Why did I lose the mark?

The answer that scoresIn ice, each water molecule is hydrogen bonded to four neighbouring molecules, and those bonds hold the molecules at fixed distances apart, building an open lattice with large empty spaces in it. When ice melts, the lattice collapses and the molecules pack more closely together, so liquid water contains more mass in the same volume than ice does. Ice therefore has a lower density (about 917 kg m-3) than sea water (about 1025 kg m-3) and floats. Because the ice floats, it forms a layer on the surface that insulates the water beneath from the cold air, and it provides a habitat on its underside for ice algae and the animals that graze on them.

The answer that scoresWhen sodium chloride is added to water, the polar water molecules surround the ions at the surface of the lattice. Their slightly negative oxygen ends are attracted to the Na+ ions and their slightly positive hydrogen ends to the Cl- ions. This attraction is stronger than the attraction holding the ions in the lattice, so the ions are pulled out one at a time and become surrounded by water molecules, spread evenly through the liquid. The ions have been separated, not melted and not chemically changed, which is why the solution conducts electricity and why evaporating the water leaves the original sodium chloride behind.

The answer that scoresAs the temperature of water rises, the solubility of oxygen falls, because the dissolved oxygen molecules gain kinetic energy and escape from the water surface faster than new oxygen dissolves from the air. Cold water therefore holds more dissolved oxygen than warm water, which is why polar seas are oxygen-rich and why warm tropical surface water holds the least. Salinity has the same effect in the same direction: oxygen is less soluble in salt water than in fresh water. For a marine organism this matters because warm, salty water holds little oxygen at the same time as the organism's metabolism, and therefore its oxygen demand, is highest - so warm shallow water can become dangerously low in oxygen.

The answer that scoresSea water is a mixture of different elements and compounds - water, dissolved salts such as sodium chloride, magnesium sulfate and calcium carbonate, dissolved gases such as oxygen and carbon dioxide, and organic matter. It is a mixture because each substance keeps its own identity, the proportions vary from place to place and with depth, and the components can be separated by physical means: evaporating the water leaves the salts behind, and filtering removes the suspended particles. A compound, by contrast, is two or more elements chemically joined in fixed proportions, like the water molecule itself.

The answer that scoresInside a water molecule, each hydrogen atom is joined to the oxygen atom by a covalent bond, a shared pair of electrons. Because oxygen holds the shared electrons more strongly, each hydrogen end of the molecule is slightly positive and the oxygen end is slightly negative. The slightly positive hydrogen of one water molecule is then attracted to the slightly negative oxygen of a neighbouring molecule, and that attraction is a hydrogen bond. It is weak compared with a covalent bond, but each molecule can form up to four of them, they break and re-form constantly, and together they explain water's solvent action, its density behaviour and its high specific heat capacity.

The answer that scoresWeathering is the breakdown of rock in place by physical, chemical or organic processes; the products stay where they formed, so a cliff can be weathered without any rock being removed. Erosion is the removal of that material and its transport by a moving agent - water, ice, wind or gravity. On a coast the two work together: salt crystallisation and freeze-thaw loosen blocks of rock from the cliff, and the next storm's waves erode them away and move them along the shore.

The answer that scoresThe littoral zone is the intertidal region of a shoreline, between the highest and lowest spring tide marks - the full vertical range that the tide can reach over the spring-neap cycle. It is covered and uncovered twice a day, so its organisms must tolerate drying out, changing temperature and salinity, and wave action. Examples include rocky shores, sandy shores, muddy shores, estuaries and deltas.

The answer that scoresSpring tides occur at new moon and at full moon, when the Earth, Moon and Sun are in a straight line, so the gravitational pulls of the Moon and Sun act in the same direction and their bulges add together, giving the greatest range. Neap tides occur at the first and third quarters, when the Sun and Moon are at right angles as seen from Earth, so their pulls partly cancel and the range is at its smallest. Each occurs about twice a month, in every season.

The answer that scoresThe fluid leaving a vent is hot, acidic and rich in dissolved metals, silica and sulfate. When it meets cold seawater it cools almost instantly, and the solubility of the metal sulfides and silica falls, so they precipitate as fine particles. The particles are deposited around the vent opening and on one another, and the deposit builds upwards and outwards into a chimney with a throat through which the fluid still flows. Growth continues while the flow is strong and stops, and the chimney is eventually dissolved or broken down, when the flow shifts or the throat blocks.

The answer that scoresVent fluid can reach 300-400 °C, but because it is under the enormous pressure of 2 000-3 000 m of seawater it does not boil; it remains a liquid, expands and becomes much less dense than the surrounding cold water. That density difference makes it rise quickly out of the vent as a buoyant jet. The plume forms where the jet mixes with cold seawater: the dissolved metals react with sulfide and precipitate as fine particles, and the plume rises hundreds of metres before spreading sideways with the current.

The answer that scoresDuring El Nino the Pacific trade winds weaken and sometimes reverse, so the warm surface water that is normally piled up in the west spreads back eastwards and the thermocline deepens in the eastern Pacific. Because the wind is no longer pulling surface water away from the Peruvian coast, upwelling is suppressed, nutrient supply falls, plankton production collapses and the anchoveta fishery fails. Rainfall shifts with the warm water, bringing floods to Peru and Ecuador and drought to Indonesia and eastern Australia. In La Nina the trade winds strengthen, the eastern Pacific becomes colder than normal, upwelling and productivity increase, and the rainfall pattern intensifies in the west.

The answer that scoresCommensalism is a symbiotic relationship in which one organism benefits and the other is neither helped nor harmed. In the whale and barnacle example, the barnacle gains a hard surface to attach to in the open ocean, transport across large distances, and a flow of plankton-rich water over its feeding legs as the whale swims. The whale gains nothing and loses nothing measurable, because its skin is thick and the barnacles are small, so the benefit pattern is + / 0 and the relationship is commensal. Mutualism would require both partners to benefit, as in a boxer crab, which gains the stinging defence of its anemones while the anemones are carried to new feeding areas.

The answer that scoresEnergy is lost at each transfer for four named reasons. Most is released as heat during respiration, because the energy an organism needs for movement, growth and maintenance is eventually lost as heat that the next trophic level cannot use. Some of an organism is never eaten - shells, bones, scales and tough structural tissue. Some of what is eaten is not digested and is egested in faeces. Some is lost as nitrogenous waste during excretion. Typically only about 10 per cent of the energy is passed on, so the energy content falls steeply at each trophic level, which is why a pyramid of energy always narrows upwards and why food chains are short.

The answer that scoresPyramids of energy, numbers and biomass are different diagrams and only the last two can be inverted. A pyramid of energy shows the energy passing through each trophic level per unit area per unit time, and because respiration, egestion, excretion and uneaten parts all remove energy, each level contains less than the one below, so it is always upright. A pyramid of numbers counts individuals and can be inverted: one large kelp plant can support many grazing sea urchins, and a single host fish can carry hundreds of parasitic copepods, so the higher level is wider. A pyramid of biomass is usually upright but can also be inverted in the sea where phytoplankton have a small standing crop yet multiply and are eaten so quickly that their total production is large.

The answer that scoresThe dissolved nutrient reservoir is depleted by uptake into organisms, so in the sunlit surface layer of the open ocean nitrate and phosphate are quickly used up by phytoplankton and locked into biomass. Dead organisms and faecal pellets sink out of the photic zone as marine snow, so the nutrients they contain are released by decomposition in deep water, out of reach of the phytoplankton. Productivity in surface water is therefore limited by the nutrient in shortest supply, and only where upwelling, run-off, tectonic input, dissolving gases, excretion and decomposition return nutrients to the surface does productivity rise. That is why the central ocean gyres are unproductive blue water while coastal upwelling zones turn green and support huge fisheries.

The answer that scoresProductivity is limited by the nutrient in shortest supply, so adding more of a nutrient that is already available makes no difference. If nitrate is plentiful but phosphate is scarce, adding nitrate will not increase the rate of photosynthesis, because the producers cannot build DNA, membranes or ATP without phosphorus. Limitation can also come from light, which falls with depth and turbidity, and from temperature. This is why nutrient enrichment only raises productivity where the added nutrient is the one in short supply, and why the same addition to an already nutrient-rich coastal water may have no measurable effect at all.

The answer that scoresPlankton is a diverse collection of generally microscopic organisms that have limited motility and drift in water currents. It includes phytoplankton, which are producers such as diatoms and dinoflagellates that photosynthesise and absorb dissolved nutrients, and zooplankton, which are consumers such as larvae, copepods and jellyfish. Because they cannot swim against a current, their distribution is controlled by the water movement rather than by the organisms themselves.

The answer that scoresA shark is a cartilaginous fish. Its skeleton is made of cartilage rather than bone, its gills open to the outside through separate gill slits instead of being covered by an operculum, its skin is covered in denticles rather than thin scales, and it has no swim bladder. It does have a lateral line and the same five fins as a bony fish, which is why those features cannot be used to tell the groups apart.

The answer that scoresA value of D = 0.66 means the habitat has moderately high species diversity: there is a good chance that two individuals sampled at random belong to different species. The value runs from 0, where all the individuals belong to one species, towards 1, where the species are equally abundant, so 0.66 indicates that several species are present with no single species completely dominating - and it should be compared only with a sample of the same area.

The answer that scoresrs = 0.91 shows a strong positive correlation between sediment organic content and worm density: as organic content rises, worm density tends to rise too. This does not prove causation, because both variables could be controlled by another factor such as the amount of fine sediment or the strength of the current. Only a controlled experiment, in which organic content is varied while everything else is held constant, could show that the relationship is causal.

The answer that scoresA workable pair uses an absolute feature: '1a Plant body with gas bladders along the blades -> kelp' and '1b Plant body without gas bladders -> go to pair 2'. Presence or absence of a structure does not depend on the age or size of the specimen, so the key still works for a small kelp and a large sea lettuce.

The answer that scoresCorals are animals in the phylum Cnidaria that live as sessile colonies of polyps. Each polyp has tentacles with nematocysts, a mouth, and a stomach, and it is a consumer that captures zooplankton. Many reef-building corals also live in a mutualistic symbiosis with zooxanthellae, single-celled algae inside their tissues; the algae photosynthesise and supply the polyp with sugars, glycerol and amino acids, while the polyp supplies carbon dioxide, nutrients from its waste and a protected position in bright shallow water. The polyp secretes a calcium carbonate skeleton, and the reef is the accumulated skeleton of many generations of polyps, which is why a reef is a biological structure rather than a rock.

The answer that scoresReef-building corals need warm water of about 20 to 30 °C, shallow water generally less than 50 m deep, clear water with little suspended sediment so that light reaches the zooxanthellae, and water that is low in nutrients, because nutrient enrichment encourages algae to grow over the coral and cloud the water. Salinity must be fully marine and stable at around 35, so reefs do not form off large river mouths, and there must be hard substrate for the larvae to settle on and moderate water movement to supply oxygen and remove sediment. The requirement for low nutrients is why reefs are found in clear tropical seas rather than in turbid coastal water.

The answer that scoresThe benthic zone is the sea floor itself, from the shore down to the bottom of the deepest trench, so it underlies all of the water-column zones rather than lying beneath them. Its light level depends on the depth of water above it: the benthic zone under a shallow reef is brightly lit and covered in algae and corals, while the benthic zone on the abyssal plain is permanently dark and its animals depend on organic debris settling from above. The deepest water-column zone is the abyssopelagic, from about 4000 m to 6000 m, which is aphotic, near freezing and under very high pressure.

The answer that scoresSandy shores have a relatively low biodiversity because the substrate is unstable and shifting, so nothing can attach to it and the whole community of seaweeds, barnacles, mussels and anemones found on rocky shores is absent. The sediment is porous, so water drains away at low tide and the surface dries, heats and cools, and the variation in temperature and salinity through the tidal cycle is greater than on rock. Below the surface of fine sand the sediment is waterlogged and oxygen-poor, and the abrasive, moving sand damages delicate organisms. Only animals that can burrow, or that keep a connection to the surface with siphons, can live there, so the number of species is small - although the individuals of those few species can be very numerous.

The answer that scoresRhizophora mangle has prop roots that grow out from the trunk and branches and arch down into the mud. They brace the tree in soft, shifting sediment where an ordinary root system could not hold it upright, and they carry lenticels so that oxygen can be taken in from the air and supplied to the roots below, which is necessary because the waterlogged mud is low in oxygen. Salt is excluded at the root, where the membranes act as a filter and keep most sodium and chloride ions out so that the tree can absorb water from saline soil, and any salt that does enter is stored in older leaves that are then shed. Reproduction is viviparous: the seed germinates on the parent tree into a long propagule, which drops, floats, disperses and then roots in the mud.

The answer that scoresWater potential is a measure of the number of water molecules in a solution and their freedom to move. Pure water has a water potential of zero, and adding solute lowers it, so a concentrated solution has a more negative water potential than a dilute one. Osmosis is the net movement of water from a region of higher water potential (less negative, closer to zero) to a region of lower water potential (more negative), through a partially permeable membrane. So water does move towards the more concentrated solution, but that solution is the one with the lower water potential. For example, a marine fish in sea water loses water because its body fluids are more dilute, and therefore have a higher water potential, than the sea around it.

The answer that scoresFacilitated diffusion is a passive process: glucose, amino acids and ions move down their concentration gradient through specific channel or carrier proteins in the membrane, and no ATP is used. It is needed because these substances are polar or charged and cannot cross the phospholipid bilayer. Active transport is different in two ways: it moves substances against their concentration gradient, and it uses ATP from respiration to do so. It works through carrier proteins, which bind the substance, use the energy from ATP to change shape, and release it on the other side of the membrane. A gill cell pumping chloride ions out into sea water, against a gradient, is active transport; glucose entering a cell down its gradient through a carrier is facilitated diffusion.

The answer that scoresPure water has a water potential of zero, which is higher than the water potential inside the cell, so water enters the plant cell by osmosis and the vacuole expands. As it expands it pushes the cytoplasm and membrane outwards against the cell wall, and the wall resists, producing an opposing pressure. The cell becomes turgid and stays intact, which is what gives non-woody plant tissue and algal fronds their firmness. An animal cell has no cell wall, so when water enters it swells and eventually the surface membrane ruptures and the cell bursts. In the opposite situation, a plant cell in a solution of lower water potential loses water, the vacuole shrinks and the membrane pulls away from the wall - plasmolysis - while an animal cell simply shrinks and becomes crenated.

The answer that scoresAn osmoconformer is an organism whose body fluid concentration follows that of the surrounding water - the marine mussel is the example, because its body fluids are isosmotic with sea water and it does not regulate them. A tuna is an osmoregulator: its body fluids are much more dilute than sea water, so its blood has a higher water potential than the sea and water is lost by osmosis across the gills. To replace it, the tuna drinks sea water and absorbs the water in its gut, and it gets rid of the salts that come in with it by actively excreting ions across its gills, while producing only a small volume of urine. Because it must hold its internal concentration at roughly a third of sea water, the tuna is also stenohaline - it tolerates only a narrow range of salinity and cannot move into estuaries.

The answer that scoresSurface area to volume ratio falls as an organism gets bigger. A cube 1 cm on a side has a surface area of 6 cm² and a volume of 1 cm³, a ratio of 6:1, but a cube 4 cm on a side has a surface area of 96 cm² and a volume of 64 cm³, a ratio of only 1.5:1. The surface is where gases are exchanged and the volume is the tissue that needs supplying, so a small organism has plenty of surface for each unit of tissue and a short diffusion distance to its centre, and simple diffusion is enough. A large fish has a small surface for a large volume and a long diffusion distance, so diffusion through the skin cannot supply its oxygen. It needs a specialised exchange surface with a large area and a thin barrier - the gills - and a transport system to carry the gases between the gills and the tissues.

The answer that scoresRed algae contain the phycobilin phycoerythrin, which absorbs green light at about 540-570 nm and passes the energy to chlorophyll a in the reaction centre. Water absorbs red light strongly, so red light is lost in the top 10 m, while blue-green light penetrates furthest - to 150-200 m in clear ocean water. Phycoerythrin reflects red light, which is why the alga looks red, but the light it actually harvests is green. This is why red algae grow deeper than green algae, whose chlorophyll a and b absorb red and blue-violet and are therefore restricted to shallow water where those wavelengths are still present.

The answer that scoresThe light-independent stage takes place in the stroma and does not absorb light itself, but it uses the ATP and reduced NADP made in the thylakoid membranes of the grana. Carbon dioxide is fixed by rubisco, and the fixed carbon is reduced to a simple sugar using the energy from ATP and the hydrogen and electrons from reduced NADP. In darkness the light-dependent stage stops, no more ATP or reduced NADP is produced, and the Calvin cycle therefore stops too once those carriers are used up. So it is light-independent for its inputs but not independent of light overall.

The answer that scoresIn the light-dependent stage, light absorbed by chlorophyll a photoactivates the pigment, raising electrons to a higher energy level so that they are emitted and pass along the electron transport chain in the thylakoid membrane. The electrons lost by chlorophyll are replaced by the photolysis of water: 2H2O -> 4H+ + 4e- + O2. The electrons reduce NADP, the protons contribute to the gradient used to make ATP, and oxygen is released as a by-product. Carbon dioxide plays no part in this stage; it is fixed in the stroma by rubisco in the light-independent stage.

The answer that scoresRf = distance moved by the pigment / distance moved by the solvent front. Both distances are measured from the origin line, and because one is divided by the other the Rf has no units and can never exceed 1. Carotene, the most soluble pigment, travels furthest and has the highest Rf; chlorophyll b is the least soluble of the four and stays nearest the origin with the lowest Rf. The value is then compared with published Rf values obtained with the same solvent and the same type of paper, which is how each band is identified.

The answer that scoresChemosynthesis is the fixation of carbon using the chemical energy released when dissolved inorganic substances are oxidised. At hydrothermal vents the water carries hydrogen sulfide, methane, hydrogen and iron compounds leached from the hot crust, and chemosynthetic bacteria oxidise these substances and use the energy released to fix carbon dioxide into organic compounds. The bacteria are the producers of the vent community, so grazers such as shrimp and the symbiotic hosts that house the bacteria depend on them, and a food chain exists in complete darkness. Temperature affects the rate of the bacteria's reactions, but it is not the energy source.

The answer that scoresA simple life cycle has no larval stage: the embryo develops inside the female, and the young is born with the adult body plan and simply grows and matures, with no metamorphosis. Marine mammals such as whales and dolphins are the example. A complex life cycle has one or more larval stages that do not resemble the adult, each separated by metamorphosis - a shore crab hatches as a planktonic zoea, moults into a megalopa, and then into a crab. The larva is the dispersal stage, so a complex cycle trades enormous larval mortality for the ability to colonise new ground.

The answer that scoresExternal fertilisation in a tuna releases millions of eggs into open water with no parental care. Most eggs are unfertilised or eaten, and the larvae are prey for almost everything larger, so survival per egg is extremely low - the strategy works only because the numbers are enormous, and it fails if the adults are removed before they spawn. Internal fertilisation in a whale gives each egg a high chance of being fertilised and protects the embryo, and the single calf is fed milk and protected for years, so survival per young is very high. Neither is better: external fertilisation is cheap per offspring but wasteful, while internal fertilisation and care are expensive but reliable. The practical consequence is that a high-fecundity fish stock can recover faster than a whale population, which takes decades.

The answer that scoresA quota is a total allowable catch, ideally set from stock assessment so that the catch does not exceed what recruitment replaces. Its strength is that it is directly linked to the biology and can be checked at landing. Its weaknesses are that it depends on accurate reporting - fish caught over the quota may be discarded at sea, and high-grading wastes the rest - that it does nothing for the habitat or for non-target species caught with the target, and that a quota set too high for political reasons still allows overfishing. It works only alongside mesh and size rules to protect juveniles, spatial protection for habitat, and monitoring and penalties that make cheating risky.

The answer that scoresBenthic trawling drags a weighted net with otter boards across the seabed. It removes large numbers of demersal fish and scallops, including juveniles and undersized fish, and its bycatch of non-target species is high and mostly discarded dead. Its habitat impact is the serious one: the gear ploughs and compacts the sediment, uproots and smashes slow-growing structure-forming organisms such as sponges, soft corals and maerl beds, and flattens the three-dimensional cover that juveniles need, so the nursery function of the ground is lost. Resuspended sediment can smother adjacent reefs and seagrass. Recovery of these habitats takes decades, which is why the damage outlasts the fishing.

The answer that scoresAquaculture reduces pressure on wild stocks only where the crop is low in the food chain or the feed is not derived from wild fish. Mussels filter natural plankton and add no feed, so they substitute for wild-caught protein with very little environmental cost. Salmon and shrimp, by contrast, are fed pellets containing fishmeal and fish oil made from wild-caught anchoveta, sardine and similar species, so farming them can add to the pressure on wild stocks rather than relieve it. Intensive systems also discharge nutrients, faeces, uneaten feed and medicines, which can enrich the water and smother the seabed, while extensive systems discharge very little. Whether aquaculture is environmentally positive therefore depends on the species, the system and the siting, not on aquaculture as a whole.

The answer that scoresMost plastics are non-biodegradable, so they persist in the sea for decades or longer. They are broken down physically and chemically rather than biologically: UV radiation in sunlight breaks the polymer chains and makes the plastic brittle, wave action grinds the fragments against sand and rock, and wind action moves them across the surface and off beaches. The rate depends on temperature, because these processes are faster when it is warm, so a plastic item fragments quickly in warm shallow tropical water and persists for much longer in the cold, dark water of the deep sea or the Arctic. The result is that one large item becomes thousands of secondary microplastic particles, which are small enough to be taken up by plankton and to enter the food chain.

The answer that scoresBioaccumulation is the build-up of a persistent, fat-soluble substance in the tissues of an individual organism over its lifetime, because it takes the substance in faster than it can excrete or break it down. Biomagnification is the increase in concentration of that substance at each successive trophic level, because a predator eats many prey and retains the contaminant load of every one of them. So mercury released from the combustion of fossil fuels is converted to methylmercury in the sediment, bioaccumulates in the plankton that absorb it, and biomagnifies through zooplankton, small fish and predatory fish such as tuna to the people who eat them, which is why the concentration in a tuna is far higher than the concentration in the water it swims in.

The answer that scoresCarbon dioxide from the atmosphere dissolves in sea water and reacts with water to form carbonic acid (CO2 + H2O -> H2CO3). The carbonic acid dissociates to release hydrogen ions and hydrogen carbonate ions (H2CO3 -> H+ + HCO3-), and the hydrogen ions combine with carbonate ions already in the water (H+ + CO3 2- -> HCO3-). The hydrogen ion concentration therefore rises, so the pH falls and the water becomes less alkaline, and the availability of carbonate ions falls at the same time. That matters because hard corals and shelled organisms build their skeletons and shells from calcium carbonate, so calcification becomes slower and more costly, and in cold or deep water existing shell can dissolve. Acid rain is a different process, caused by sulphur dioxide and nitrogen oxides in the atmosphere, and it is not the cause of ocean acidification.

The answer that scoresReef-building corals live in symbiosis with single-celled algae called zooxanthellae, which live in their tissues, give them their colour and supply most of their energy by photosynthesis. When the sea surface temperature stays about 1 °C or more above the normal summer maximum for weeks, the coral expels the zooxanthellae as a stress response, the colony turns white, and because it has lost most of its energy supply it is effectively starving. If the water cools within a few weeks the coral can take the algae back and recover, but if the heat persists the coral dies. Because recovery takes a decade or more, reefs that bleach repeatedly never fully recover, and the loss of the coral also removes the three-dimensional structure that fish and invertebrates depend on.

The answer that scoresCITES is the Convention on International Trade in Endangered Species, and it regulates or bans international trade in the species listed in its appendices, such as the humphead wrasse and several species of shark and ray. It is valuable because it works across borders and can be enforced at customs, and because it closes the market that drives the trade. But it only covers international trade: a species can still be caught for domestic sale, killed as bycatch or lose its habitat, and none of those is addressed by a CITES listing. It also depends on every member state passing and enforcing the law, funding the inspections and reporting the trade, and many do not have the capacity. That is why conservation needs CITES alongside MPAs, no-take zones, national legislation, ecotourism income and control of invasive species, rather than relying on one agreement.

The answer that scoresNitrate and phosphate from fertiliser are nutrients, and when they reach coastal water they cause eutrophication. The nutrients allow a rapid bloom of phytoplankton and seaweeds, and when that growth dies the bacteria that decompose it respire and use up the dissolved oxygen in the water. Oxygen concentrations fall until the water is hypoxic, and fish, crustaceans and shellfish suffocate or leave the area, which leaves a dead zone in which little that needs oxygen can survive. The decay also releases more nutrients, which can feed another bloom, and the shading by the bloom kills seagrass, so the habitat and the food web are damaged as well as the water quality.