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Date May 2021 Marks available 2 Reference code 21M.2.HL.TZ1.3
Level Higher level Paper Paper 2 Time zone Time zone 1
Command term Explain Question number 3 Adapted from N/A

Question

The map shows the widespread distribution of coral reef ecosystems (indicated by black dots) in the world’s oceans. Death of coral reefs is related to increasing atmospheric carbon dioxide concentrations.

[Source: National Oceanic and Atmospheric Administration, 2021. Where Reef Building Corals Found. [map online] Available at: https://oceanservice.noaa.gov/education/tutorial_corals/media/supp_coral05a.html [Accessed 20 May 2021].]

Explain how increased atmospheric carbon dioxide concentrations can lead to coral death.

Markscheme

a. carbon dioxide dissolves in oceans/seawater ✔

b. carbonic acid formed/equation/lowers pH/makes water acidic ✔

c. prevents deposition of calcium carbonate/causes calcium carbonate to dissolve ✔

d. skeleton of (hard) corals degraded ✔

e. carbon dioxide is a greenhouse gas/causes warming/increases temperatures ✔

f. warmer oceans cause corals to expel zooxanthellae ✔

g. bleaching due to death/expulsion of mutualistic organisms/algae ✔

Allow zooxanthellae instead of algae in mpf.

Reject reacts and diffuses instead of dissolves in mpa.

Examiners report

Answers relating to global warming and therefore rising sea temperatures and also answers relating to ocean acidification were accepted. Many candidates knew that carbon dioxide causes acidification but fewer knew about the resulting problems with mineral deposition to form the skeleton in a hard coral.

Syllabus sections

Core » Topic 4: Ecology » 4.4 Climate change
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