| Date | May 2017 | Marks available | 1 | Reference code | 17M.2.SL.TZ1.1 |
| Level | Standard level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | Outline | Question number | 1 | Adapted from | N/A |
Question
Native oyster populations are decreasing where rivers meet the ocean along the northwest coast of North America. These oyster populations are being attacked by a gastropod.
It is known that oysters and gastropods have hard parts composed of calcium carbonate and that ocean acidification is increasing. Studies were carried out using juvenile oysters and gastropods to investigate the effects of acidification on the decrease in the population of oysters.
The first step was to raise oysters in two different mesocosms. One had seawater at a normal concentration of CO2 and the other had sea water with a high concentration of CO2. Gastropods were raised in two further mesocosms with normal and high CO2 concentrations respectively.
A juvenile gastropod will attack a juvenile oyster by using its tongue-like structure (radula) to drill a hole through the oyster shell. Once the hole has been drilled, the gastropod sucks out the soft flesh. Researchers investigated the shell thickness at the site of the drill hole in relation to the size of the oyster. The results are seen in this graph.
Equal numbers of oysters raised in seawater with a normal CO2 concentration and in seawater with a high CO2 concentration were then presented together to the gastropod predators in seawater with a normal CO2 concentration. The same numbers of oysters from the two groups were also presented together to the gastropods in seawater with a high CO2 concentration. The bar charts show how many of the oysters were drilled by the gastropods and the mean size of drilled oysters.
Outline how acidified sea water could affect the shells of the oyster.
Outline the trends shown in the data in the graph.
Estimate how much smaller drilled oysters raised in seawater at a high CO2 concentration were than drilled oysters raised in seawater at a normal CO2 concentration.
Deduce from the data in the bar charts which factors were and were not correlated significantly with the number of oysters drilled by the gastropods.
Suggest reasons for the differences in the numbers of oysters drilled, as shown in the bar charts.
The radula in a gastropod is hard but not made of calcium carbonate. Outline how this statement is supported by the drilling success of the gastropods in seawater with normal or high CO2 concentrations.
Using all the data, evaluate how CO2 concentrations affect the development of oysters and their predation by gastropods.
Markscheme
Shells might dissolve/deteriorate / become smaller/thinner/weaker / OWTTE
OR
shell formation reduced / more difficult
a. positive correlation between shell thickness and shell size
OR
as shell thickness increases, shell size «also» increases
b. (positive correlation) occurs at two different CO2 concentrations / both high and normal concentrations
c. trend for thickness is «slightly» lower with high CO2
«approximately» 0.2 mm2
OR
«approximately» 40 % «smaller»
unit required
a. significant factor: concentration of CO2 in which oysters were raised
b. insignificant factor: concentration of CO2 at which oysters were presented to gastropods
a. (because) shells are thinner/smaller when the oyster is raised in high CO2/lower pH
OR
«because» lower pH/higher acidity prevents/reduces deposition of calcium carbonate
b. gastropods target smaller/thinner-shelled oysters more
c. gastropods can eat/drill thin-shelled/smaller oysters at a faster rate (and move onto another)
d. eating smaller oysters «from high CO2 environments» means given population of gastropods require more oysters for same food intake
a. data shows that similar numbers are drilled regardless of conditions
b. since radulas are not affected by acidification
OR
radulas not made of calcium carbonate so (remain) strong/successful at drilling
a. the data/trend lines indicate that a higher CO2 concentration diminishes the shell thickness, making gastropod predation more successful
OR
the bar graphs suggest that oysters raised in a higher CO2 concentration are smaller, making gastropod predation more successful
b. CO2 concentrations «during feeding» do not change the occurrence of drilling/predation «by gastropods»
c. «limitation» no information about how exaggerated the CO2 concentrations were
OR
«limitation» no information about numbers of gastropods used «in each setting»
