Ocean acidification — Paper 4 · A Level Marine Science 9693

4 (a) Planting of mangroves and building artificial reefs can assist in the rehabilitation of marine species. Discuss the advantages and disadvantages of each of these methods. … … … … … … … … … … … … … … … … … [7] (b) Describe and explain the possible impacts of increasing atmospheric carbon dioxide on marine ecosystems. … … … … … … … … … … … … … … … … … … … … [8] [Total: 15]

15 marks

Mark scheme: 4(a) advantages minimum of two: a (mangrove / artificial reef) reduced coastal erosion / AW ; b (mangrove) stabilises substrate ; c (mangrove) acts as a nursery ground / breeding site for species / habitats for juveniles ; d (artificial reef) acts as a shelter for young fish ; e (artificial reef) provides habitat for attachment of organisms ; f (mangrove / artificial reef) provides safety from predators ; g (mangrove / artificial reef) provides food for food chains / establishes food chains ; h (mangrove / artificial reef) increases fish catch for locals ; i (mangrove) provides oxygen ; j (mangrove) removes carbon dioxide ; k (artificial reef) encourages tourism / diving ; disadvantages minimum of two: l (mangroves may cause) low oxygen due to leaf decay ; m (mangroves may be) invasive / damage other areas / grow out of control ; n (mangroves may) lose land with economic value (e.g. beaches) / AW ; 1 (artificial reefs) may alter water currents affecting coastline ; 2 (artificial reefs) may get in the way of fishing boats ; 3 (artificial reefs) may release toxins / TBT / AW ; 4 (artificial reefs) may damage sea bed / physical destruction of habitats ; 5 (artificial reefs) may cause aggregation of wrong species / AW ; 7 A correct named toxin Question Answer Marks Guidance 4(b) any eight of: a increased global temperature / global warming / enhanced greenhouse effect ; b long wave IR light trapped ; c glacier / ice cap melt causing sea level rise ; d loss of habitats ; e altered salinities ; f food chain effect ; g extinction ; h migration / range changes ; i altered primary productivity / increased photosynthesis ; j acidification / carbonic acid ; k coral bleaching ; l erosion of shells / AW ; m expelling zooxanthellae ; 8

This question in 9693/41 Oct/Nov 2017

4 Increasing ocean acidification may be a threat to marine biodiversity. (a) (i) Describe how carbon dioxide reacts with water to reduce the pH and availability of carbonate ions. … … … … … … [3] (ii) Explain the impact of acidification on shelled organisms. … … … … [2] (b) Fig. 4.1 shows the changes in atmospheric carbon dioxide concentration and ocean pH around Hawaii. 400 8.40 8.35 375 8.30 atmospheric CO2 8.25 350 CO2 in 8.20 atmosphere pH / parts per 325 million ocean pH 8.15 8.10 300 8.05 275 8.00 1958 1964 1970 1976 1982 1988 1994 2000 2006 2012 year Fig. 4.1 Discuss the extent to which the data in Fig. 4.1 support the theory that fossil fuel use by humans is causing ocean acidification. … … … … … … … … [4] (c) Scientists have suggested that conservation of kelp forests may help to reduce the impact of ocean acidification. They measured the light intensity and pH of water at a depth of two metres in a kelp forest around the coast of Norway during the summer months. The results are shown in Fig. 4.2. 8.25 8.20 8.15 maximum 8.10 daily pH 8.05 8.00 7.95 0 100 200 300 400 500 600 maximum light intensity / arbitrary units Fig. 4.2 (i) Use your knowledge of photosynthesis to explain why the maximum daily pH was affected by the maximum light intensity. … … … … [2] (ii) The investigation was carried out in the Arctic Ocean during the summer months. Discuss whether the investigation supports the use of kelp forests in reducing global ocean acidification. … … … … … … [3] (iii) Fig. 4.3 shows a light micrograph of cells in the surface of a kelp frond. A B C Fig. 4.3 Draw the group of cells A, B and C as shown in Fig. 4.3 in the space below. Do not label your diagram. [3] [Total: 17]

17 marks

Mark scheme: 4(a)(i) any 3 from: forms H2CO3 / carbonic acid ; (H2CO3) dissociates / AW, into H+ and HCO3- / hydrogen ions and hydrogen carbonate ions ; increased H+ causes an increase in acidity ; H+ reacts with CO3- ions / removes / reacts / combines with carbonate ions ; 4(a)(ii) any 2 from: fewer carbonate ions in water ; shells become thinner / weaker / lose mass / dissolve / less calcium carbonate / AW ; more susceptible to, predation, wave action / damage / AW ; 2 4(b) as atmospheric carbon dioxide rises, pH falls / there is a negative correlation (between carbon dioxide and pH) / AW ; plus any 3 from: fossil fuel use releases carbon dioxide ; ocean pH, has a lot of fluctuations / AW ; fluctuations in oceanic pH do not match the atmospheric CO2; carbon dioxide may not be from human use ; data does not give any information on fossil fuels use / AW ; pH data only from 1988 ; other factors may be causing the change in pH / correlation does not mean causation ; data are only limited to Hawaii / other areas need to be checked / no statistical analysis ; 4 Question Answer Marks 4(c)(i) any 2 from: as light intensity increase, pH increases / AW ; (photosynthesis) uses carbon dioxide / more carbon dioxide is taken up / AW ; less carbon dioxide dissolved in water reduces acidity / less H+ present / less carbonic acid present ; 2 4(c)(ii) any 3 from: increased light intensity reduces carbon dioxide concentration data are close to line of best fit / strong correlation ; at low light intensity there is less pH reduction / AW ; only shows summer months / cold water area / one area near Norway / AW ; no control experiment / AW ; in winter / with less light so would not work as well / AW ; other factors could affect photosynthesis (e.g. temperature / sediment in water) ; cannot grow kelp forests everywhere / kelp cannot survive in other areas ; kelp could become an invasive species and damage the environment ; 3 4(c)(iii) three cells touching and double layer (in appropriate areas) for cell wall ; thin, continuous line with no shading ; size and proportions correct – minimum size, cell length longer than width and bottom of cell B is above bottom of cell A ; 3

This question in 9693/42 May/June 2022

4 Increasing ocean acidification may be a threat to marine biodiversity. (a) (i) Describe how carbon dioxide reacts with water to reduce the pH and availability of carbonate ions. … … … … … … [3] (ii) Explain the impact of acidification on shelled organisms. … … … … [2] (b) Fig. 4.1 shows the changes in atmospheric carbon dioxide concentration and ocean pH around Hawaii. 400 8.40 8.35 375 8.30 atmospheric CO2 8.25 350 CO2 in 8.20 atmosphere pH / parts per 325 million ocean pH 8.15 8.10 300 8.05 275 8.00 1958 1964 1970 1976 1982 1988 1994 2000 2006 2012 year Fig. 4.1 Discuss the extent to which the data in Fig. 4.1 support the theory that fossil fuel use by humans is causing ocean acidification. … … … … … … … … [4] (c) Scientists have suggested that conservation of kelp forests may help to reduce the impact of ocean acidification. They measured the light intensity and pH of water at a depth of two metres in a kelp forest around the coast of Norway during the summer months. The results are shown in Fig. 4.2. 8.25 8.20 8.15 maximum 8.10 daily pH 8.05 8.00 7.95 0 100 200 300 400 500 600 maximum light intensity / arbitrary units Fig. 4.2 (i) Use your knowledge of photosynthesis to explain why the maximum daily pH was affected by the maximum light intensity. … … … … [2] (ii) The investigation was carried out in the Arctic Ocean during the summer months. Discuss whether the investigation supports the use of kelp forests in reducing global ocean acidification. … … … … … … [3] (iii) Fig. 4.3 shows a light micrograph of cells in the surface of a kelp frond. A B C Fig. 4.3 Draw the group of cells A, B and C as shown in Fig. 4.3 in the space below. Do not label your diagram. [3] [Total: 17]

17 marks

Mark scheme: 4(a)(i) any 3 from: forms H2CO3 / carbonic acid ; (H2CO3) dissociates / AW, into H+ and HCO3- / hydrogen ions and hydrogen carbonate ions ; increased H+ causes an increase in acidity ; H+ reacts with CO3- ions / removes / reacts / combines with carbonate ions ; 4(a)(ii) any 2 from: fewer carbonate ions in water ; shells become thinner / weaker / lose mass / dissolve / less calcium carbonate / AW ; more susceptible to, predation, wave action / damage / AW ; 2 4(b) as atmospheric carbon dioxide rises, pH falls / there is a negative correlation (between carbon dioxide and pH) / AW ; plus any 3 from: fossil fuel use releases carbon dioxide ; ocean pH, has a lot of fluctuations / AW ; fluctuations in oceanic pH do not match the atmospheric CO2; carbon dioxide may not be from human use ; data does not give any information on fossil fuels use / AW ; pH data only from 1988 ; other factors may be causing the change in pH / correlation does not mean causation ; data are only limited to Hawaii / other areas need to be checked / no statistical analysis ; 4 Question Answer Marks 4(c)(i) any 2 from: as light intensity increase, pH increases / AW ; (photosynthesis) uses carbon dioxide / more carbon dioxide is taken up / AW ; less carbon dioxide dissolved in water reduces acidity / less H+ present / less carbonic acid present ; 2 4(c)(ii) any 3 from: increased light intensity reduces carbon dioxide concentration ; data are close to line of best fit / strong correlation ; at low light intensity there is less pH reduction / AW ; only shows summer months / cold water area / one area near Norway / AW ; no control experiment / AW ; in winter / with less light so would not work as well / AW ; other factors could affect photosynthesis (e.g. temperature / sediment in water) ; cannot grow kelp forests everywhere / kelp cannot survive in other areas ; kelp could become an invasive species and damage the environment ; 3 4(c)(iii) three cells touching and double layer (in appropriate areas) for cell wall ; thin, continuous line with no shading ; size and proportions correct – minimum size, cell length longer than width and bottom of cell B is above bottom of cell A ; 3

This question in 9693/43 May/June 2022

6 Oysters have a complex life cycle with a larval stage and an adult stage. The larvae live in the open water until they settle on a substrate and mature into adults. It has been suggested that ocean acidification might affect the settling of oyster larvae onto substrates. Plan an ethical, laboratory investigation that you could do to investigate the effect of changing the pH on the settling of oyster larvae onto a substrate. Your plan should: • include a clear statement of the hypothesis • identify the independent, dependent and standardised variables • include full details of the method so that another person can follow it • describe how you would analyse your results • be safe and ethical. … … … … … … … … … … … … … … … … … … … … … … … … … … … … [11] The boundaries and names shown, the designations used and the presentation of material on any maps contained in this question paper/insert do not imply official endorsement or acceptance by Cambridge Assessment International Education concerning the legal status of any country, territory, or area or any of its authorities, or of the delimitation of its frontiers or boundaries.

11 marks

Mark scheme: 6 (hypothesis) clear statement of hypothesis ; and any 10 of: (independent variable) 1 independent variable identified as pH ; 2 use at least five different pHs ; (dependent variable) 3 dependent variable identified as number of larvae that settle / AW ; 4 counting number that settle in a set time ; (standardised variables and methods) (max 4) 5 same volume of tank / water ; 6 same salinity / stated salinity / AW ; 7 maintain temperature / monitor temperature / AW ; 8 same species of oyster / larvae / age of larvae / number of larvae / AW ; 9 same mass of food / type of food / AW ; 10 maintain oxygen by bubbling air through ; 11 same substrate type / same area / volume of substrate / AW ; 12 use of random sampling method to count larvae ; 13 same light intensity by placing under lamp / in same area of light / AW ; (safety and ethics) 14 wear eye protection / gloves  when handling solutions / buffers / acids / larvae / AW ; 15 no extreme pH / temperature used / allow larvae to acclimatise / return oysters to wild / do not release chemicals into sea / do not take larvae from wild stocks / AW ; (analysis) (max 3) 16 at least two replicates and finding means / medians ; 17 graph of number settled against pH ; 18 correct statistical test (e.g. chi squared / t-test / standard error, depending on comparison) / use of standard deviations / error bars ; 19 results table with headings drawn and independent variable in left column ;

This question in 9693/41 May/June 2023

4 Foraminifera are microscopic, single-celled marine organisms. Some species of foraminifera produce shells made from calcium carbonate. Fig. 4.1 shows the shell of a species of foraminifera. Fig. 4.1 Scientists investigated the effect of carbon dioxide concentration in the water on the length of the shells and on population growth of foraminifera. Tanks of sea water were set up with different concentrations of carbon dioxide. Foraminifera were added to each of the tanks. Oxygen was bubbled into each tank and food was added daily. The mean shell lengths of the foraminifera and the population density were determined after eight weeks. The results are shown in Table 4.1. Table 4.1 carbon dioxide mean shell length / mm mean population density concentration / ppm / foraminifera per cm3 10 0.85 1800 25 0.82 1750 50 0.84 1500 110 0.52 950 125 0.25 250 (b) The foraminifera have shells made from calcium carbonate. Explain why increasing the carbon dioxide concentration affects the growth of the foraminifera. … … … … … … [3] (c) There are many different species of foraminifera in areas of coral reef. The species diversity of foraminifera is thought to be affected by factors such as temperature, carbon dioxide concentration, and calcium ion concentration. Some scientists think that global warming will affect the species diversity of foraminifera. Plan a laboratory-based investigation that you could do to investigate the effect of temperature on the species diversity of a sample of foraminifera that grows around coral. You are provided with a mixed starter culture of 20 different species of foraminifera, sea water, microscopes, coral around which the foraminifera live, and other standard laboratory equipment. Your plan should: • include a clear statement of the hypothesis • identify the independent, dependent and standardised variables • include full details of the method so that another person can follow it • describe how you would analyse your results • be safe and ethical. … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … [11] [Total: 19]

19 marks

Mark scheme: 4(a) 1 both y axes labelled with units ; 2 two linear y axes with suitable scales so that lines cover at least half grid ; 3 points correctly plotted ; 4 neat lines joining points with no extrapolation ; 5 key for both lines ; 5 Question Answer Marks 4(a) Question Answer Marks 4(b) any 3 of: 1 carbon dioxide dissolves in water ; 2 forms, carbonic acid / H2CO3 / acidification occurs / pH drops / AW ; 3 (carbonic dissociates) into, bicarbonate (ions) / HCO3- (and H+ ions) ; 4 (H+) reacts with, CO32- / carbonate ions ; 5 loss of calcium carbonate from shells / shells dissolve / weaker shells produced / poor shell growth / AW ; 3 4(c) Hypothesis (h): increase in temperature will reduce diversity / AW ; any 10 of: Independent variable (i) 1 independent variable identified as temperature / AW ; 2 at least five stated temperatures ; Dependent variable (d) 3 dependent variable identified as species diversity / AW ; 4 (count) number of different species / AW ; 5 (count) number of (individuals of) each species of foraminifera / AW ; Standardised variables (s) max 3 6 same stated time (more than one week) ; 7 same pH ; 8 same salinity / same calcium / mineral ions / AW ; 9 same number of foraminifera at start / same volume of starter culture / initial diversity / AW ; 10 same quantity of food / nutrients / AW ; 11 same volume of water / volume of tanks / AW ; 12 same carbon dioxide / oxygen ; 13 same size / mass / amount of coral / AW ; 11 Question Answer Marks 4(c) Method marks (m) max 2 14 method of changing temperature (e.g. water-bath) / AW ; 15 use of pipette / syringe / AW ; 16 count / identify foraminifera using, microscope / haemocytometer / AW ; 17 use of key to identify foraminifera / AW ; Analysis (a) max 2 18 use Simpson’s index of diversity / AW ; 19 plot graph of diversity index against temperature ; 20 calculation of correlation coefficient / Spearman’s rank ; 21 example of correct results table ; 22 repeats and calculate mean / median / standard deviation / standard error ; Ethical / Health and safety (e) 23 identification of any risk with method to minimise / statement that experiment is low risk / AW ; 24 treat foraminifera ethically by using acceptable range of temperatures / return foraminifera to sea / replace coral / AW ;

This question in 9693/41 May/June 2024

5 In 2010, the total global output of carbon dioxide from the combustion of fossil fuels was estimated to be 33 000 000 000 kg. Some scientists have suggested that by 2030 the emission of carbon dioxide from fossil-fuel combustion could increase. (a) (i) Calculate the predicted mass of carbon dioxide that will be released in 2030 if emissions from fossil-fuel combustion increases by 25%. State the unit. … [2] (ii) Increased carbon dioxide output could lead to global warming. Give two possible consequences of global warming for the marine environment. 1 … 2 … [2] (iii) Describe how high levels of atmospheric carbon dioxide could reduce pH and carbonate availability in sea water. … … … … … … [3] (b) Increasing acidification of water may affect the shell growth of molluscs such as mussels. Plan a laboratory‑based investigation that you could do to investigate the effect of changing the pH of sea water on the rate of growth of mussels. You are provided with standard laboratory apparatus and materials. Your plan should: • include a clear statement of the hypothesis • identify the independent, dependent and standardised variables • include full details of the method so that another person can follow it • describe how you would analyse your results • be safe and ethical. … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … [12] [Total: 19]

19 marks

Mark scheme: 5 (a)(i) 41 250 000 000 ; kg ; 2 Question Answer Marks 5(a)(ii) any 2 of: 1 flooding of coastal areas / sea level rise / AW ; 2 habitat loss ; 3 coral bleaching / AW ; 4 changes in distribution of species / migration of species / AW ; 5 changes in circulation of sea currents ; 6 loss of biodiversity / extinction of species / loss of biodiversity / AW ; 7 spread of disease / pathogens / pests / AW ; 2 5(a)(iii) any 3 of: 1 carbon dioxide dissolves ; 2 forms H2CO3 / carbonic acid ; 3 dissociates into H+ and HCO3 – (hydrogen carbonate) ; 4 H+ ions react with CO32– ions ; 3 Question Answer Marks 5(b) lower pH will reduce rate of growth of mussels ; and any 11 of: independent variable 1 independent variable: pH ; 2 at least five stated pHs used ; 3 mussels placed into beakers ; dependent variable 4 dependent variable: length / mass of mussels / growth of mussel / AW ; 5 length measured with ruler / mass with balance ; standardised variables (max 3) 6 volume of water / volume of tanks ; 7 volume of pH buffer added ; 8 temperature ; 9 salinity / mineral ions / AW ; 10 quantity of food / nutrients / feeding regime / AW ; 11 oxygen / carbon dioxide concentration ; 12 same mussel, species / age / size / AW ; method marks (max 3) 13 air bubbled through ; 14 pH buffers used ; 15 salt weighed out and added to water (to maintain salinity) ; 16 method for measuring mussels ; ethics and safety 17 identification of any risk with method to minimise / statement that experiment is low risk / AW; 18 ensure no extreme temperature / pH used so mussels are not harmed / ensure mussels are replaced / ensure that 19 mussels are fed / do not release waste chemicals into the drains / AW ; 12 Question Answer Marks 5(b) analysis (max 3) 20 rate of growth calculated as, length / mass, divided by time ; 21 plot graph of mussel length / mass / growth rate against pH ; 22 calculation of correlation coefficient / Spearman’s rank ; 23 example of correct results table ; 24 repeats and calculate mean / median / standard deviation / standard error ;

This question in 9693/42 May/June 2024

5 In 2010, the total global output of carbon dioxide from the combustion of fossil fuels was estimated to be 33 000 000 000 kg. Some scientists have suggested that by 2030 the emission of carbon dioxide from fossil-fuel combustion could increase. (a) (i) Calculate the predicted mass of carbon dioxide that will be released in 2030 if emissions from fossil-fuel combustion increases by 25%. State the unit. … [2] (ii) Increased carbon dioxide output could lead to global warming. Give two possible consequences of global warming for the marine environment. 1 … 2 … [2] (iii) Describe how high levels of atmospheric carbon dioxide could reduce pH and carbonate availability in sea water. … … … … … … [3] (b) Increasing acidification of water may affect the shell growth of molluscs such as mussels. Plan a laboratory‑based investigation that you could do to investigate the effect of changing the pH of sea water on the rate of growth of mussels. You are provided with standard laboratory apparatus and materials. Your plan should: • include a clear statement of the hypothesis • identify the independent, dependent and standardised variables • include full details of the method so that another person can follow it • describe how you would analyse your results • be safe and ethical. … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … [12] [Total: 19]

19 marks

Mark scheme: 5 (a)(i) 41 250 000 000 ; kg ; 2 Question Answer Marks 5(a)(ii) any 2 of: 1 flooding of coastal areas / sea level rise / AW ; 2 habitat loss ; 3 coral bleaching / AW ; 4 changes in distribution of species / migration of species / AW ; 5 changes in circulation of sea currents ; 6 loss of biodiversity / extinction of species / loss of biodiversity / AW ; 7 spread of disease / pathogens / pests / AW ; 2 5(a)(iii) any 3 of: 1 carbon dioxide dissolves ; 2 forms H2CO3 / carbonic acid ; 3 dissociates into H+ and HCO3 – (hydrogen carbonate) ; 4 H+ ions react with CO32– ions ; 3 Question Answer Marks 5(b) lower pH will reduce rate of growth of mussels ; and any 11 of: independent variable 1 independent variable: pH ; 2 at least five stated pHs used ; 3 mussels placed into beakers ; dependent variable 4 dependent variable: length / mass of mussels / growth of mussel / AW ; 5 length measured with ruler / mass with balance ; standardised variables (max 3) 6 volume of water / volume of tanks ; 7 volume of pH buffer added ; 8 temperature ; 9 salinity / mineral ions / AW ; 10 quantity of food / nutrients / feeding regime / AW ; 11 oxygen / carbon dioxide concentration ; 12 same mussel, species / age / size / AW ; method marks (max 3) 13 air bubbled through ; 14 pH buffers used ; 15 salt weighed out and added to water (to maintain salinity) ; 16 method for measuring mussels ; ethics and safety 17 identification of any risk with method to minimise / statement that experiment is low risk / AW; 18 ensure no extreme temperature / pH used so mussels are not harmed / ensure mussels are replaced / ensure that 19 mussels are fed / do not release waste chemicals into the drains / AW ; 12 Question Answer Marks 5(b) analysis (max 3) 20 rate of growth calculated as, length / mass, divided by time ; 21 plot graph of mussel length / mass / growth rate against pH ; 22 calculation of correlation coefficient / Spearman’s rank ; 23 example of correct results table ; 24 repeats and calculate mean / median / standard deviation / standard error ;

This question in 9693/43 May/June 2024

3 Oysters are shelled molluscs that have a complex life cycle. The adults are sessile organisms that anchor to a substrate. Fig. 3.1 shows some adult oysters. Fig. 3.1 (a) Outline the importance of having a complex life cycle for organisms such as oysters. … … … … … … [3] (b) Scientists investigated the effects of temperature and pH on the survival of oyster larvae. Oyster larvae were placed into tanks of water at different temperatures and pHs. The percentages of larvae surviving were calculated after two days and then again after 15 days. Fig. 3.2 and Fig. 3.3 show the results. 100 Key 90 day 2 day 15 80 70 60 percentage of 50 larvae surviving 40 30 20 10 0 20 25 27 30 35 temperature / °C Fig. 3.2 100 Key 90 day 2 day 15 80 70 60 percentage of 50 larvae surviving 40 30 20 10 0 6.5 7.0 7.5 8.0 8.2 8.5 pH Fig. 3.3 (i) The scientists placed 500 larvae in each condition. Calculate the number of larvae that did not survive from day 2 to day 15 when placed at a temperature of 30 °C. … [2] (ii) Use Fig. 3.2 and Fig. 3.3 to state the optimum temperature and optimum pH for the survival of oyster larvae. optimum temperature … °C optimum pH … [1] (c) Fishers in India reported declining harvests of adult oysters from an area of coastal water after 2011. (i) Table 3.1 shows the temperature and pH of the water during 2009 in this area. Table 3.1 month temperature / °C pH February 25 8.0 April 27 7.9 June 34 8.2 August 30 7.0 October 32 6.5 December 28 7.0 Use Table 3.1 to plot a line graph to show the temperature and the pH from February to December. [5] (ii) Around the coasts of India, oysters spawn throughout the year but have two peak periods of breeding, in April and August. Discuss the reasons for the reduction in the number of oysters that the fishers harvested after 2011. Use information in Table 3.1, Fig. 3.2 and Fig. 3.3 to support your answer. … … … … … … … … [4] (d) Explain why the use of fossil fuels places future oyster populations at risk. … … … … … … [3] [Total: 18]

18 marks

Mark scheme: 3(a) any 3 from: 3 1 larvae can move to other areas / larvae allow distribution to other areas / AW ; 2 reduced competition / AW ; 3 (reduced competition for) food / nutrients / AW ; 4 idea that larvae and adults occupy different niches / AW ; 5 increased genetic diversity (if oysters spread to other areas) / AW ; 6 reduces spread of disease (as population density is lower) ; 3(b)(i) 180 = 2 marks 2 36 (%) = 1 mark OR 320 survived = 1 mark 3(b)(ii) (optimum temperature) = 27 (°C) 1 optimum pH = 8(.0) ; 3(c)(i) 1. linear y axes for both temperature and pH, labelled with units, and horizontal axis as month ; 5 2. all three scales enable plots to cover at least half grid ; 3. plots correct +/- ½ square for temperature ; 4. plots correct +/- ½ square for pH ; 5. points joined with straight lines and key ; 3(c)(ii) any 4 from: 4 1 in April, conditions enable survival / there are optimal conditions / oysters can breed / conditions are ideal for breeding / AW ; 2 August has a temperature of 30 oC and pH of 7(.0) / April has a temperature of 27 oC and pH of 7.9 / AW ; 3 in August, conditions reduce larvae settling / kill larvae / few larvae survive / will not settle in August / too acidic for survival / AW ; 4 there is only one successful breeding season (per year) / AW ; 5 idea that few oyster larvae become adults / it takes time to produce adults (so effects are only seen in 2011) / AW ; 6 few other months have ideal conditions for larvae / AW ; 7 overfishing / pollution / AW, may be causing the fall ; 3(d) any 3 from: 3 1 release of carbon dioxide / AW ; 2 causes (enhanced) greenhouse effect / increased temperature / global warming / AW ; 3 acidification of water / AW ; 4 (acid) reduces oyster shell formation / erodes shells / dissolves shells / AW ; 5 larvae do not survive / larvae cannot settle / fewer adult oysters to breed (in future) / AW ;

This question in 9693/41 May/June 2025

3 Carbon dioxide produced by the combustion of fossil fuels may lead to ocean acidification. (a) Describe how carbon dioxide reacts with water to increase the acidity. … … … … [2] (b) Fig. 3.1 shows a photograph of a free-swimming planktonic mollusc, Limacina helicina. This shelled species of mollusc is an important part of many Arctic food chains. Fig. 3.1 Scientists investigated the effect of changing pH on the mass of calcium carbonate deposited into the shells of Limacina helicina during larval development. Three larvae were incubated in each of two tanks of sea water, one at pH 8.1 and the other at pH 7.8. The mass of calcium carbonate deposited into the shells was measured every two hours for six hours. Fig. 3.2 shows the results. 3.0 line of best fit for pH 8.1 2.0 mass of calcium line of best fit for pH 7.8carbonate deposited in shell / μmol g–1 1.0 Key reading for individual animals at pH 8.1 reading for individual 0 animals at pH 7.8 0 1 2 3 4 5 6 7 time / hr Fig. 3.2 (i) Use the line of best fit in Fig. 3.2 to calculate the mean rate of calcium deposition for the mollusc larvae when placed at pH 7.8. State the unit. Show your working. … [3] (ii) A pH of 8.1 is the normal pH of water from the Arctic Ocean where Limacina helicina is found. Discuss the possible impact of ocean acidification on Arctic food webs. Use Fig. 3.2 to support your answer. … … … … … … … … [4] (c) Renewable energy installations such as wind turbines may help to reduce dependency on fossil fuels. Some scientists are concerned that the noise generated by the building of offshore wind turbines may affect the behaviour of marine mammals such as porpoises. An offshore wind turbine area was built in an area of the North Sea during 2006. Scientists recorded the number of days over six-month periods that porpoises were seen in the area: • before construction, in 2005 • during construction, in 2006 • after construction had finished, in 2007. To see if there was a significant difference in porpoise numbers during these three periods, a chi-squared ( χ 2) test was performed. Table 3.1 shows the chi-squared test results. Table 3.1 year number of expected (O – E) (O – E)2 (O – E)2 / E days that number of porpoises days that were detected porpoises (O) were detected (E) 2005 56 49 7 49 1.0 (before construction) 2006 54 49 5 25 0.5 (during construction) 2007 36 49 (after construction) (i) Complete Table 3.1. [1] (ii) Use Table 3.1 to calculate the value of chi-squared ( χ 2). Use the formula: (O – E)2 χ 2 = Σ E where, χ 2 = chi-squared value O = observed values E = expected values Σ = sum of … [1] (iii) The scientists made the null hypothesis: ‘There is no difference in the number of days that the area was visited by porpoises before, during or after construction of the offshore wind turbines.’ Table 3.2 shows the critical values of chi-squared. Table 3.2 probability degrees of freedom 0.50 0.10 0.05 0.01 1 0.455 2.706 3.841 6.635 2 1.386 4.605 5.991 9.210 3 2.366 6.251 7.815 11.345 4 3.357 7.779 9.488 13.277 Use your answer to 3(c)(ii) and Table 3.2 to assess the impact of the wind turbines on the porpoises. … … … … … … … … [4] (d) The use of renewable energy could reduce the risk of ocean acidification. State two other benefits of increasing the use of renewable energy installations. 1 … … 2 … … [2] [Total: 17]

17 marks

Mark scheme: 3(a) any 2 of: 2 1 forms carbonic acid / H2CO3 / AW ; 2 (dissociates) into H+ and HCO3– / results in high concentration of H+ / AW ; 3(b)(i) 0.22 to 0.25 ;; 3  mol g-1 hr-1 ; One mark for correct values of pH change and time 3(b)(ii) any 4 of: 4 1 rate of calcium carbonate deposition is lower at pH 7.8 / AW ; 2 as less carbonate (available) / CO32– AW ; 3 so weaker shells / AW ; 4 lower survival / populations fall / easily predated / AW ; 5 lower primary productivity (due to low pH) / less photosynthesis by producers / AW ; 6 less food / biomass / energy to pass along food chain (for higher trophic levels) / AW ; 7 algae populations could increase if fewer snails to consume them / AW ; 8 range of points on graph overlap / large range of masses / points are far from lines of best fit / weak correlation / AW ; 9 sample size is low ; 3(c)(i) 1 year number expected (O–E) (O–E)2 (O–E)2 / E of days number that of days porpoises that were porpoises detected were (O) detected (E) 2005 (before 56 49 7 49 1 construction) 2006 (during 54 49 5 25 0.5 construction) 2007 (after 36 49 –13 169 3.4(5) ; construction) 3(c)(ii) 4.9(5) ; 1 3(c)(iii) any 4 of: 4 1 null hypothesis is, not rejected / accepted, / porpoises are not harmed by wind turbine construction ; 2 calculated value is lower than critical value / AW ; 3 identified critical value as 5.991 ; 4 greater than 5% probability that the difference is due to chance ; 5 no significant difference / porpoise movement is not significantly affected ; 3(d) any 2 of: 2 1 (reduced dependency on fossil fuels so) reduced greenhouse effect / less climate change / less global warming / less risk of sea level rises / AW ; 2 will not run out / renewable / sustainable / AW ; 3 low risk of acid rain ; 4 low risk of oil spills ; 5 no habitat damage from (oil) drilling / AW ;

This question in 9693/42 May/June 2025

3 Carbon dioxide produced by the combustion of fossil fuels may lead to ocean acidification. (a) Describe how carbon dioxide reacts with water to increase the acidity. … … … … [2] (b) Fig. 3.1 shows a photograph of a free-swimming planktonic mollusc, Limacina helicina. This shelled species of mollusc is an important part of many Arctic food chains. Fig. 3.1 Scientists investigated the effect of changing pH on the mass of calcium carbonate deposited into the shells of Limacina helicina during larval development. Three larvae were incubated in each of two tanks of sea water, one at pH 8.1 and the other at pH 7.8. The mass of calcium carbonate deposited into the shells was measured every two hours for six hours. Fig. 3.2 shows the results. 3.0 line of best fit for pH 8.1 2.0 mass of calcium line of best fit for pH 7.8carbonate deposited in shell / μmol g–1 1.0 Key reading for individual animals at pH 8.1 reading for individual 0 animals at pH 7.8 0 1 2 3 4 5 6 7 time / hr Fig. 3.2 (i) Use the line of best fit in Fig. 3.2 to calculate the mean rate of calcium deposition for the mollusc larvae when placed at pH 7.8. State the unit. Show your working. … [3] (ii) A pH of 8.1 is the normal pH of water from the Arctic Ocean where Limacina helicina is found. Discuss the possible impact of ocean acidification on Arctic food webs. Use Fig. 3.2 to support your answer. … … … … … … … … [4] (c) Renewable energy installations such as wind turbines may help to reduce dependency on fossil fuels. Some scientists are concerned that the noise generated by the building of offshore wind turbines may affect the behaviour of marine mammals such as porpoises. An offshore wind turbine area was built in an area of the North Sea during 2006. Scientists recorded the number of days over six-month periods that porpoises were seen in the area: • before construction, in 2005 • during construction, in 2006 • after construction had finished, in 2007. To see if there was a significant difference in porpoise numbers during these three periods, a chi-squared ( χ 2) test was performed. Table 3.1 shows the chi-squared test results. Table 3.1 year number of expected (O – E) (O – E)2 (O – E)2 / E days that number of porpoises days that were detected porpoises (O) were detected (E) 2005 56 49 7 49 1.0 (before construction) 2006 54 49 5 25 0.5 (during construction) 2007 36 49 (after construction) (i) Complete Table 3.1. [1] (ii) Use Table 3.1 to calculate the value of chi-squared ( χ 2). Use the formula: (O – E)2 χ 2 = Σ E where, χ 2 = chi-squared value O = observed values E = expected values Σ = sum of … [1] (iii) The scientists made the null hypothesis: ‘There is no difference in the number of days that the area was visited by porpoises before, during or after construction of the offshore wind turbines.’ Table 3.2 shows the critical values of chi-squared. Table 3.2 probability degrees of freedom 0.50 0.10 0.05 0.01 1 0.455 2.706 3.841 6.635 2 1.386 4.605 5.991 9.210 3 2.366 6.251 7.815 11.345 4 3.357 7.779 9.488 13.277 Use your answer to 3(c)(ii) and Table 3.2 to assess the impact of the wind turbines on the porpoises. … … … … … … … … [4] (d) The use of renewable energy could reduce the risk of ocean acidification. State two other benefits of increasing the use of renewable energy installations. 1 … … 2 … … [2] [Total: 17]

17 marks

Mark scheme: 3(a) any 2 of: 2 1 forms carbonic acid / H2CO3 / AW ; 2 (dissociates) into H+ and HCO3– / results in high concentration of H+ / AW ; 3(b)(i) 0.22 to 0.25 ;; 3  mol g-1 hr-1 ; One mark for correct values of pH change and time 3(b)(ii) any 4 of: 4 1 rate of calcium carbonate deposition is lower at pH 7.8 / AW ; 2 as less carbonate (available) / CO32– AW ; 3 so weaker shells / AW ; 4 lower survival / populations fall / easily predated / AW ; 5 lower primary productivity (due to low pH) / less photosynthesis by producers / AW ; 6 less food / biomass / energy to pass along food chain (for higher trophic levels) / AW ; 7 algae populations could increase if fewer snails to consume them / AW ; 8 range of points on graph overlap / large range of masses / points are far from lines of best fit / weak correlation / AW ; 9 sample size is low ; 3(c)(i) 1 year number expected (O–E) (O–E)2 (O–E)2 / E of days number that of days porpoises that were porpoises detected were (O) detected (E) 2005 (before 56 49 7 49 1 construction) 2006 (during 54 49 5 25 0.5 construction) 2007 (after 36 49 –13 169 3.4(5) ; construction) 3(c)(ii) 4.9(5) ; 1 3(c)(iii) any 4 of: 4 1 null hypothesis is, not rejected / accepted, / porpoises are not harmed by wind turbine construction ; 2 calculated value is lower than critical value / AW ; 3 identified critical value as 5.991 ; 4 greater than 5% probability that the difference is due to chance ; 5 no significant difference / porpoise movement is not significantly affected ; 3(d) any 2 of: 2 1 (reduced dependency on fossil fuels so) reduced greenhouse effect / less climate change / less global warming / less risk of sea level rises / AW ; 2 will not run out / renewable / sustainable / AW ; 3 low risk of acid rain ; 4 low risk of oil spills ; 5 no habitat damage from (oil) drilling / AW ;

This question in 9693/43 May/June 2025

4 Environmental groups have warned that increasing ocean acidification is causing the shells of marine molluscs to become thinner. Fig. 4.1 shows a photograph of the shell of a marine mollusc species called Neptunea contraria. Fig. 4.1 (a) Make a large drawing of the shell of Neptunea contraria shown in Fig. 4.1. Do not include the markings on the shell. Do not label your drawing. [4] (b) (i) Describe how carbon dioxide reacts with water to affect the pH. … … … … [2] (ii) Explain why excess atmospheric carbon dioxide can cause the shells of marine molluscs to become thinner. … … … … [2] (c) The temperature of water may affect the speed at which mollusc shells are damaged by acidity. Plan a laboratory-based investigation that you could do to determine the effect of temperature on the loss of mass of mollusc shells due to acidic water. You are provided with standard laboratory apparatus and materials. Your plan should: • include a clear statement of the hypothesis • identify the independent, dependent and standardised variables • include full details of the method so that another person can follow it • describe how you would analyse your results • be safe and ethical. … … … … … … … … … … … … … … … … … … … … [11] [Total: 19]

19 marks

Mark scheme: 4(a) thin, unbroken, clear lines with no shading, no stippling and no overhanging lines ; 4 correct proportions of spirals + aperture ; (aperture is approximately half width of shell and height shell is in proportion) minimum of five spirals and aperture shown ; at least size of original image ; 4(b)(i) any 2 of: 2 carbon dioxide dissolves in water ; forms carbonic acid / H2CO3 ; produces / dissociates into, H+ (ions) and HCO3– (ions) ; 4(b)(ii) any 2 of: 2 acid / H+, reacts with carbonate ions / AW ; less calcium carbonate deposited in shells / AW ; shells dissolve / poor shell growth / AW ; 4(c) hypothesis 11 1 higher temperature increases loss of mass of shells / AW ; plus any 10 of: independent variable 2 temperature ; 3 at least five temperatures ; dependent variable 4 mass / weight, of shell ; 5 measured before and after ; control variables (max 3) 1 same age / number / species of mollusc shell or mollusc species / shape of shell / AW ; 2 same pH (if stated pH, must be lower than 7) ; 3 same volumes of water ; 4 same volume of buffer solution ; 5 same carbon dioxide concentration / AW ; 6 same salinity ; 7 same time / stated time ; 4(c) method (max 3) 8 use of water bath / method of maintaining temperature of water ; 9 using balance to measure mass ; 10 control pH by adding buffer ; 11 reference to drying shells before (re-)weighing ; 12 reference to placing shell in water as a control / AW ; analysis (max 3) 13 replicate the experiment 3 times and calculate mean / median / standard deviation / AW ; 14 plot a graph of change in mass against temperature / AW ; 15 calculate percentage change in mass / AW ; 16 calculate rate of loss by dividing change in mass by time / AW ; 17 use of a suitable named statistical test ; 18 example of correct results table ; safety and ethics 19 identification of any risk with method to minimise / AW ; 20 do not take shells from wild molluscs as this could damage ecosystem / dispose of chemicals correctly / AW ;

This question in 9693/41 Oct/Nov 2025

4 Environmental groups have warned that increasing ocean acidification is causing the shells of marine molluscs to become thinner. Fig. 4.1 shows a photograph of the shell of a marine mollusc species called Neptunea contraria. Fig. 4.1 (a) Make a large drawing of the shell of Neptunea contraria shown in Fig. 4.1. Do not include the markings on the shell. Do not label your drawing. [4] (b) (i) Describe how carbon dioxide reacts with water to affect the pH. … … … … [2] (ii) Explain why excess atmospheric carbon dioxide can cause the shells of marine molluscs to become thinner. … … … … [2] (c) The temperature of water may affect the speed at which mollusc shells are damaged by acidity. Plan a laboratory-based investigation that you could do to determine the effect of temperature on the loss of mass of mollusc shells due to acidic water. You are provided with standard laboratory apparatus and materials. Your plan should: • include a clear statement of the hypothesis • identify the independent, dependent and standardised variables • include full details of the method so that another person can follow it • describe how you would analyse your results • be safe and ethical. … … … … … … … … … … … … … … … … … … … … [11] [Total: 19]

19 marks

Mark scheme: 4(a) thin, unbroken, clear lines with no shading, no stippling and no overhanging lines ; 4 correct proportions of spirals + aperture ; (aperture is approximately half width of shell and height shell is in proportion) minimum of five spirals and aperture shown ; at least size of original image ; 4(b)(i) any 2 of: 2 carbon dioxide dissolves in water ; forms carbonic acid / H2CO3 ; produces / dissociates into, H+ (ions) and HCO3– (ions) ; 4(b)(ii) any 2 of: 2 acid / H+, reacts with carbonate ions / AW ; less calcium carbonate deposited in shells / AW ; shells dissolve / poor shell growth / AW ; 4(c) hypothesis 11 1 higher temperature increases loss of mass of shells / AW ; plus any 10 of: independent variable 2 temperature ; 3 at least five temperatures ; dependent variable 4 mass / weight, of shell ; 5 measured before and after ; control variables (max 3) 1 same age / number / species of mollusc shell or mollusc species / shape of shell / AW ; 2 same pH (if stated pH, must be lower than 7) ; 3 same volumes of water ; 4 same volume of buffer solution ; 5 same carbon dioxide concentration / AW ; 6 same salinity ; 7 same time / stated time ; 4(c) method (max 3) 8 use of water bath / method of maintaining temperature of water ; 9 using balance to measure mass ; 10 control pH by adding buffer ; 11 reference to drying shells before (re-)weighing ; 12 reference to placing shell in water as a control / AW ; analysis (max 3) 13 replicate the experiment 3 times and calculate mean / median / standard deviation / AW ; 14 plot a graph of change in mass against temperature / AW ; 15 calculate percentage change in mass / AW ; 16 calculate rate of loss by dividing change in mass by time / AW ; 17 use of a suitable named statistical test ; 18 example of correct results table ; safety and ethics 19 identification of any risk with method to minimise / AW ; 20 do not take shells from wild molluscs as this could damage ecosystem / dispose of chemicals correctly / AW ;

This question in 9693/42 Oct/Nov 2025

4 Environmental groups have warned that increasing ocean acidification is causing the shells of marine molluscs to become thinner. Fig. 4.1 shows a photograph of the shell of a marine mollusc species called Neptunea contraria. Fig. 4.1 (a) Make a large drawing of the shell of Neptunea contraria shown in Fig. 4.1. Do not include the markings on the shell. Do not label your drawing. [4] (b) (i) Describe how carbon dioxide reacts with water to affect the pH. … … … … [2] (ii) Explain why excess atmospheric carbon dioxide can cause the shells of marine molluscs to become thinner. … … … … [2] (c) The temperature of water may affect the speed at which mollusc shells are damaged by acidity. Plan a laboratory-based investigation that you could do to determine the effect of temperature on the loss of mass of mollusc shells due to acidic water. You are provided with standard laboratory apparatus and materials. Your plan should: • include a clear statement of the hypothesis • identify the independent, dependent and standardised variables • include full details of the method so that another person can follow it • describe how you would analyse your results • be safe and ethical. … … … … … … … … … … … … … … … … … … … … [11] [Total: 19]

19 marks

Mark scheme: 4(a) thin, unbroken, clear lines with no shading, no stippling and no overhanging lines ; 4 correct proportions of spirals + aperture ; (aperture is approximately half width of shell and height shell is in proportion) minimum of five spirals and aperture shown ; at least size of original image ; 4(b)(i) any 2 of: 2 carbon dioxide dissolves in water ; forms carbonic acid / H2CO3 ; produces / dissociates into, H+ (ions) and HCO3– (ions) ; 4(b)(ii) any 2 of: 2 acid / H+, reacts with carbonate ions / AW ; less calcium carbonate deposited in shells / AW ; shells dissolve / poor shell growth / AW ; 4(c) hypothesis 11 1 higher temperature increases loss of mass of shells / AW ; plus any 10 of: independent variable 2 temperature ; 3 at least five temperatures ; dependent variable 4 mass / weight, of shell ; 5 measured before and after ; control variables (max 3) 1 same age / number / species of mollusc shell or mollusc species / shape of shell / AW ; 2 same pH (if stated pH, must be lower than 7) ; 3 same volumes of water ; 4 same volume of buffer solution ; 5 same carbon dioxide concentration / AW ; 6 same salinity ; 7 same time / stated time ; 4(c) method (max 3) 8 use of water bath / method of maintaining temperature of water ; 9 using balance to measure mass ; 10 control pH by adding buffer ; 11 reference to drying shells before (re-)weighing ; 12 reference to placing shell in water as a control / AW ; analysis (max 3) 13 replicate the experiment 3 times and calculate mean / median / standard deviation / AW ; 14 plot a graph of change in mass against temperature / AW ; 15 calculate percentage change in mass / AW ; 16 calculate rate of loss by dividing change in mass by time / AW ; 17 use of a suitable named statistical test ; 18 example of correct results table ; safety and ethics 19 identification of any risk with method to minimise / AW ; 20 do not take shells from wild molluscs as this could damage ecosystem / dispose of chemicals correctly / AW ;

This question in 9693/43 Oct/Nov 2025