Aim for FiveEnvironmental Science

Design an investigation

Ocean acidification and oyster larvae

  • Units 3 and 9
  • 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

A shellfish hatchery has had trouble raising young oysters in recent years. Marine scientists suspect that rising carbon dioxide (CO₂) in seawater is the cause.

Figure 1: Model of seawater carbonate chemistry

The model shows CO₂ moving from the atmosphere into the ocean surface. In the water, the following reactions are shown with arrows:

CO₂ + H₂O ⇌ H₂CO₃ (carbonic acid)

H₂CO₃ ⇌ H⁺ + HCO₃⁻ (hydrogen ion + bicarbonate)

H⁺ + CO₃²⁻ ⇌ HCO₃⁻ (hydrogen ion + carbonate ion forms bicarbonate)

A separate arrow shows oysters taking carbonate ions (CO₃²⁻) and calcium ions (Ca²⁺) from the water to build calcium carbonate (CaCO₃) shells.

Source: Simplified model

Planned investigation

The scientists will fill nine identical tanks with filtered seawater and add the same number of newly hatched oyster larvae to each. Air with a CO₂ concentration of 280 ppm will be bubbled through three tanks, 420 ppm through three tanks, and 800 ppm through three tanks. After 14 days, they will measure the shell length of 50 larvae from each tank.

Source: Hypothetical study design

Suggested time: 23 minutes

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Part (a)

1 point

Using Figure 1, describe how an increase in atmospheric CO₂ reduces the carbonate ions available to oysters.

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Part (b)

1 point

Identify the independent variable in this investigation.

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Part (c)

1 point

Identify the dependent variable in this investigation.

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Part (d)

1 point

State a testable hypothesis for this investigation.

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Part (e)

1 point

Identify one variable that should be kept the same in all nine tanks.

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Part (f)

1 point

Explain why the scientists included tanks with air at 280 ppm CO₂.

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Part (g)

1 point

An adult oyster releases millions of eggs, and most of the larvae die within their first few weeks. Identify the type of survivorship curve that describes oysters.

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Part (h)

1 point

Ocean temperatures are also rising. Describe one effect of ocean warming on coral reefs.

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Part (i)

1 point

Propose one realistic solution that would reduce ocean acidification.

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Part (j)

1 point

Explain how the solution you proposed in part (i) would reduce ocean acidification.

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