on human health.

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Part (g)

1 point

Propose one realistic way to reduce the amount of mercury entering the lake.

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