| Date | November 2016 | Marks available | 1 | Reference code | 16N.2.HL.TZ0.1 |
| Level | Higher level | Paper | Paper 2 | Time zone | TZ0 |
| Command term | Comment | Question number | 1 | Adapted from | N/A |
Question
Coral reefs are among the most spectacular ecosystems on Earth. They support a rich diversity of life and provide economic benefits to the people who use them. In Papua New Guinea in the Pacific Ocean north of Australia the following data were collected. Coral cover is the percentage of the reef surface covered by live hard coral.
In order to test the effect of temperature, live samples of a species of coral, Pocillopora damicornis, were placed in an experimental chamber at a constant pH, water depth and low light. All the coral samples were started at 26°C and half of them were rapidly increased to 30°C.
The pie charts show the percentage of live and dead tissues at the end of the experiment.
Acidification of the world’s oceans is an increasing threat to the health of oceanic life including coral reefs. Corals perform calcification to create their calcium carbonate exteriors. An experiment was conducted on Heron Island, Southern Great Barrier Reef, Australia. For the experiment the pH was altered by dissolving carbon dioxide in the water. Three different coral species were used, with each test group at two different temperature ranges and three different pH values. The white line in each photograph represents 5 cm.
Calculate the difference in coral cover in 1996 and 2002. No working required.
Describe the evidence that the ocean temperature has an effect on coral cover.
Suggest causes for the changes in ocean temperature.
Identify one advantage of conducting this experiment in the laboratory rather than in the ocean.
Comment on whether the experimental data supports the observed data from the ocean.
(i) Describe the trend in calcification when the pH is decreased at 25 –26°C.
(ii) In environmental studies, a critical value is the level at which a population declines or shows signs of poor health. Suggest a critical pH for P. onkodes.
(iii) Using all of the data, comment on the hypothesis that ocean acidification in warming seas will have the same effect on all species of coral.
Suggest another marine animal that has parts made of calcium carbonate and may therefore be damaged due to ocean acidification.
Outline causes of ocean acidification.
Discuss the need for international cooperation to solve the problems of declining coral populations.
Markscheme
60 (%)
a. coral cover decreases as temperature rises (between 1996 and 1998/2000 and 2002) / negative correlation between temperature and coral cover / coral cover highest when temperature is lowest/vice versa
b. coral cover remains constant when temperature drops (between 1998/1999 and 2000)/remains (nearly) constant when temperature stops rising (between 2002 and 2003)
c. no proof of causation / only a correlation / other factors could be affecting the coral
Do not award this mark for “inversely proportional”, but the mark can still be awarded if other parts of the answer give one of the alternative parts of the mark point.
a. increased carbon dioxide/methane in the atmosphere / carbon dioxide emissions from burning of fossil fuels / other specific source of a named greenhouse gas
b. increased greenhouse effect / more heat/long wave radiation trapped in the atmosphere
c. heat transfer from atmosphere to ocean / ocean absorbs heat from atmosphere
No marks for increased CO2 in the oceans, global warming or climate change.
The idea of an increase must be included, not just greenhouse effect or heat trapping.
control of variables/pH/light/temperature / no predators of coral
a. supports because there is more dead coral/less % cover at the higher temperature
b. (experimental data) does not support (observed data) because experimental temperatures were (all) higher/rose much faster
The answer must make it clear whether or not the data provides support.
(i) less calcification in all three/each species (as pH decreased)
(ii) 7.6 / 7.7 / 7.8
Accept any pH that is 7.6 or higher, but lower than 7.9.
(iii)
a. greater reduction in calcification as pH drops at the higher temperature in P. onkodes than on the other two species (so hypothesis not supported)
This answer is based on the larger drop in calcification between 8.2 and 7.6 at both temperatures in onkodes than the other two species.
b. net loss in calcification at lowest pH and highest temperature in P. onkodes whereas there is still calcification in the other two species (so hypothesis not supported)
This answer is based only on whether there are positive values for calcification or negative.
c. warming reduces calcification at all pH levels in A. intermedia but not in the other two species (so hypothesis not supported)
This answer is based on the drop in calcification at each pH when the temperature rises in intermedia, whereas in the other species there is a rise at one or more of the pHs.
d. combined effect of acidification and warming is a larger reduction inncalcification in A. intermedia than in the other two species (so hypothesis not supported)
This answer is based on the larger overall drop in calcification between pH 8.2 at 25/26°C and 7.6 at 28/29°C.
e. more calcification as temperature rises at lower pH/pH 7.9 and 7.6 in P. lobata whereas there is less in the other two species (so hypothesis not supported)
The answer must either state pHs 7.9 and 7.6 or specify lower pH or greater acidification.
f. more calcification as pH drops from 8.2 to 7.9 at higher temperature in P. lobata whereas there is a drop/no rise in the other two species (so hypothesis not supported)
The answer must state the two pH values and state higher temperature or 28-29°C.
Mollusca/named marine mollusc with a shell/crustacean/named marine crustacean/Porifera/sponges/named calcareous marine sponge
Reject terrestrial examples. Reject sea shells, shellfish. Specific named examples must be verified if it is uncertain whether they have calcified parts.
a. carbon dioxide makes an acid/carbonic acid in water
b. (carbon dioxide from) burning fossil fuels/forest fires
c. carbon dioxide forms solution with/dissolves into water/oceans/rain
Do not award a mark for stating only that carbon dioxide causes ocean acidification.
Do not award marks for methane sources or sources of unspecified greenhouse gases or statements about increased carbon dioxide in the atmosphere.
a. international cooperation needed to reduce carbon dioxide emission/concentrations
b. carbon dioxide produced anywhere increases the greenhouse effect/global warming/ocean acidification/health of coral everywhere
c. ocean currents/tides/wind move carbon dioxide/acid/heat around the world / oceans of the world are interconnected/part of one overall system
d. (some) coral reefs are in international waters (or words to that effect) / coral reefs cannot be protected by single national governments alone
e. the more groups of people/nations/corporations that reduce their carbon emissions, the lower the impact on coral will become / not enough for one country/group/corporation to reduce carbon dioxide emissions
f. sharing of technology/research/information/resources
g. aid to poorer/developing countries (to help with coral conservation)
h. reference to an economic/ecological benefit of conserving coral reefs
