Cambridge A Level Marine Science 9693 — 2022 May/June Paper 4 · Variant 2

1 Desalination plants are industrial facilities that produce fresh water from sea water. A group of students investigated the effect of a desalination plant on the salinity of sea water and the percentage cover of the seabed with seagrass. They measured the salinity of the sea water and percentage cover of the seabed with seagrass every 5 m from the desalination plant over a total distance of 30 m. The results are shown in Table 1.1. Table 1.1 distance from desalination salinity of sea water / ppt percentage cover of plant / m seabed with seagrass 5 45 25 10 41 27 15 34 58 20 33 85 25 32 95 30 32 95 (a) (i) Draw a graph to show how the salinity of the sea water and percentage cover of the seabed with seagrass change with distance from the desalination plant. Join your points with ruled, straight lines. [5] (ii) Use your graph and Table 1.1 to suggest an explanation for the distribution of the seagrass. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (b) Outline how salmon perform osmoregulation in regions of high salinity. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] [Total: 10]

Mark scheme: 1(a)(i) linear axes that use at least one y axis scale half the grid ; all axes labelled ; plots correct ; points joined by straight lines ; key for both lines / labelled lines; 5 1(a)(ii) any 2 from: seagrass cannot live in areas with high salinity / less seagrass grows in areas with high salinity / seagrass cannot live in areas of concentrated brine / ORA ; high salinity / water near desalination plant, has a low water potential ; water is lost from seagrass cells (by osmosis) / cells lose turgidity / AW ; toxins are released from desalination plant / AW ; 2 1(b) any 3 from: salmon body (cells) have higher water potential than seawater so lose water (by osmosis) ; salmon drink water ; excrete / release, salt / AW ; produce concentrated urine ; active pumping (of salt) into water ; 3

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2 The effect of mercury from untreated sewage released into the ocean was investigated. Table 2.1 shows the mean concentrations of mercury in the body tissues of species of fish with different diets that live in areas where untreated sewage is released. Table 2.1 diet of fish mean concentration of mercury in body tissues of fish / μg g–1 plants 72 small invertebrates 122 plants and invertebrates 97 small fish 382 mixed small and large fish 453 dead fish 597 (a) Use Table 2.1 to suggest an explanation for the different mean concentrations of mercury in the body tissues of fish with different diets. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [4] (b) (i) Tuna have a diet of mixed small and large fish. Many health authorities advise pregnant women to eat no more than 340 g of tuna each week. Use Table 2.1 to calculate the mass of mercury found in 340 g of tuna. Give your answer to three significant figures. ..................................................... μg [2] (ii) Suggest why pregnant women are advised to restrict the amount of tuna in their diet. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] [Total: 8]

Mark scheme: 2(a) any 4 from: mercury passes along food chains / AW ; mercury of is not broken down / bioaccumulation / excreted ; biomagnification occurs (of mercury through the food chains / trophic levels) / mercury builds up ; fish from higher trophic levels / fish that eat other fish / predators, have higher levels of mercury / fish that consume dead organisms have high levels / ORA ; dead fish may have high concentrations which killed them / were predator species / AW ; plants obtain mercury from substrate / do not obtain mercury from other trophic levels / do not feed on other organisms ; 2(b)(i) 154 000 (g) ;; 2 Question Answer Marks 2(b)(ii) any 2 from: tuna are top predators / high trophic levels / at top of food chain and accumulate mercury / have high concentrations of mercury ; mercury could harm baby / is toxic / nervous system problems / AW ; mercury concentration in baby would be high (due to smaller size of baby) / what is safe for adult is not for baby 2

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3 A student investigated the rate of respiration of a species of coral. The coral was a species that contained zooxanthellae. A piece of the coral was placed into a tank filled with one cubic decimetre of sea water. The concentration of oxygen dissolved in the water was measured every five minutes for 25 minutes. The coral was kept in darkness throughout the experiment. The results are shown in Fig. 3.1. 10.0 9.5 concentration of oxygen in water 9.0 / mg dm–3 8.5 8.0 0 5 10 15 20 25 time / min Fig. 3.1 (a) (i) Give the balanced chemical (symbol) equation for aerobic respiration. ..................................................................................................................................... [2] (ii) Explain why the experiment was carried out in darkness. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (iii) Use Fig. 3.1 to calculate the mean rate of change of oxygen concentration over the first 15 minutes of the experiment. Show your working and state the correct unit. .......................................................... [3] (b) In a second investigation, a pump was used to circulate the water over the coral polyps. The rate of oxygen uptake by the coral polyps was measured at different speeds of water current. The results are shown in Fig. 3.2. 0.60 0.55 0.50 0.45 0.40 rate of oxygen uptake / mg min–1 0.35 0.30 0.25 0.20 0.15 0.10 0 2 4 6 8 10 12 water current speed / arbitrary units Fig. 3.2 (i) Describe the effect of increasing water current speed on the rate of oxygen uptake. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (ii) Suggest reasons for the effect of increasing water current speed on the rate of oxygen uptake. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] (c) Coral polyps use simple diffusion for gaseous exchange. Agar cubes that contain alkali and indicator solution can be used as a model for diffusion. The agar cubes can be placed into hydrochloric acid. As the hydrochloric acid diffuses into the agar, it neutralises the alkali and changes the indicator from pink to colourless. This is shown in Fig. 3.3. acid agar cube area of agar that acid has not reached Fig. 3.3 The time taken for the agar to turn colourless when placed into hydrochloric acid is a measure of the rate of diffusion of acid to the centre of the agar cubes. Plan an investigation into the effect of temperature on diffusion of hydrochloric acid into agar cubes. Your plan should: • include a clear statement of the hypothesis • identify the key variables • include full details of the method • describe how you would analyse your results • be safe and ethical. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ........................................................................................................................................... [12] [Total: 24]

Mark scheme: 3(a)(i) 6O2 + C6H12O6  6CO2 + 6H2O 2 3(a)(ii) coral / zooxanthellae would not photosynthesise / photosynthesis would not occur ; so no oxygen is released / oxygen levels are not affected (by photosynthesis) ; 2 3(a)(iii) 10 – 8.5 (= 1.5) 8.5 – 10.0 = (1.5) / 15 = 0.1 ; negative gradient (minus) (–0.1) ; mg dm-3 min-1 ; 3 3(b)(i) increase ; level off / decrease from, 7 / 8 / 9 (a.u.) ; 2 3(b)(ii) any 3 from: maintains / steeper, diffusion / concentration gradient ; movement of tentacles (increasing surface area contact with oxygen in water) ; another factor begins to limit rate (at speed of 7) ; such as, surface area of tentacles, rate of oxygen use by coral / temperature / AW ; 3 Question Answer Marks 3(c) hypothesis: increasing temperature increases speed that agar changes colour / increases rate of diffusion / reduces time taken for agar to change colour / AW ; plus any 11 from: variables: independent variable: temperature ; at least five stated temperatures ; dependent variable: time taken for block to change colour / rate of diffusion ; record time taken for block to change colour with stop clock / timer ; controls: max 3 from: cubes have same surface area / volume ; same concentration of acid / pH ; same volume of acid ; same concentration of alkali in block ; same concentration of agar ; method: use of water bath / AW ; left for period of time to reach temperature ; cut agar with scalpel / knife and measure with ruler ; analysis max 3 from: repeats, and means calculated / anomalies identified; rate calculated by inverse of time / AW ; graph of rate / time taken against temperature ; appropriate statistical test ; 12 Question Answer Marks 3(c) safety: max 1 from: safety with cutting away from the body / take care with scalpel correct safety precaution linked with acid, e.g. eye protection care with hot water baths ; ethics max 1 from: do not let acid / indicator / alkali drain into natural water ; no living things involved / affected so few ethical issues ;

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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]

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

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5 Lionfish are a species of carnivorous fish naturally found in the Pacific Ocean near Indonesia. Lionfish are now found in the western Atlantic Ocean and Caribbean Sea and are classed as an invasive species. (a) State what is meant by an invasive species, as defined by the IUCN. ................................................................................................................................................... ............................................................................................................................................. [1] (b) Fig. 5.1 shows the change in population of lionfish between 2004 and 2010 on an area of coral reef near the Bahamas in the western Atlantic Ocean. 50 45 40 35 mean 30 number of lionfish 25 counted per survey 20 15 10 5 0 2004 2005 2006 2007 2008 2009 2010 year Fig. 5.1 (i) Suggest explanations for the changes in population of lionfish on this coral reef between 2004 and 2010 shown in Fig. 5.1. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] (ii) Fig. 5.1 has error bars that represent the standard deviation. Explain what the standard deviations in Fig. 5.1 show about the data. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2] (c) Fig. 5.2 shows the percentage change in abundance of other species of fish and algae growing on the same coral reef between 2004 and 2010. The species of fish were classed as: • small fish species that are prey of lionfish • small fish species that are not prey of lionfish • large fish species that are competitors of lionfish • large fish species that are not competitors of lionfish +100 +80 +60 +40 +20 percentage change 0 in abundance –20 –40 –60 –80 –100 small prey small non-prey large large algae species species competitor non-competitor species species Fig. 5.2 Discuss the change in abundance of the different species of fish and algae shown in Fig. 5.2. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] (d) Scientists investigated the effect of removing lionfish on the population of damselfish on a reef. Divers physically removed lionfish from an area of reef each week for a period of six months. The population of damselfish on the reef was then recorded. The population of damselfish on an area of identical reef where lionfish were not removed was also recorded. Both reefs had the same initial populations of damselfish. The scientists carried out a chi-squared test to see if removing the lionfish caused a change in the population of damselfish. They made the following null hypothesis: Removing the lionfish did not affect the number of damselfish on the reef. The results are shown in Table 5.1. Table 5.1 reef area number of expected (O – E) (O – E)2 (O – E)2 damselfish number of E (O) damselfish (E) lionfish 420 390 30 900 2.308 removed no lionfish 360 390 removed (i) Complete Table 5.1. [1] (ii) Use the formula to calculate the chi-squared value for the results. (O – E)2 chi-squared = Σ E Σ = sum of (total) O = observed values E = expected values .......................................................... [1]

Mark scheme: 5(a)(i) a species that has been introduced outside its natural past or present distribution and has become problematic / AW ; 1 5(b)(i) any 3 from: between 2004 and 2006 / up to 2006, (slow population increase) as lionfish, adapt to area / have few to breed / take time to establish / AW ; from 2004, increase as lionfish have abundant food / few predators / high breeding rate / outcompete other species / AW ; plenty of niches available for lionfish ; from 2007 / 2008 population stabilises / levels off / falls, due to competition / as food limiting / predators arrive / control methods / harvesting / AW ; 3 Question Answer Marks 5(b)(ii) any 2 from: wide variation in number of lionfish sightings in years between 2007 and 2010 ; less / little variation in number of lionfish sightings in years 2004 / 2005 / 2006 ; no overlap between 2006 and 2007 shows (significant) difference / AW ; overlap between 2007 and 2010 shows no (significant) difference / AW ; 2 5(c) any 3 from: decrease in (small) prey species as they are eaten / predated (by lionfish) ; increase in (small) non-prey species due to less competition (from other fish) / more algae to eat ; large competitors decrease as lionfish consume food / take territory / AW ; large non-competitors show similar numbers so maintain niche / are unaffected / have enough food ; algae increases, as small herbivores / small prey species, are, consumed / are fewer ; 3 5(d)(i) Reef area Number of damselfish (O) Expected number of damselfish (E) (O–E) (O–E)2  2 ( ) O E E Lionfish removed 420 390 30 900 2.308 No lionfish removed 360 390 –30 900 2.308 ; 1 5(d)(ii) 4.616 ; 1 5(d)(iii) any 3 from: the null hypothesis is rejected ; there is a significant difference (in the number of damselfish) ; the calculated value is greater than the critical value ; correct reference to 3.841 / critical value for 0.05 and 1 degree of freedom ; there is a probability of less than, 0.05 / 5, that the difference is due to chance ; 3 Question Answer Marks 5(d)(iv) any 2 from: marine ecosystems are large ; species cross (national) national borders ; species are migratory / breed in different locations ; ocean currents move species / pollutants around the globe / AW ; trade of species occurs between countries / ref to trade restrictions ; some areas of ocean / High Seas have no national ownership 2

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