QCAA Marine Science Changes on the reef
15 sample questions with marking guides and sample answers
Explain how land-based flooding impacts the salinity and turbidity of a nearby reef.
Reveal Answer
Flooding increases freshwater plumes that result in decreased salinity. It also results in increased turbidity due to increased suspended sediments.
| Descriptor | Marks |
|---|---|
Identifies that flooding increases fresh water and decreases salinity | 1 |
Identifies that increased flooding increases turbidity due to increased suspended sediment | 1 |
Describe the impact that changes in turbidity from land-based flooding would have on the zooxanthellae and coral of a nearby reef.
Reveal Answer
Increased turbidity would decrease light availability. Insufficient light decreases the rate of photosynthesis by zooxanthellae. Increased turbidity would also increase sedimentation, which would smother both the zooxanthellae and the coral.
Decreased light and increased sediment cause the zooxanthellae to be expelled from the coral, resulting in bleaching.
| Descriptor | Marks |
|---|---|
Identifies that light availability decreases | 1 |
Indicates that rate of photosynthesis in zooxanthellae will decrease | 1 |
Identifies sediment increases and will smother coral and zooxathallae | 1 |
Indicates that zooxanthellae will be expelled and coral will bleach | 1 |
Identify the relationship between atmospheric CO concentration, ocean pH and temperature.
Increased atmospheric temperature has led to increased atmospheric CO concentration, ocean temperature and pH.
Decreased ocean pH has led to increased atmospheric temperature and decreased atmospheric CO concentration.
Increased atmospheric CO concentration has led to decreased ocean pH and increased ocean temperature.
Decreased ocean temperature has led to increased atmospheric CO concentration and ocean pH.
Reveal Answer
Increased atmospheric temperature has led to increased atmospheric CO concentration, ocean temperature and pH.
Incorrect. Increased atmospheric is the primary driver of rising temperatures, not the reverse. Additionally, higher levels cause ocean acidification, which decreases ocean pH rather than increasing it.
Decreased ocean pH has led to increased atmospheric temperature and decreased atmospheric CO concentration.
Incorrect. Decreased ocean pH is a direct result of increased atmospheric dissolving into seawater, not the cause of atmospheric temperature or changes.
Increased atmospheric CO concentration has led to decreased ocean pH and increased ocean temperature.
Correct. Higher atmospheric enhances the greenhouse effect, raising ocean temperatures, and dissolves into the water to form carbonic acid, which lowers ocean pH.
Decreased ocean temperature has led to increased atmospheric CO concentration and ocean pH.
Incorrect. Global ocean temperatures are currently increasing, not decreasing. Furthermore, rising atmospheric leads to a decrease in ocean pH, not an increase.
Which of the following strategies improves the short-term resilience of coral reefs against ocean acidification?
monitoring coastal run-off, such as nitrogen inputs
investigating the effect of CO on planktonic organisms
promoting the growth of calcium carbonate organisms to buffer the system
implementing catchment management plans to reduce nutrient load in run-off
Reveal Answer
monitoring coastal run-off, such as nitrogen inputs
Monitoring provides valuable data but is a passive action; it does not actively reduce stressors or improve the resilience of the reef.
investigating the effect of CO on planktonic organisms
Investigating the effects of CO is a research activity that builds knowledge, but it does not directly mitigate the impacts of ocean acidification on coral reefs.
promoting the growth of calcium carbonate organisms to buffer the system
Ocean acidification decreases the availability of carbonate ions, making it inherently difficult for calcium carbonate organisms to grow, so this is not a practical short-term mitigation strategy.
implementing catchment management plans to reduce nutrient load in run-off
Actively reducing local stressors, such as excess nutrient loads from run-off, improves the overall health of the reef ecosystem, thereby increasing its short-term resilience to global stressors like ocean acidification.
Shelford's law of tolerance can be used to explain coral abundance, thermal tolerance and species
distribution.
resilience.
diversity.
size.
Reveal Answer
distribution.
Shelford's law of tolerance states that an organism's success is determined by its minimum, maximum, and optimum environmental factors, which directly dictates its geographic distribution.
resilience.
Resilience refers to an ecosystem's or species' ability to recover from a disturbance, whereas Shelford's law focuses on the environmental limits within which a species can survive.
diversity.
Species diversity describes the variety of different species in a community, while Shelford's law applies to the environmental tolerance limits of individual species rather than community-level metrics.
size.
Although environmental stress can affect growth, Shelford's law is primarily used to explain population abundance and where a species can live, not the physical size of individual organisms.
Identify the system that enables ocean water to resist pH change when exposed to increased atmospheric carbon dioxide (CO).
Reveal Answer
Ocean water resists pH change because of the presence of the carbonate buffering system.
| Descriptor | Marks |
|---|---|
identifies carbonate buffering system | 1 |
Identify one natural and two anthropogenic causes of increased atmospheric CO.
Reveal Answer
Natural: decaying organic matter
Anthropogenic: burning of fossil fuels and deforestation
| Descriptor | Marks |
|---|---|
identifies a natural source of atmospheric | 1 |
identifies an anthropogenic source of atmospheric | 1 |
identifies a second anthropogenic source of atmospheric | 1 |
Describe how increased atmospheric CO affects levels of hydrogen and carbonate ions in ocean waters.
Reveal Answer
Carbon dioxide is absorbed from the atmosphere and reacts with ocean water to form carbonic acid. Carbonic acid forms bicarbonate and excess hydrogen ions. Bicarbonate ions can split further to release hydrogen and carbonate ions, increasing levels for both.
| Descriptor | Marks |
|---|---|
describes the reaction of carbon dioxide and water to form carbonic acid | 1 |
describes the breakdown of carbonic acid () to release bicarbonate () and hydrogen () ions | 1 |
describes the increase in carbonate () ions | 1 |
describes the increase in hydrogen ions | 1 |
Explain what implications a change in levels of carbonate ions from increased atmospheric CO has for shell-forming organisms.
Reveal Answer
Shell-forming organisms require carbonate ions to grow their shells. The carbonate buffering system leads to an increase in carbonate ions when there is more in the atmosphere. Since carbonate is required to buffer excess hydrogen ions, there is less available for shell growth, resulting in reduced growth and weaker shells.
| Descriptor | Marks |
|---|---|
explains that carbonate is required to grow shells | 1 |
identifies the implications are reduced/slower growth | 1 |
identifies the implications are weaker shells | 1 |
Ocean acidification accelerates reef erosion rates through
lowered density of corals.
reduced mechanical erosion.
increased crustose coralline algae.
decreased susceptibility of calcifiers to grazers.
Reveal Answer
lowered density of corals.
Ocean acidification reduces the availability of carbonate ions, causing corals to build weaker, less dense skeletons that are more easily broken down.
reduced mechanical erosion.
Weaker coral skeletons resulting from acidification actually increase, rather than reduce, their vulnerability to mechanical erosion from waves and storms.
increased crustose coralline algae.
Crustose coralline algae are calcifying organisms that help cement reefs together; ocean acidification decreases their growth and abundance rather than increasing it.
decreased susceptibility of calcifiers to grazers.
The less dense, fragile skeletons caused by acidification actually increase the susceptibility of calcifying organisms to grazing and bioerosion.
Explain the concept of coral bleaching in terms of Shelford's law of tolerance.
Reveal Answer
Shelford's law of tolerance indicates the impact of thermal stress on the abundance and distribution of corals.
Corals live in a narrow thermal range close to the upper limit of their optimal range, therefore a small increase in thermal stress causes corals to shift into the zone of stress.
When corals are exposed to temperatures at or beyond their bleaching threshold temperature, they enter the upper zone of stress.
When corals become stressed for prolonged periods above their bleaching threshold, the zooxanthellae leave the coral and bleaching occurs.
| Descriptor | Marks |
|---|---|
identifies that Shelford's law of tolerance indicates the impact of thermal stress on corals | 1 |
identifies that corals live in a narrow optimal thermal range | 1 |
indicates exposure to temperatures above their bleaching threshold causes coral to enter upper zone of stress | 1 |
explains that prolonged periods in the upper zone of tolerance results in zooxanthellae being expelled, causing coral bleaching | 1 |
Why does the carbonate system operate on geological timescales?
Decomposition of organic matter takes longer in the ocean than in air.
The atmosphere acts as a carbon sink, making diffusion into the ocean difficult.
The system depends on the burning of fossil fuels to release carbon from sedimentary rocks.
Calcium carbonate sediments stored in the deep ocean do not easily mix with upper ocean layers.
Reveal Answer
Decomposition of organic matter takes longer in the ocean than in air.
While decomposition rates vary, the biological pump and organic matter breakdown operate on much shorter timescales (days to centuries) compared to the geological timescales of the carbonate system.
The atmosphere acts as a carbon sink, making diffusion into the ocean difficult.
The ocean is actually a massive carbon sink, and gas exchange between the atmosphere and surface ocean occurs relatively quickly (years to decades), not on geological timescales.
The system depends on the burning of fossil fuels to release carbon from sedimentary rocks.
The natural carbonate system operates independently of human activities; burning fossil fuels is a recent anthropogenic disruption, not a driver of the natural geological carbon cycle.
Calcium carbonate sediments stored in the deep ocean do not easily mix with upper ocean layers.
Deep ocean circulation and the weathering or dissolution of calcium carbonate sediments take thousands to millions of years, driving the carbonate system's geological timescale.
A resilient reef will respond to ocean acidification by
maintaining condition in the short term before declining.
immediately declining before returning in another form.
maintaining condition indefinitely.
immediately declining.
Reveal Answer
maintaining condition in the short term before declining.
This is correct because ecological resilience allows the reef to absorb initial stress and maintain its structure temporarily, but chronic stressors like ocean acidification will eventually overwhelm its physiological limits.
immediately declining before returning in another form.
This is incorrect because a resilient reef would resist immediate decline; immediate degradation indicates high vulnerability rather than resilience.
maintaining condition indefinitely.
This is incorrect because no reef can withstand a continuously worsening stressor indefinitely, as ocean acidification fundamentally depletes the carbonate ions corals need to build their skeletons.
immediately declining.
This is incorrect because immediate decline is a characteristic of a highly sensitive or vulnerable ecosystem, whereas a resilient reef has the capacity to buffer against initial disturbances.
The average sea surface temperature (SST), the maximum monthly mean (MMM) and the bleaching threshold temperature for coral in two regions of the Great Barrier Reef were recorded.
| Great Barrier Reef region | SST (°C) | MMM (°C) | Bleaching threshold temperature (°C) |
|---|---|---|---|
| Far Northern | 27.5 | 29.0 | 30.0 |
| Southern | 26.5 | 28.0 | 29.0 |
Compare the potential changes in coral cover and resilience in the Far Northern and Southern regions if the MMM increased to 29.2 °C.
Reveal Answer
If average MMM increased to 29.2 ºC, both regions would experience an increase in temperature stress.
Thermal stress in the Southern region would increase beyond the bleaching threshold temperature, increasing the risk of bleaching and reducing coral cover.
The Far Northern region would remain close to its optimum range and below its bleaching threshold so coral cover would be maintained.
Corals that survive bleaching in the Southern region would be more resilient to future bleaching events.
| Descriptor | Marks |
|---|---|
identifies a similarity | 1 |
identifies a difference | 1 |
identifies the significance for coral cover | 1 |
identifies the significance for coral resilience | 1 |
Increases in atmospheric carbon dioxide can influence ocean chemistry by
decreasing carbonic acid and decreasing pH.
decreasing carbonic acid and increasing pH.
increasing carbonic acid and decreasing pH.
increasing carbonic acid and increasing pH.
Reveal Answer
decreasing carbonic acid and decreasing pH.
Incorrect. While the pH does decrease, absorbing more from the atmosphere actually increases the formation of carbonic acid, rather than decreasing it.
decreasing carbonic acid and increasing pH.
Incorrect. Dissolving in water forms carbonic acid, so its levels increase. Additionally, the resulting release of ions lowers the pH, rather than raising it.
increasing carbonic acid and decreasing pH.
Correct. As the ocean absorbs more , it reacts with water to form more carbonic acid (), which dissociates to release ions, thereby lowering the ocean's pH.
increasing carbonic acid and increasing pH.
Incorrect. While carbonic acid levels do increase, acids release ions into the solution, which causes the pH to decrease (become more acidic), not increase.
Changes to ocean chemistry and pH were estimated using modelling, and the effects of these changes on calcified marine organisms are shown.
| Pre-industrial | 1990 | 2 × pre-industrial | 4 × pre-industrial | 6 × pre-industrial | |
|---|---|---|---|---|---|
| Atmospheric concentration of CO | 280 ppm | 380 ppm | 560 ppm | 1120 ppm | 1680 ppm |
| Average pH of surface oceans | 8.18 | 8.07 | 7.92 | 7.65 | 7.49 |
| Calcite saturation | 5.3 | 4.4 | 3.3 | 1.9 | 1.3 |
| Aragonite saturation | 3.4 | 2.8 | 2.1 | 1.2 | 0.9 |
| Organisms | Form of calcium carbonate | Habitat |
|---|---|---|
| Macroalgae | Aragonite or calcite | Benthic |
| Corals: warm water cold water | Aragonite Aragonite | Benthic Benthic |
| Crustaceans | Calcite | Benthic or planktonic |
Not all members of the group are calcified.
Describe the consequences of ocean acidification and predict the impact on the complexity and diversity of the coral reef ecosystem.
Reveal Answer
As the average pH of the ocean decreases from 8.18 to 7.49, the calcite saturation decreases from 5.3 to 1.3 (75%) and aragonite saturation decreases from 3.4 to 0.9 (74%).
This would increase the dissolution (erosion) of existing reef structures, resulting in mortality of carbonate reef builders and the destruction of habitats.
As pH decreases, aragonite organisms, such as corals and some macroalgae, would be more affected than calcite organisms.
This would result in a higher proportion of calcite organisms surviving, such as crustaceans, until they reach their pH limit.
Therefore, ocean acidification decreases the complexity and diversity of coral reef ecosystem.
| Descriptor | Marks |
|---|---|
Identifies that as pH decreases, both calcite and aragonite saturations decrease | 1 |
Links the decrease in pH to increased dissolution of carbonate reef builders | 1 |
Identifies that as pH decreases, aragonite organisms, such as corals and some macroalgae, will be affected more than calcite organisms | 1 |
Indicates that the reef ecosystem would have a higher proportion of calcite organisms | 1 |
Indicates that the consequence of ocean acidification is a reduction in complexity and diversity of the coral reef ecosystem | 1 |
Explain how resilience may partially offset ocean acidification responses in the short term.
Reveal Answer
Resilience varies between marine organisms and from one reef to another.
A reef’s resilience is a measure of how well it is able to resist ocean acidification and its impacts.
Reef resilience is a short term phenomenon. It decreases over time as reefs are repeatedly impacted by acidification events.
| Descriptor | Marks |
|---|---|
Explains that resilience varies between organisms and/or reefs | 1 |
Explains that resilience impacts the reef’s ability to resist the impact of ocean acidification | 1 |
Explains that resilience decreases over time or is limited to the short term | 1 |
Describe two conditions necessary for a bleached reef to recover.
Reveal Answer
The reef must not be exposed to any other stressors (e.g. cyclone, COTS, pollution or increased water temperature) while it recovers.
Recovery is also aided by the presence of calcifiers, which encourages site selection by other corals.
| Descriptor | Marks |
|---|---|
describes a condition necessary for reef recovery | 1 |
describes a second condition necessary for reef recovery | 1 |
Ocean water resists changes in pH more than fresh water because ocean water has a greater concentration of
hydrogen ions.
carbonate ions.
dissolved carbon dioxide.
dissolved sodium chloride.
Reveal Answer
hydrogen ions.
While the concentration of hydrogen ions determines the pH of a solution, it does not inherently provide buffering capacity to resist changes in pH.
carbonate ions.
Carbonate and bicarbonate ions act as a natural buffer system in ocean water, absorbing excess hydrogen or hydroxide ions to resist significant changes in pH.
dissolved carbon dioxide.
Although dissolved carbon dioxide forms carbonic acid as part of the ocean's buffer system, it is the carbonate and bicarbonate ions that actively neutralize added acids to stabilize the pH.
dissolved sodium chloride.
Sodium chloride is a neutral salt that dissociates into sodium and chloride ions, neither of which have buffering properties to resist changes in pH.
Explain what affects the abundance of different forms of calcium carbonate in deep sea floor sediments.
Reveal Answer
Calcium carbonate is present in two separate forms (aragonite and calcite) in differing amounts depending on the organism that made it.
As calcium carbonate shells of dead organisms sink, their solubility increases with depth and pressure.
Aragonite is more soluble than calcite, so it will dissolve first.
Therefore, the position of the CCD and the ratio of aragonite and calcite in dead organisms will affect the composition of seafloor sediment.
| Descriptor | Marks |
|---|---|
identifies calcite and aragonite as the two forms of calcium carbonate | 1 |
explains solubility is affected by depth or pressure | 1 |
identifies that aragonite is more soluble than calcite | 1 |
explains how the position of the CCD affects the calcium carbonate composition in deep sea floor sediments | 1 |