Marine Science Marine

We are working with Cambridge International towards endorsement of this title. AS & A Level AS Marine Science Marine

COURSEBOOK Marine Science for Cambridge International AS & A Level

COURSEBOOK

Matthew Parkin, Melissa Lorenz, Claire Brown & Jules Robson

Completely Cambridge Cambridge University Press works with Cambridge Assessment International Education and experienced authors to produce high-quality endorsed textbooks and digital resources that support Cambridge Teachers and encourage Cambridge Learners worldwide. To nd out more about Cambridge University Press visit cambridge.org/cambridge-international DRAFT

Second edition Cambridge Elevate edition

Original material © Cambridge University Press 2019. This material is not final and is subject to further changes prior to publication. We are working with Cambridge International towards endorsement of this title. Contents Contents

How to use this series 00 Practical skills 1 Experimental planning including making How to use this book 00 estimates, predictions and hypotheses 00 Introduction 00 2 Presentation of data and observations 00 3 Evaluation of procedures and data 00 Introduction to command words 00 4 Analysis of data and conclusions 00

1 Water 6 Physiology of marine organisms 1.1 Particle theory and bonding 00 6.1 General cell structure 00 1.2 Solubility in water 00 6.2 Movement of substances 00 1.3 Density and pressure 00 6.3 Gas exchange 00 2 Earth processes 6.4 Osmoregulation 00 2.1 Tectonic processes 00 7 Energy 2.2 Weathering, erosion and sedimentation 00 7.1 Photosynthesis 00 2.3 Tides and ocean currents 00 7.2 Chemosynthesis 00 3 Interactions in marine ecosystems 7.3 Respiration 00 3.1 Interactions 00 8 Fisheries for the future 3.2 Feeding relationships 00 8.1 Life cycles 00 3.3 Nutrient cycles 00 8.2 Sustainable fisheries 00 4 Classification and biodiversity 8.3 Marine aquaculture 00 4.1 The classification of marine organisms 00 9 Human impacts on marine ecosystems 4.2 Key groups of marine organisms 00 9.1 Ecological impacts of human activities 00 4.3 Biodiversity 00 9.2 Global warming and its impact 00 4.4 Populations and sampling techniques 00 9.3 Ocean acidification 00 5 Examples of marine ecosystems 9.4 Conservation of marine ecosystems 00 5.1 The open ocean 00 Glossary 00 5.2 The tropical coral reef 00 Index 00 5.3 The rocky shoreDRAFT 00 5.4 The sandy shore 00 Acknowledgements 00

Chapter 3 Interactions in marine ecosystems

LEARNING OUTCOMES

In this chapter you will learn how to: • describe the three main types of symbiotic relationship and describe examples of each one • represent feeding relationships as food chains or food webs and be able to describe the organisms in these relationships in terms of their trophic level • explain how photosynthesisDRAFT provides energy to the food chain and summarise the process as a word equation • give the word equation for respiration • define productivity and explain how high productivity can affect food chains • explain why energy is lost at each stage of the food chain

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

CONTINUED

• draw pyramids of number, energy and biomass and be able to explain and interpret their shapes • explain what a nutrient is and be able to give the biological roles of nitrogen, carbon, magnesium, calcium and phosphorus • draw simple diagrams to show how carbohydrates, proteins and lipids are made up from smaller molecules • explain how nutrients in the ocean are depleted and replenished • describe the different stages in the carbon cycle including combustion, photosynthesis, decomposition, fossil fuels and the formation and weathering of rocks.

BEFORE YOU START

• In small groups, think of as many ways as you can in which organisms of different species can interact. Divide your list into positive and negative interactions. • In pairs, discuss what you already know about food chains. If you can, draw and label a simple food chain. • Write down any examples of nutrients you have heard of and the reasons you think they are needed by living organisms.

THE IMPORTANCE OF PHYTOPLANKTON

Around three billion people rely on marine the oxygen you breathe will have come from their ecosystems for their food. These people obtain the photosynthesis. majority of the protein in their diet from farmed or wild caught fish and other seafood. What they NASA, the United States’ civilian space programme, may not realise is that, ultimately, much of the monitors the levels of phytoplankton in the oceans energy in their food has come from tiny organisms using satellite imagery, comparing the colour called phytoplankton. This word comes from the of the water to the colour of chlorophyll and Greek meaning ‘drifting plants’ Phytoplankton other pigments. NASA has found that, overall, are tiny, microscopic organisms which float in the phytoplankton populations have declined over the upper sunlit layers of the water. Like land plants, last decade. This may be because the surface of they contain chlorophyll which enables them to the ocean is increasing in temperature. This in turn photosynthesise. They carry out more than half decreases the mixing between the nutrient-rich of all the photosynthesis on Earth, producing up to lower layers of water and the upper sunlit layers. 160 billion tonnes of carbohydrates every year. Fewer nutrients means less growth and reproduction of phytoplankton. In contrast, marine dead zones The ability of the phytoplankton to capture the are formed where excess fertilisers enter the water. energy in sunlight and to fix it into carbohydrates In these areas productivity reaches such a peak is essential to life in theDRAFT oceans. Many of the that the ecosystem becomes unbalanced. The consumers must gain their energy from either phytoplankton population initially increases very eating the phytoplankton directly, or from eating rapidly in a phenomenon known as an algal bloom. other animals who have eaten the phytoplankton. As the cells die and are decomposed by bacteria the Even if you do not eat seafood you are benefitting levels of oxygen fall. Eventually the oxygen levels from these tiny organisms, as more than half of become so low that little life can remain.

3 Interactions in marine ecosystems

CONTINUED

Questions done about this problem and the impacts on 1 Marine dead zones are often formed when land and ocean ecosystems. agricultural fertilisers enter the water. This 2 Think about why an organisation such as NASA reduces the population of fish available to catch might need to monitor the phytoplankton in the and use as food. However, if we do not use the ocean. Discuss, in groups, the reasons that this fertilisers there will be a reduction in the yield might be important and whether you think it is a of food crops. Discuss, in pairs, what should be good use of resources.

to each other. The word comes from a Greek term which 3.1 Interactions means ‘living together’. The smaller organism is the symbiont and the larger organism is the host. There are Ecological interactions describe how a pair of organisms several different types of symbiosis including: living together in a community can affect one another. Some of these interactions are mutualistic and have • parasitism – where the symbiont benefits but a positive effect on both organisms. Others such as the host is damaged; for example, copepods and predation and parasitism are harmful to one of the marine fish species involved. Competition is harmful to both • commensalism – where the symbiont benefits but organisms as they are both trying to use the same the host remains unaffected; for example, manta resources. However, all of these relationships are vital to rays and remora fish the ecosystem as they allow the transfer of energy from one organism to the next. • mutualism – where both organisms benefit from the relationship, for example, boxer crabs and Symbiosis is a relationship between two or more anemones. organisms of different species which live physically close

KEY WORDS

phytoplankton: microscopic photosynthetic mutualism (mutualistic): a relationship between organisms that live in the upper, sunlit layers of water two different organisms where both organisms benefit chlorophyll: a pigment found in plants and algae that is used to absorb sunlight for photosynthesis predation: a relationship between two organisms where a predator hunts, kills and eats a prey animal photosynthesis (photosynthetic, photosynthesise): the process of using light energy to synthesise parasitism: a relationship between two organisms glucose from carbon dioxide and water where the parasite obtains benefit at the expense of the host carbohydrate: organic compounds occurring in living tissues that contain carbon, hydrogen and competition: a relationship between two organisms oxygen, for example, starch, cellulose and sugars; where both species are negatively affected as they carbohydrates can be broken down in the process are trying to use the same resources of respiration to release energy symbiosis: a relationship between two or more consumer: an animal which feeds on other organisms of different species which live physically organisms to gain energyDRAFT from food close to each other algal bloom: a rapid increase in a population of commensalism: a relationship between two algae organisms where one organism benefits and the other is neither harmed nor benefitted community: all the different populations interacting in one habitat at the same time

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

The parasitic relationship The commensal relationship between copepods and between manta rays and marine fish remora fish Copepods are tiny shrimp-like crustaceans found both Remora fish (also known as suckerfish) have a modified in freshwater and saltwater. They are one of the most dorsal fin which can create suction and attach the fish abundant life forms on Earth and there are thousands of to smooth surfaces. The skin of a manta ray is one such different species. About half of the species are parasitic surface (Figure 3.2). The remora fish is able to cling with and these cause major problems for the aquaculture of a force of three times its own weight and it has even marine fish as infection can lead to substantial economic been studied as way to create new adhesive technology. losses. An example of copepod parasites are the sea lice which include more than 500 species. Two of the most common are Lepeophtheirus salmonis and Caligus elongatus.

Figure 3.2: A manta ray with two attached remora fish.

This is an example of phoresis, a type of commensal Figure 3.1: Sea lice attached to a juvenile salmon. relationship where the host remains unaffected and the symbiont uses the host for transport. In this case, the manta ray is unaffected but the remora benefits as it Sea lice are ectoparasites which means they live on does not have to use its own energy to travel. It is also the outside of their host as shown in Figure 3.1. possible for this relationship to become mutualistic as (Endoparasites, such as tapeworm, are found inside there are cases where the remora have been recorded the host’s body.) Sea lice feed on mucus, tissues and eating the parasites which affect the rays. blood. This can lead directly to the death of the host fish if enough lice are attached, particularly if they are KEY WORDS attached to the gills or to the more vulnerable juvenile fish. Even a lesser infestation will reduce the growth rate ectoparasite: a parasite, such as a flea or a of the fish and may leave them open to infection through louse, which lives on the outside of its host the damaged tissues. The lice not only cause problems for farmed species but canDRAFT spread to wild populations. endoparasite: a parasite, such as a tapeworm, In doing so, they can act as vectors for other diseases which lives inside the body of its host including infectious salmon anaemia, a virus which phoresis: a commensal relationship where one causes severe losses to fish farming. In this relationship, organism attaches itself to another in order to the fish (host) is negatively affected, and the sea louse travel (parasite) gains food and is positively affected.

3 Interactions in marine ecosystems

The mutualistic relationship Host Symbiont between boxer crabs and mutualism parasitism anemones commensalism Boxer crabs (sometimes known as pom-pom crabs) are small crabs (less than 2 cm across) from the genus Lybia. Table 3.1: Different interspecies relationships. Its name comes from the anemones which the crabs hold in their claws which look like boxing gloves or 2 Compare parasitism and commensalism. pom-poms (Figure 3.3). 3 Give an example of a symbiotic relationship. Name two organisms which are involved in this type of relationship and list the cost or benefit to each organism.

3.2 Feeding relationships Ultimately, all life on Earth is dependent on the energy that can be fixed into carbohydrates by autotrophic organisms. An autotroph is able to make its own food by forming organic compounds from simple inorganic molecules. Marine ecosystems contain both photosynthetic and chemosynthetic organisms. Photosynthetic organisms capture the energy in the Figure 3.3: Boxer crab holding anemones. sunlight, whereas chemosynthetic organisms are able to use the energy in chemicals that are dissolved in the water.

As this is a mutualistic relationship, both organisms Photosynthesis can only take place in the sunlit upper benefit. Anemones have stinging cells called cnidocytes layer of the ocean. Therefore, about 90% of all marine life on their tentacles. The crab makes use of these by is found in this area. The ability of the chemosynthetic holding the anemones in its claws and using them for organisms to produce carbohydrates is an important defence. The anemone gains easy access to food and adaptation to living in extreme conditions. They are will also use its tentacles like mops to pick up debris found near hydrothermal vents which are fissures or gaps and food from the crab’s hiding place. A crab without in the ocean floor. There is no light, so photosynthesis an anemone may try to steal one from another crab. is not possible. It was originally thought that the only Recently it has been found that most crabs carry two way in which energy could reach these lower parts of the anemones which are clones of each other. This suggests ocean was when organisms died and their remains fell to that, once a crab has one anemone, it splits it into the bottom. This view was not challenged until the vent two, one for each claw. Crabs have also been observed communities were discovered in the 1970s. directly feeding their anemones. The evidence, therefore, seems to be that the crabs actively feed and cultivate KEY WORDS their anemones, rather like looking after a pet. autotroph (autotrophic): an organism that can capture the energy in light or chemicals and Test yourself use it to produce carbohydrates from simple DRAFTmolecules such as carbon dioxide 1 Complete Table 3.1 using the symbols below to show the effects of different interspecies chemosynthesis (chemosynthetic): the relationships on both the host and the symbiont: production of organic compounds by bacteria or • 0 species is unaffected other living organisms using the energy derived from reactions with inorganic chemicals • – species is harmed • + species benefits

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

reefs. Carnivores are animals which feed only on other KEY WORDS animals. Many of these animals are also predators heterotroph (heterotrophic): an organism that which means that they catch and kill their prey. Seals cannot make its own food and instead relies on are predators which feed on a range of organisms consuming other organisms; all animals, fungi including squid, shrimp and fish. Omnivores, by and protozoans are heterotrophic, as well as contrast, eat both plants and animals. Many species of most bacteria crab are omnivorous, feeding on algae, juvenile fish and small invertebrates. In marine ecosystems, zooplankton primary productivity: the rate of production are important consumers and include copepods, of new biomass through photosynthesis or foraminifera and krill. Non-parasitic copepods are chemosynthesis small herbivores that feed on diatoms. Foraminifera food chain: a way to describe the feeding are single-celled animals with calcium carbonate shells relationships between organisms which are often omnivorous and feed on algae and copepods. Krill are shrimp-like omnivores that feed on herbivore: an animal which feeds only on other zooplankton species and phytoplankton. Krill producers (plants or phytoplankton) are an important food source for birds, fish, seals and carnivore: an animal which feeds on other animals baleen whales. predator: an animal which hunts, kills and eats All organisms will eventually die and the nutrients in other animals their bodies will be broken down by decomposers. These are bacteria and fungi which are able to break down prey: an animal which is eaten by predators dead organic matter. The decomposers are, therefore, the omnivore: an animals which feeds on other last stage in any feeding relationship, returning nutrients animals and on producers to the environment where they can be absorbed by the producers. decomposers: bacteria and fungi which break down dead organic matter Predation Other organisms known as consumers have to obtain One of the most important feeding relationships is that their energy by feeding on the autotrophs. These are between a predator and its prey. A predator is an animal also known as heterotrophic organisms. The primary that catches, kills and eats another animal. Predators productivity of an ecosystem relates to how much can be secondary, tertiary or quaternary consumers. energy is fixed into carbohydrates (new organic matter). Marine predators include sharks and carnivorous fish Estuaries, swamps and marshes are the most productive that eat zooplankton (planktivores) or fish (piscivores). ecosystems per unit area. However, the most productive Predators are often well adapted by being fast and agile. ecosystems overall are the oceans, because they cover They may also have camouflage, large teeth, poison or such a high proportion of the Earth’s surface. the ability to hunt in packs. Consumers can be put into different categories Prey are the animals that are eaten by the predators. A depending on how close they are to the producer in well-adapted prey animal will often have camouflage, the food chain. Primary consumers feed directly on the defensive spines, the ability to hide in safe places, or the producer, secondary consumers feed on the primary ability to flee. consumer, tertiary consumers feed on the secondary Predator–prey relationships may be an example of consumers, and quarternary consumers feed on coevolution. Here, the predator and prey species have the tertiary consumers. It is very rare to find more evolved together in response to changes in each other’s consumers than this due to the decrease in the available morphology and physiology. energy at each stage of theDRAFT process. Predator–prey relationships are crucial for keeping a There are different types of consumer depending on healthy balance of populations within the ecosystem. the food they eat. Herbivores are animals which feed For example, without starfish (Figure 3.4) there would only on plants and algae (producers). For example, be no natural predators to control the numbers of parrotfish and angelfish graze on the algae on coral mussels, sea urchins and shellfish. If the populations

3 Interactions in marine ecosystems

Food chains and food webs These feeding relationships between organisms can be represented by food chains and food webs. A food chain is a linear relationship beginning with the producer and then moving through the primary consumers and predators. A food web shows all the different interrelated feeding relationships within one ecosystem. In both food chains and food webs, arrows represent the direction in which energy, biomass and nutrients are transferred. An example of a food web is shown in Figure 3.5.

KEY WORDS

Figure 3.4: Starfish eating mussels. food web: a way to show all the different feeding relationships in an ecosystem of these organisms became too large, they would potentially be able to destroy a kelp forest. This would biomass: the mass of living material in an area; it negatively affect the other species feeding on or living can be measured as dry mass (without the water) within the kelp. or wet mass (with the water)

blue whale killer whale

leopard seal

elephant seal

seagull

salmon penguin

squid krill DRAFTzooplankton crab

phytoplankton seaweed

Figure 3.5: An example of a marine food web showing the flow of energy between organisms.

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

An example of a marine food chain is: KEY WORDS dinoflagellates → shrimp → tuna → marlin → shark trophic level: the position an organism occupies The organism at the end of the food chain (the shark in the food chain or food web in the above example) is known as a top carnivore or apex predator: an organism at the end of the an apex predator. These organisms have no natural food chain which has no natural predators predators of their own. photoautotroph: an organism which is able to use light energy to synthesise organic compounds Productivity Primary productivity is the rate of production of new biomass (living material) per unit area by autotrophic The ‘feeding level’ in a food chain is called the trophic organisms. This takes place through either photosynthesis level. The first trophic level is made up of the producers, or chemosynthesis. These processes allow light or the second trophic level is the primary consumers, the chemical energy to be fixed into useable organic third trophic level is the secondary consumers, and molecules, and as such are the basis of all food chains so on. Within a food chain each organism can only and food webs. The main way in which energy is fixed occupy one trophic level. However, in a food web, is through photosynthesis. On land, the majority of organisms may occupy more than one trophic level. This photosynthesis is carried out by green plants, but in the is particularly true of omnivores who may be feeding oceans it is mainly carried out by phytoplankton. Most on producers (making them part of the second trophic of these tiny algae are single-celled and simply float with level) (a)and other consumers (third or fourth trophic the current(b) in water. As well as the phytoplankton, there level). Organisms which are feeding on more than one are much larger algae (macroalgae), such as kelp and species are not dependent on only one food source. This rooted plants called seagrass. These organisms are all means that, if the population of one prey species or photoautotrophs, meaning they are able to make their own producer declines, there will be an alternative source of food using light from the sun. food. This also means that the numbers of organisms of each species in a food web are interrelated. Photosynthesis In general, food chains can be summarised as: Photosynthesis is a process in which two inorganic producer → primary consumer → secondary consumer compounds, carbon dioxide and water, are combined → tertiary consumer → quaternary consumer to produce glucose. Glucose is a useable organic compound. Oxygen is produced as a by-product. or light first trophic level → second trophic level → third trophic carbon dioxide + water —————→ glucose + oxygen level → fourth trophic level → fifth trophic level chlorophyll (a) (a) (c) (b) (b) (a) DRAFT(b) (c)

Figure 3.6: Important marine photoautotrophs: (a) phytoplankton; (b) kelp; (c) seagrass. (c) (c)

3 Interactions in marine ecosystems

photic zone 200 m thermocline (pycnocline) 1000 m water depth

deep water

Figure 3.7: Single-celled alga with green chloroplasts abyssal plain visible. 5000 m

Figure 3.8: Layers of water in the ocean. The energy to do this comes from sunlight and must be absorbed by pigments in the plants or algae. The most common pigment is chlorophyll, which is found in Light organelles called chloroplasts (Figure 3.7). To show that Photosynthesis can only take place within a relatively light and chlorophyll are both necessary, they can be thin layer of the ocean which has enough light. This written above and below the arrow in the equation. You sunlit zone is called the photic zone and is the only layer can read more about the different pigments needed for where enough light penetrates for photosynthesis to take photosynthesis in Chapter 7. place (Figure 3.8). This means that the vast majority of the biomass in the open ocean is contained within the Factors affecting photosynthesis upper 200 m of water. The rate of photosynthesis can be affected by several The sunlight is scattered and absorbed by the water. The different factors. These include: amount of light reflected will depend on the state of • temperature the water. When there are waves, more light is reflected because the waves act like lenses and focus the light. • concentration of carbon dioxide When the light penetrates the surface of the water it is • nutrients refracted because light travels more slowly in water than in the air. Finally, solid particles within the water also • amount of light. scatter and absorb the light. In a marine environment, the most important factors are The sunlight which is absorbed by the water also likely to be the availability of nutrients and light. increases the temperature. When the temperature Temperature and carbon dioxide increases, the molecules of water have more kinetic energy and move more quickly, so the warm water In marine environments there is always an abundance of is less dense and, therefore, more buoyant than cold water, and the water contains dissolved carbon dioxide. water. The thin layer of warm water floats on top of Although the reactions of photosynthesis are affected the colder deep water; the transition between the two by the temperature, the temperature of each area in the is called the thermocline. It can also be referred to as ocean is very stable and so has little effect on the rate of a pycnocline, which is simply related to the different photosynthesis. DRAFTdensities of the layers rather than the temperature. Nutrients Algae and plants both need nutrients in the form of KEY WORD mineral ions in order to grow. A lack of a particular nutrient, therefore, affects the rate of productivity of new photic zone: the surface layer of the ocean which biomass because it affects the rate of growth. Later in this receives sunlight chapter you can read about the roles of the major nutrients.

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

dioxide is turned into useable organic molecules using the KEY WORDS energy stored in dissolved chemicals such as hydrogen deep chlorophyll maximum (DCM): the sulfide. The chemicals dissolve in heated water in the maximum concentration of chlorophyll below the undersea crust as it makes its way back to the surface surface of a body of water to emerge from the vents. Chemoautotrophs are species of bacteria that are able to make their own food using chemoautotroph: an organism which is able chemical energy. Each species uses different chemicals as to use chemical energy to synthesis organic their energy source and produces different sugars. materials Chemosynthetic bacteria were first discovered in extremophile: an organism that is adapted to hydrothermal vents in the ocean floor in 1977. The survive extreme temperature, pressure, salinity vents are found at depths varying from around 2000 m or pH in the Galapagos Ridge to 7700 m in the Mid-Atlantic Ridge. At these depths there is no light, there are respiration: the process by which all living things no phytoplankton and, therefore, there can be no release energy from their food by oxidising photosynthesis. Chemosynthesis is the only way in which glucose life is possible in such an inhospitable environment. The species able to survive here are all examples of extremophiles, which means that they are able to survive There is little mixing between the two layers because very harsh conditions. At these vents there is extremely a source of energy (such as wind) is needed to push high pressure, as well as water temperatures that can the warm water down. This is very important to the vary from 2 °C to 400 °C. phytoplankton as it keeps them floating near the surface where they have access to light. Up to 75% of animal species at hydrothermal vents depend on mutualistic relationships with Without the thermocline, there would be much more chemosynthetic bacteria for at least some of their mixing of the water and currents would carry the food. For example, mussels at these vents have phytoplankton down and away from the light. This mutualistic bacteria living in their gills but are also would reduce the rate of photosynthesis and, therefore, able to filter feed. the productivity. However, the thermocline also prevents nutrient-rich water from mixing with the upper layers and, therefore, limits the productivity. Generally, the Similarities and differences deeper the water, the higher the nutrient levels but between photosynthesis and the lower the light. There is normally a point near the chemosynthesis thermocline where the productivity is highest as there is enough light for photosynthesis and enough nutrients Both photosynthesis and chemosynthesis use carbon for growth. This level is known as the deep chlorophyll dioxide and require an energy source to produce sugars. maximum (DCM) because it is the area with the highest In photosynthesis, oxygen is produced as a by-product. concentration of chlorophyll. In chemosynthesis, the by-products vary depending on the chemicals that are used, although sulfur is often The light varies with the seasons, particularly at high produced. There is, therefore, only one possible equation latitudes. In spring, the average length of the day and for photosynthesis compared with several different the intensity of light both increase. This is clearly an equations for chemosynthesis. In both processes, the advantage in terms of photosynthesis, and productivity sugars produced are used to provide metabolic energy is, therefore, higher in spring and summer than it is in through respiration, or built up into the other chemicals winter. Often it is nutrient availability that limits the needed by the organism which adds to the biomass. rate of photosynthesis in spring and summer, and light which limits it during autumnDRAFT and winter. Respiration Respiration is the process by which all living things Chemosynthesis release the chemical energy stored in organic molecules There are some ecosystems, such as those found around such as carbohydrates. This energy is then used to carry hydrothermal vents, where light is not available for out all the different metabolic reactions within the productivity. Chemosynthesis is a process where carbon organism. Aerobic respiration requires a supply

3 Interactions in marine ecosystems

of oxygen and glucose and produces carbon dioxide and water. KEY WORDS glucose + oxygen → carbon dioxide + water gross primary production (GPP): the amount of light or chemical energy fixed by producers in a As well as the useable energy the organism needs, given length of time in a given area respiration produces heat energy, which is transferred to the environment. net primary production (NPP): the amount of energy that is left over after respiration to be The link between photosynthesis made into new plant biomass and respiration secondary production: the rate of production Primary productivity is the amount of new biomass of new biomass by consumers, using the energy made by the producers. But not all of this biomass gained by eating producers is available for the consumers to eat. Some of the carbohydrate produced is not stored but is oxidised For example, the higher the rate of growth of during respiration to provide energy. Gross primary producers, the higher the amount of chlorophyll production (GPP) is the amount of energy that primary present. Net primary production and gross primary producers are able to fix in a given length of time and production are usually given as units of energy per within a given area. Net primary production (NPP) is the unit area per unit of time, for example kJ m−2 year−1. amount left over to create new biomass after respiration However, the units used vary depending on the method (R) has been taken into account. This can be shown by of measurement chosen. the equation: NPP = GPP – R Rate of photosynthesis This net primary production is available to pass on to The rate of photosynthesis can be found by looking the consumers. Secondary production is the amount at the change in either the oxygen or carbon dioxide of biomass produced by heterotrophs after eating the concentrations. If photosynthesis is taking place, producers. So, the more productive an ecosystem, the there will be a decrease in the concentration of more energy is available to pass along the food chain. carbon dioxide and an increase in the concentration In the marine environment, there is no large-scale of oxygen. accumulation of biomass as there is in savannahs and Because the majority of marine producers are single- forests on land. However, the reproductive rate of the celled phytoplankton, they can be easily kept in a phytoplankton is very high so there is a constant source closed system such as a bottle. When the bottle is in of new organisms which photosynthesise. Carbon the light, both photosynthesis and respiration will dioxide from the atmosphere dissolves in the water and take place. When the bottle is in the dark, there will is then available for photosynthesis. When it is fixed into be no photosynthesis but respiration will continue. glucose it is stored as phytoplankton biomass. Much If we assume that the rate of respiration remains of this ‘locked up’ carbon dioxide sinks to the floor of relatively constant, we can compare the readings of the ocean when organisms die. This process is discussed bottles kept in the dark and light to work out the rate later in this chapter in Section 3.3. of photosynthesis (Figure 3.9). The oxygen level needs to be taken using a dissolved oxygen sensor. There are Measuring productivity three readings to take: Primary productivity can be estimated in several • an initial reading before the experiment begins different ways including: • the reading in the light bottle at the end of the • using the rate of photosynthesisDRAFT of producers experiment • using the rate of increase in the biomass of • the reading in the dark bottle at the end of the producers experiment. • using satellite imagery to measure the amount of chlorophyll.

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

In this example:

–3 –1 NPP = 19 – 8 = 11 mg O2 dm h –3 –1 O2 photosynthesis Respiration = 8 – 2 = 4 mg O dm h O2 2 and respiration are O2 GPP = 19 – 2 = 17 mg O dm–3 h–1 O2 O2 occurring in this bottle 2 O 2 This technique can be extended to investigate the effect O O2 2 of light on productivity. Samples are removed from start of experiment end of experiment different depths in the water and placed into pairs of (light) light and dark bottles. The bottles are then suspended at the same depth the samples were removed from. The calculations are carried out as described to work out the net primary production, gross primary production and respiration at each depth. O2 only respiration is occurring Generally, the productivity increases moving towards O 2 in this bottle the deep chlorophyll maximum and then decreases as the amount of light begins to limit the rate of O 2 photosynthesis (Figure 3.9). At the point where the start of experiment end of experiment rates of respiration and photosynthesis are equal, (dark) there is no change in the amounts of carbon dioxide or oxygen, and the net productivity is zero. The light Figure 3.9: The light- and dark-bottle method for intensity at this depth is known as the compensation measuring productivity. The bottles on the left represent point. Immediately below this depth, there is still light start of experiment and the bottles on the right represent available but producers are unable to survive because end of experiment. the rate of respiration would be greater than the rate of photosynthesis. This part of the photic zone is sometimes called the disphotic zone. Around 90% of Mass of oxygen / mg dm–3 h–1 marine life is, therefore, found above the depth of the Start of End of compensation point. This upper area, with sufficient experiment experiment light for photosynthesis is called the euphotic zone. Light bottle 8 19 Changes in biomass Dark bottle 8 2 Scientists can measure the rate of accumulation of biomass by harvesting producers after a set amount Table 3.2: A sample set of results from the light- and dark- of time, drying them to remove variations in the water bottle method. content, and then finding the mass. If they know the size

The results would be tabulated as shown in Table 3.2. KEY WORDS In the dark bottle, the only process taking place is respiration, so the mass of oxygen in this bottle compensation point: the light intensity at decreases. In the light bottle, the rate of photosynthesis which the rate of photosynthesis and the rate of is higher than the rate of respiration, so the mass of respiration are equal oxygen increases. disphotic zone: The layer of water in the ocean The difference in the mass of oxygen at the start and where there is a low level of light which is not the mass in the light bottleDRAFT at the end is net primary sufficient for photosynthesis. It is sometimes also productivity. The difference between the oxygen at the referred to as the twilight zone start and the oxygen in the dark at the end is respiration. The gross primary production is the difference euphotic zone: The upper 80 m of the water between the light and the dark bottles at the end of the where there is enough light for photosynthesis to experiment. take place

3 Interactions in marine ecosystems

−3 −1 In Figure 3.11, the most productive areas are found productivity / mg O2 dm h 0 1 2 3 4 in the tropics and at the higher latitudes, which are shown in green and orange. The least productive areas, shown in purple and blue, tend to be where there is a 20 smaller supply of nutrients from the deeper waters, net respiration productivity perhaps because of wind patterns. There are problems 40 with this method of measurement, though, because the m

/ relationship between chlorophyll concentration and

gross productivity biomass is not fixed. It depends on the individual species 60

depth present and their adaptations.

80 compensation The satellites can only indicate relatively shallow depths depth and cannot penetrate the entire euphotic zone where 100 production is taking place. However, the satellite images give a very useful summary of differences in productivity and enable scientists to monitor any changes. In 2016, Figure 3.10: Productivity at different depths as measured researchers at Sheffield University discovered that water by the light- and dark-bottle method. from melting giant icebergs in the Southern Ocean contains nutrients that increase the growth of the local of the area the producers came from, they can work out phytoplankton (Figure 3.12). These giant icebergs can be the biomass per unit area per year to give an estimate more than 18 km in length. Scientists studied the satellite of the net primary production. As the producers would images and found that the increased productivity caused have been respiring while growing, scientists are unable by these icebergs lasts for at least a month after they pass to measure the gross primary production. There are through an area and can extend for up to 600 miles. difficulties with this method, however, as you cannot measure the biomass that has already been consumed by heterotrophic organisms. This may also be true for the light- and dark-bottle method if small heterotrophic organisms are not sieved out before the experiment begins. Satellite imagery Scientists can monitor the productivity of the oceans by using satellite imagery to measure the colour of the surface layers of water. This can be used to follow the meltwater meltwater changes in chlorophyll concentration and, therefore, the containing containing amount of producers present (Figure 3.11). nutrients nutrients

plankton

Figure 3.12: The meltwater from icebergs increases productivity.

The influence of changes in productivity DRAFTon the food chain The higher the productivity, the more biomass accumulated by the producers and, therefore, the more 0.007 0.02 0.05 0.14 0.37 1.0 2.7 biomass available for the consumers to eat. This means Figure 3.11: False-colour picture showing annual mean that, in general, higher productivities lead to more amount of chlorophyll a in the oceans (NASA aqua modis). abundant populations of consumers, and longer food chains. The most productive areas of the oceans tend

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

never reaches the producers because it is reflected back KEY WORD into space. Of the light that does reach the ocean, some eutrophication: the process by which a body of is absorbed, reflected or scattered by the water. The water becomes enriched in dissolved nutrients remainder is available to the producers – but even then it (such as nitrates and phosphates) that stimulate cannot all be used. Some light is the wrong wavelength the growth of producers, usually resulting in the for the pigments of producers to absorb. Chlorophyll, depletion of dissolved oxygen for example, absorbs red and blue light but reflects green light (which is why it appears to be green). Of the light that is the correct wavelength, some will miss the to be those areas with high levels of nutrients from chloroplasts and still not be absorbed (Figure 3.13). upwelling. In tropical areas, there are high levels of light Photosynthesis itself is not completely energy efficient. but it is also warm, which leads to a strong thermocline Energy is lost as heat during the various chemical and little mixing of nutrients from deeper waters. In reactions which take place during the process. It has contrast, polar waters are nutrient rich because it is been estimated that producers worldwide only fix about very cold and there is only a weak thermocline. Polar 0.06% of the total solar energy available. In aquatic waters can have long food chains as the productivity ecosystems this figure can be as high as 1%. is high enough to maintain populations at several different trophic levels. It is not unusual to have five or Some of the glucose produced in photosynthesis is used even six trophic levels in these food chains. However, in respiration. This means that only the net production productivity is only high in the summer when the light of biomass is available to the next trophic level. The levels are higher. energy stored in biomass is passed to heterotrophic organisms when they ingest, digest and absorb the There does come a point when productivity can be too nutrients from the producers. These nutrients can then high. This leads to effects that are similar to the process be assimilated into new biomass. If the producers are of eutrophication seen in fresh-water ecosystems. If the phytoplankton, then the entire cell is usually ingested levels of nutrients increase too much or too rapidly, by the primary consumers, passing on all the available phytoplankton may rapidly increase in a phenomenon energy. However, in the case of macroalgae and rooted known as an algal bloom. plants such as seagrass, there are parts of the producer In these circumstances, up to five million cells per litre that are not eaten (the roots, for example). The energy can be produced, which is damaging to the ecosystem. stored in these parts is not available to the next trophic This density of algae is so high that it can clog the level, although it may later re-enter the ecosystem gills of fish so that they are unable to obtain enough through decomposition when the plant dies. oxygen. Once the algal cells die, they are broken down Secondary production is the production of new biomass by bacterial decomposers so there is also an increase by the consumers. It can involve animals eating the in bacterial populations. The bacteria respire and grow phytoplankton, macroalgae and seagrass, or animals and use up the oxygen in the water, which can lead to eating other animals. Decomposers such as bacteria and hypoxic conditions (lacking oxygen). This also kills fungi break down dead organic matter to obtain the heterotrophic organisms because without oxygen they nutrients they need. This also releases nutrients back cannot respire. into the ecosystem. If the algal species involved also produce toxins, the Secondary production depends on: effects can be even worse because the organisms that ingest them will be poisoned. This can cause mass • the biomass available in the producers mortality in aquatic organisms such as dolphins, • the amount of energy lost through respiration by manatees and whales, as well as food poisoning in the consumers people who eat contaminated shellfish. DRAFT• the amount of energy lost in waste products such as urine (Figure 3.14). Energy transfer through the Most salt-water fish only lose small amounts of urine food chain and excrete most of their nitrogenous waste through their gills in the form of ammonia. Undigested food is Only a small amount of the radiation from the Sun is egested as faeces. fixed by the Earth’s producers. Some of the radiation

3 Interactions in marine ecosystems

incident solar energy 100% energy transformed to heat in photosynthetic reactions reflected unsuitable wavelength water

absorbed by chloroplasts

gross oxidation in producer primary mitochondria production glucose

net productivity

water

transmitted respiratory heat

Figure 3.13: The fate of light energy falling on producers in the ocean.

These energy transfers can also be expressed as a urine and other excreted waste products of metabolism, formula: and P is the energy left over for the production of new biomass by the animal. C = P + R + F + U The energy of production (P) is then available to pass on Where C is the energy consumed, R is the energy used in to the next trophic level. respiration, F is the energy in faeces, U is the energy in

heat loss through respiration

energy used for life processes

total energy in food consumed DRAFTenergy stored in tissues

energy lost in waste products (faeces and excretory products)

Figure 3.14: Energy transfer into and out of a consumer.

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

Typically, the efficiency of transfer between trophic ectothermic organisms ranges from 5% to 15%, and the levels is around 10%, but it varies depending on: efficiency of transfer from endothermic organisms ranges from 1% to 5%. The efficiency of transfer between the • how much of the food is eaten different trophic levels determines how many levels there • how easy it is for the consumer to digest and are in the ecosystem. The higher the efficiency of transfer, assimilate the nutrients the more trophic levels the ecosystem can support. • how much energy is used for movement • how much is lost in the waste products of metabolism. Illustrating feeding Some organisms are easier to digest and assimilate than others: generally, consumers find it easier to assimilate the relationships energy in other animals than the energy in producers. If We can show the relationships between the different it is easier to assimilate the energy, then more of it will be trophic levels using pyramids of number, biomass and passed to the next consumer. In addition, some organisms energy. These are made up of horizontal bars arranged at each trophic level escape being eaten and the energy in a pyramid shape to show a particular food chain. stored in their biomass will never pass to the next level. They can be drawn to scale or simply sketched to give Most fish are ectothermic, which means that their body an idea of the changes as energy is transferred along temperature varies with the environmental temperature. the food chain. Producers are always at the bottom, The ocean sunfish, for example, is a secondary consumer followed by primary consumers, secondary consumers that eats zooplankton, tiny animals present in the water and tertiary consumers. Although energy is transferred that feed on the phytoplankton. The ocean sunfish to decomposers once the producers and consumers die, is ectothermic and does not use energy in respiration it is not often shown on the pyramid. to keep its body temperature higher than that of the surrounding water. Pyramids of number Tuna are endothermic, which means that they must A pyramid of numbers simply shows the number of expend energy maintaining their body temperature. organisms present in each trophic level at a particular Tuna occupy two trophic levels in the same food web, moment in time. The size of each horizontal bar is as secondary consumers eating zooplankton, and as proportional to the number of organisms. In theory, tertiary consumers eating small crustaceans that have this should be quite simple but, in practice, it is actually already fed on the zooplankton. rather difficult. It is often hard to estimate accurately the number of organisms present, and even once this has Small sharks feed on both the sunfish and the tuna been achieved it can be difficult to show them to scale. but the efficiency of energy transfer is higher from the For example, a typical oceanic food chain is: sunfish. Assuming that the sunfish and tuna take in similar amounts of energy, the tuna use more of this phytoplankton → zooplankton → lantern fish→ squid in respiration to keep warm so there is less to pass to → lancet fish → marlin → shark the sharks. In general, the efficiency of transfer from There could be millions of cells of phytoplankton and only one or two sharks. Finding a scale to show this is KEY WORDS impossible. For this reason, many pyramids of numbers are sketched rather than drawn to scale (Figure 3.15). ectothermic: an organism that maintains its In addition, much of the phytoplankton is consumed body temperature by exchanging heat with its very quickly after it is produced. Thus, if the numbers surroundings present in an ecosystem are counted after most have been eaten, the pyramid will be inverted (upside down) endothermic: an organism that maintains and it will look as though there are fewer phytoplankton its body temperature byDRAFT generating heat in than zooplankton. The number of organisms in an metabolic processes ecosystem will also vary depending on factors such as pyramid of numbers: a diagram that shows the the time of year or the amount of fishing. This means number of organisms in each trophic level of a that the pyramid can only show the numbers in each food chain trophic level at a particular moment in time. Pyramids of number also do not take into account the size of

3 Interactions in marine ecosystems

(a) (b) seasea lice lice

salmonsalmon

secondarysecondary consumersconsumers krillkrill

primaryprimary consumers consumers zooplanktonzooplankton

producersproducers phytoplanktonphytoplankton

Figure 3.15: (a) A generalised pyramid of numbers; (b) A pyramid of numbers for a marine ecosystem showing small parasites feeding off a large fish.

organisms, which can lead to odd-looking pyramids. For example, if several small parasites feed on one large fish KEY WORD you will see an inverted pyramid. pyramid of biomass: a diagram that shows the biomass present in each trophic level of a food Pyramids of biomass chain Instead of finding the number of each organism, we could measure their total biomass. This overcomes means that every individual must be found and weighed. the difficulties of having organisms of different sizes, Alternatively, the dry mass of a sample can be taken such as the parasites in the last example. It does not, and then multiplied by the total number of organisms to however, solve the issues caused by phytoplankton being give the total average dry mass. Both of these methods eaten before they can be measured. It is possible that will give an estimate of the total biomass but neither of the biomass of organisms within an ecosystem could them will be completely accurate. increase or decrease after measurements are taken which will make the pyramid inaccurate. A pyramid of biomass may still be inverted as the total amount of biomass in phytoplankton at any one time It is difficult to find the biomass of each trophic level is small because they are eaten very quickly. However, accurately. Organisms vary in the amount of water they their reproductive rate is very high so they reproduce contain, and this water does not contribute to their quickly enough to provide enough biomass to maintain biomass. For this reason, dry mass should be used, the population of consumers. In other words, the with the water removed by evaporation. To do this, amount of biomass is low but the rate of production of the organisms must be killed, which is not feasible or biomass is high. This snapshot view of the biomass at a desirable when measuring the biomass of the entire food particular moment in time is known as the standing crop chain. Instead, there are conversions available to change (Figure 3.16). the mass of living material into dry mass. This still

(a) sea lice sea lice (b) salmon salmon krill krillDRAFTzooplankton zooplankton zooplankton zooplankton phytoplanktonphytoplankton phytoplanktonphytoplankton

Figure 3.16: (a) Pyramid of biomass showing the decrease in biomass through the food chain; (b) Inverted pyramid of biomass showing the problems caused by the standing crop of rapidly reproducing phytoplankton.

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

KEY WORD Pyramids of number, biomass and energy during an algal bloom pyramid of energy: a diagram that shows the During an algal bloom we would expect the numbers, amount of energy in each trophic level of a biomass and energy of phytoplankton to increase, food chain followed by the numbers, biomass and energy of zooplankton and other consumers, as there is more energy to pass along the food chain. Once animals start Pyramids of energy to die due to the hypoxic conditions, the numbers and A pyramid of energy shows the rate of production of biomass will decrease again. As this takes place, there will biomass rather than the standing crop, so is always be less energy to pass down the food chain so the energy pyramid shaped (Figure 3.17). It involves finding the in each trophic level will also decrease. However, often energy in each trophic level of the food chain, which the phytoplankton which grow most during a bloom are is a complex procedure. Data is collected over a long inedible species of cyanobacteria. The proportion of period of time, normally a year. Often conversion edible cells in the phytoplankton decreases. This means tables are used that will convert dry biomass into that the figures for zooplankton do not increase as much energy production. The units for pyramids of energy as expected. In this situation, the bar for phytoplankton are kJ m–2 year–1 so it will not be a standing crop but a would be bigger than normal in all three types of measurement of the energy available over the entire year. pyramid. The bar for zooplankton would be smaller than Although pyramids of energy are the most difficult to expected, as with less energy and biomass passing along produce, they are probably the most useful in terms of the food chain there will also be fewer zooplankton. understanding the ecosystem.

tertiary consumers 0.1% Test yourself 4 State the two ways in which new biomass is produced in the ocean. secondary consumers 1% 5 Explain why ocean productivity is limited to the first 200 m in depth.

primary consumers 10% 6 a Calculate the amount of energy used in respiration if the GPP = 472 kJ m–2 year–1 and the NPP = 310 kJ m–2 year–1 producers 100% b Explain why organisms need to respire. 7 Explain why is it best to use dry mass to produce pyramids of biomass Figure 3.17: A generalised pyramid of energy showing the approximate transfer between tropic levels. 8 Describe what an inverted pyramid is and explain what might cause it.

CORE PRACTICAL ACTIVITY 3.1: INVESTIGATE THE EFFECT OF LIGHT INTENSITY ON THE RATE OF PHOTOSYNTHESIS

In this practical you will investigate the effect of with oxygen for respiration. This is important as light intensity on the rate of photosynthesis by there is less oxygen dissolved in the water than using an aquatic plant such as Elodea or Cabomba. there is in the air and so the roots would be at risk Aquatic plants like this haveDRAFT a special kind of spongy from the lack of oxygen available for respiration. tissue in their stems called aerenchyma which enables them to transport the oxygen produced Light is an important limiting factor for during photosynthesis around the plant. This photosynthesis and is only available in the photic means even those parts of the plant which cannot layer of the ocean. This investigation will enable photosynthesise (such as the roots) will be supplied you to investigate how the rate of photosynthesis

3 Interactions in marine ecosystems

CORE PRACTICAL ACTIVITY 3.1: CONTINUED

is affected by the light intensity by counting the 3 Why do you think we have the pondweed tube bubbles of oxygen which are released from the in a beaker of water with a thermometer? aerenchyma. The faster the rate of photosynthesis, 4 Predict the pattern of results as the lamp is the more oxygen will be produced and the more moved further away from the pondweed. bubbles you will be able to count. Method Equipment: 1 Switch all the lights off in the room. You will need: 2 Place the pondweed in the test tube with the • a cut stem of an aquatic pondweed, about cut end at the top. 5 cm long 3 Fill the test tube with sodium hydrogencarbonate • test tube solution, making sure it completely covers the • large beaker pondweed. • lamp 4 Fill the beaker with warm water. Measure and • meter ruler record the water’s temperature. • timer 5 Place the test tube of pondweed into the beaker. • thermometer 6 Position the lamp close to the beaker and measure the distance from the beaker to the • enough sodium hydrogencarbonate solution lamp. to fill the test tube. 7 Allow two minutes for the pondweed to adjust Safety Considerations to the light intensity. Sodium hydrogencarbonate is not hazardous. 8 Count the number of bubbles released over a one-minute period. It is possible, but unlikely, that an infection could 9 Wait another minute. be picked up when handling pondweed and 10 Count the bubbles for a second one-minute pondwater so follow normal hygiene procedures, period. cover cuts and wash hands afterwards. 11 If your two values are very different, repeat for The room should be dim so that only the lamps are a third time and then ignore the anomaly when affecting photosynthesis, but not completely dark calculating the mean. as that would be dangerous. 12 Move the lamp 20 cm further away and repeat Before you start all the steps. 13 Continue at further intervals of 20 cm up to 1 Name the independent and dependent 100 cm. variables in this practical. 14 At each stage, measure the temperature of the 2 Explain why the sodium hydrogencarbonate is water in the beaker and add hot or cold water added to the pondweed.DRAFTto try to keep it constant.

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

CORE PRACTICAL ACTIVITY 3.1: CONTINUED

Results Record your results in a results table as shown below (Table 3.3). Calculate the mean average light intensity at each distance.

distance from temperature / ºC number of bubbles released in one minute lamp / cm 1 2 3 (if required) mean

Table 3.3: Results table showing the effect of light intensity on photosynthesis.

You may wish to convert the distances to 5 Explain the pattern of your graph. approximate light intensities. This can be done 6 The bubbles may not have all been the same using the formula: size. Explain what effect this could have on the 1 reliability of the results. light intensities = distance2 7 Suggest how you could improve the experiment so that different sized bubbles do not affect the You should now plot a graph with the independent results. variable (light intensity or distance) on the x-axis, and the dependent variable (average number of Reflection bubbles per minute) on the y-axis. Join the points with straight lines. 1 Discuss any difficulties you had with this experiment and how you could avoid them Evaluation and conclusions in the future. If there were no difficulties then discuss what else you could investigate using 1 Describe the shape and pattern of your graph. the same apparatus. 2 If there are any anomalous results, circle them DRAFT2 Describe how has this practical has increased on the graph. your understanding of photosynthesis in the 3 State the gas we assume is inside the bubbles. ocean. If it has not altered your understanding 4 Suggest how you could test your answer to then describe what else you still need to find question 3. out to help with this.

3 Interactions in marine ecosystems

3.3 Nutrient cycles KEY WORDS Nutrient cycles show the essential movement and nutrient cycles: the movement and exchange of recycling of the elements that are necessary for elements that are essential to life, from inorganic organisms to live and grow. The carbon and nitrogen molecules, through fixation and then into living cycles are probably the best known and most clearly organisms, before being decomposed back into understood, but there are many other elements that inorganic molecules are important. These include phosphorus, calcium nutrient: and magnesium. Each of these elements is required a chemical that provides what is for a different biological role within living organisms. needed for organisms to grow, repair damaged Nitrogen is used to make amino acids, proteins and cells or tissues, release energy or for their DNA. Carbon is found in all organic compounds. metabolism Magnesium is used to make chlorophyll and is essential assimilation: the conversion of a nutrient into a for photosynthesis. Calcium is used to make bones, useable form that can be incorporated into the shells and coral skeletons. Phosphorus is used to make tissues of an organism bones and DNA. All nutrient cycles have a biotic and an abiotic phase photosynthesis into glucose. This can later be converted (Figure 3.18). A nutrient moves from the abiotic to the into the other molecules needed by the producer; for biotic phase when it is absorbed and assimilated by example, starch. It has been assimilated and is now part producers. A nutrient is a substance which is needed for of the biotic cycle. During the biotic phase, nutrients growth, repair, energy or the normal metabolism of an are moved from one organism to the next by feeding. So organism. nutrients move along the food chain from the producers to the consumers. Some will be lost from each organism by egestion and excretion and the rest will remain within The biotic phase organic compounds until the organism dies. In this In the carbon cycle, carbon dioxide (an inorganic biotic part of the cycle, nutrients will be found as organic molecule that is part of the abiotic cycle) is fixed during compounds such as carbohydrates, lipids and proteins.

consumers

biotic phase

primary producers decomposers

abiotic DRAFTphase nutrients found as inorganic ions and compounds in the atmosphere, dissolved in water, forming rocks and sediments

Figure 3.18: A generalised nutrient cycle showing the movement from the biotic to the abiotic phases.

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

Carbohydrates, lipids and proteins KEY WORDS Carbohydrates, lipids and proteins are three very important polymers in living organisms. A polymer is polymer: a large molecule made from many a large molecule made from many repeating smaller repeating sub-units sub-units called monomers. The monomers are joined monomer: the smallest unit of a polymer, together during condensation reactions where a molecule monomers are able to join chemically to form of water is removed in order to form the chemical bonds longer molecules between them. They can be broken down again by hydrolysis reactions where a molecule of water is added condensation: a reaction where two molecules to break each chemical bond. bond to form a larger molecule with water being released as a by-product Carbohydrates Carbohydrates are molecules made from carbon, hydrolysis: a reaction where a bond in a hydrogen and oxygen. They are the most abundant molecule is broken by the addition of water biological molecules on Earth and are used for energy starch: a carbohydrate made from chains of and to provide structure. glucose molecules joined together An example of a carbohydrate used for energy is starch amylose: one of the components of starch which which is the energy storage molecule in plant and algal is made from chains of glucose molecules with cells. Starch is made from many glucose molecules no branches joined together into long chains by glucosidic bonds. Starch is a mixture of two types of chains formed from amylopectin: one of the components of starch these glucose molecules. Amylose chains are unbranched which is made from chains of glucose molecules and can coil up easily, which makes them compact for with branches storage. Amylopectin chains do have branches which cellulose: an important component of plant cell (a) walls which is made from many straight chains of glucose molecules held together by hydrogen bonds

glucose means that they have more ‘end’ molecules which can be easily broken off in hydrolysis to provide glucose for respiration (Figure 3.19(a)). Starch is a suitable molecule for storage because it is insoluble. It will not affect the water potential of the cell so will not cause water to amylopectin amylose move in or out of cells by osmosis. An example of a carbohydrate used for structure is (b) cellulose which forms the basis of cell walls in plant and green algal cells. Like starch, cellulose is made from chains of glucose molecules. However, the chains hydrogen bonds in cellulose are very long and have no branches. Many chains lie alongside each other to form microfibrils, rather like the way a rope is formed from many thinner strands of thread. The individual cellulose molecules are chains of glucose moleculesDRAFT form cellulose joined together by hydrogen bonds. Many microfibrils join together to form fibrils and the fibrils in turn Figure 3.19: (a) Starch is formed from chains of glucose join together to form cellulose fibres. This makes molecules amylopectin has branched chains and amylose the cellulose molecule extremely strong and enables chains are unbranched; (b) Cellulose is formed from many plants to remain upright despite their lack of a skeletal long branches of glucose with hydrogen bonds between structure (Figure 3.19(b)). them.

3 Interactions in marine ecosystems

Proteins fatty acid Proteins are polymers formed from amino acids. Proteins contain carbon, hydrogen, oxygen, nitrogen and fattyglycerol acid sulfur. There are 20 amino acids which form proteins. glycerol The amino acids can be joined together in different fatty acid orders to form all the different proteins needed in living organisms. The order of amino acids in a protein is determined by the DNA which is found in the nucleus Figure 3.20: Triglyceride formed from three fatty acid of the cell. Thus our genetic code is really a code to chains and glycerol. build proteins. Proteins are important in all living cells, forming, among other things: Lipids • enzymes, which speed up chemical reactions Lipids are made from carbon, hydrogen and oxygen. Some types of lipid also contain phosphorus and • hormones, which are chemical messengers nitrogen. Three of the most common classes of lipid are: • part of bones, muscles, skin, cartilage and blood • fats or triglycerides, which are used for energy in animals storage, insulation and protection • part of the cell wall in plants. • phospholipids, which are the main component of The amino acids in proteins are joined together by membranes in living cells peptide bonds which are once again formed in a • steroids, which form the basis of many hormones. condensation reaction (Figure 3.21). Each chain of amino acids can be folded in a specific way to achieve Triglycerides are formed entirely from carbon, hydrogen all the different types and shapes of protein needed by and oxygen. They are made from three long fatty living organisms. acid molecules joined to a glycerol molecule by ester bonds (Figure 3.20). The ester bonds are formed by a condensation reaction and can be broken down by amino acids a hydrolysis reaction. The long fatty acid chains are hydrophobic which makes lipids insoluble in water.

KEY WORDS peptide bond triglyceride: a type of lipid which is made from a glycerol molecule joined to three fatty acid Figure 3.21: A short length of a protein formed from chains amino acids. phospholipid: a class of lipids that consist of fatty acids, glycerol and phosphate. They are a major component of cell membranes. The abiotic phase steroid: a type of lipid which forms the basis of After death, organisms must be broken down by many important hormones decomposers, which results in nutrients returning to their inorganic form and the abiotic part of the cycle. fatty acid: lipid molecules that are a major During this part of the cycle, nutrients can be found in constituent of triglycerides and phospholipids several different forms: hydrophobic: a molecule without a charge which • as ions dissolved in water for example, Mg2+, CO 2−, DRAFT 3 repels water molecules 3− − PO4 and NO3

• as gases in the atmosphere for example, CO2 • forming sediment that can later become rocks.

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

limit the rate of growth because they are found in the Reservoirs in nutrient cycles lowest concentrations in the water. This means that there A reservoir is part of the abiotic phase of the nutrient is usually slightly less than is needed by the producers. If cycle where soluble nutrients can remain for long periods the concentrations increase, the productivity increases. of time. The ocean is an important reservoir for many The average concentrations of ions dissolved in the elements. The residence time is the average time a particle water at the ocean surface are shown in Table 3.5. spends in a system. Average residence times for nutrient ions in the ocean tend to be very long because some of Average concentration Ion them fall to the bottom in faeces or dead organisms. in seawater / ppm They can remain in sediment on the ocean floor for chloride 19 345.00 thousands or even millions of years (Table 3.4). sodium 10 752.00 Average residence sulfate 2701.00 Nutrient time / years magnesium 1295.00 phosphate (phosphorus)* 20 000–100 000 calcium 416.00 magnesium 17 000 000 hydrogencarbonate 145.00 hydrogencarbonate** 100 000 nitrate 0.50 (carbon)* phosphate 0.07 nitrogen 2000 calcium 1 000 000 Table 3.5: Average concentrations of some of the ions found dissolved in seawater. *Where the nutrient is found as an ion, the element is given in parentheses **Sometimes called bicarbonate Processes that add nutrients to the Table 3.4: Approximate residence times for different surface water nutrients in the ocean. There are four main processes that add nutrients to replenish the reservoir within the surface water. The time the same nutrients spend in just the surface These are: layer of the ocean is much shorter because they are • dissolving in the water from the atmosphere constantly being used and recycled by the organisms living there. This surface reservoir is of particular • upwelling importance because it enables the high productivity • run-off of phytoplankton. After light intensity, nutrient • tectonic activity. availability is often the main limiting factor for growth of producers. The relative importance of these processes depends on each nutrient. For nutrients present in high Phytoplankton are found in the surface layer of concentrations in the atmosphere, dissolving will add the ocean where there is plenty of light. It is the more to the reservoir than run-off, for example. concentration of nutrients that determines the rate of growth. The higher the rate of growth of Dissolving of atmospheric gases phytoplankton, the higher the rate of photosynthesis and the higher the productivity. As discussed earlier, Nitrogen and carbon are both present in the Earth’s the productivity of the phytoplankton determines how atmosphere and are therefore both able to dissolve much energy can be transferred to the next trophic level. directly into the water. Nitrogen is present in the form of nitrogen gas, N , and carbon as carbon dioxide gas, In general, the amounts ofDRAFT nitrogen and phosphorus 2 CO2. The amount of gas that can dissolve in the water depends on several factors. These include: KEY WORD • the temperature of the water residence time: the average time that a particle • the atmospheric concentration of each gas spends in a particular system • the amount of mixing of water at the surface.

3 Interactions in marine ecosystems

Figure 3.22: Movement (flux) of carbon dioxide into and out of the ocean over the course of a year. Purple and blue areas are carbon sinks; yellow and red areas are carbon sources; green areas are at an equilibrium, with the same amount of carbon dioxide dissolving as being released.

In some areas, there will be more gas dissolving in the Upwelling water than there is diffusing back into the atmosphere. Upwelling is where cold water from the deep ocean is These areas are known as sinks. brought to the surface. These deep waters have higher In other areas, more gas will diffuse into the atmosphere concentrations of nutrients than those at the surface than is dissolving into the water. These areas are called because of the tendency for the remains of living things sources. to sink. So faecal matter and dead organisms sink from the surface layers to the deeper parts of the ocean. Here Generally, the overall concentration remains at an they may be broken down by decomposers and the equilibrium, with the same amount dissolving into nutrient ions returned to the water. During upwelling the ocean as is removed by diffusion back into the this nutrient-rich water rises to the surface where it atmosphere (Figure 3.22). effectively fertilises the surface layers and increases productivity. Areas with high levels of coastal upwelling KEY WORDS tend to be the most productive and have high catches of commercially important fish. It has been estimated that sink: an area where thereDRAFT is a net loss of material 25% of fish are caught from just 5% of the ocean where (for example, where more gas dissolves into the there are high levels of upwelling. ocean than diffuses into the atmosphere) Coastal upwelling is caused when winds blow parallel to source: an area where there is a net gain of the shore (Figure 3.23). This displaces the warm surface material (for example, where more gas diffuses water, which moves further offshore and has to be replaced into the atmosphere than dissolves in the ocean) by water from deeper in the ocean. If the wind is moving

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

surface winds KEY WORDS push surface water away from an area infiltration: part of the water cycle where water warmer surface water soaks into the soil from ground level and moves moves offshore underground upwelling leaching: a process during which water-soluble nutrients are removed from the soil and dissolve in water that is flowing to the sea (run-off) deeper, colder, nutrient-rich water rises up from beneath the During the water cycle, water evaporates from rivers, surface to replace the water lakes, oceans and streams. It condenses into clouds that was pushed away in the atmosphere and from there falls on the land as precipitation (Figure 3.24). Some of the precipitation enters the soil in a process called infiltration. The rate of Figure 3.23: Coastal upwelling caused by surface winds. infiltration is affected by the characteristics of the soil. Sandy soil, which is formed from large particles with relatively large gaps between them, has a high infiltration in the opposite direction and drives the water towards the rate, compared with clay soil, which has a low infiltration coastline, it is also possible for downwelling to occur. This rate. The higher the infiltration rate, the lower the rate of removes nutrients from the surface layers of the ocean. surface run-off. In other words, the more impermeable Run-off the ground, the more surface run-off there is. Run-off is part of the water cycle; water flows into As the water flows towards the sea it leaches nutrients streams and rivers and from there to the ocean. from the soil. This means that water-soluble nutrient

precipitation condensation

evaporation from plants

surface run-off evaporation from oceans, lakes DRAFTand streams

Figure 3.24: Summary of the water cycle.

3 Interactions in marine ecosystems

ions dissolve in the water. Run-off can also collect other substances as it flows, such as oil, heavy metals, KEY WORD pesticides and sewage. These all end up in the ocean. marine snow: particles of organic material that Excess nutrients in run-off can lead to marine dead fall from surface waters to the deeper ocean zones and harmful algal blooms. Marine dead zones are discussed in the extended case study. digest these proteins, using the amino acids released in Tectonic processes digestion to produce their own proteins. Small fish then Tectonic processes add nutrients to the water in two eat the zooplankton and the process continues. Once the main ways. At hydrothermal vents nutrients dissolve in nutrients have entered the food chain there are different the water as it passes over a magma chamber. When the paths they can take. Some sink to the floor as marine superheated water is released from the vent and meets snow, some are incorporated into coral reefs, and some the cold seawater, some of the mineral ions precipitate are removed by harvesting. out and form the solid chimney. However, many minerals which are essential for life remain dissolved in the water Marine snow where they are available to the organisms living near the Marine snow is the name given to the particles of vents. These are particularly important for the process of organic matter that fall from the surface of the ocean chemosynthesis which uses sulfides. Without this, there to the deeper water. It is made up of faeces from the would be no life at this depth as there is no light. organisms living in the surface layers, as well as dead Erosion and weathering are also important in adding animals, phytoplankton and zooplankton. It is called nutrients to the reservoir in the ocean. Tectonic activity marine snow because it looks like snow – small, white such as the release of magma from volcanoes as well as particles floating in the water (Figure 3.25). the formation of mountains adds fresh rock to Earth’s surface. At a convergent plate boundary between a continental and an oceanic plate, the more dense oceanic plate is subducted and pulled beneath the continental plate (see Chapter 2). In contrast, at convergent plate boundaries between two dense continental plates, a mountain can form. The Himalayas are a mountain range which formed in this way. The fresh rock added in these processes can then be eroded and weathered. Soluble nutrients will dissolve in rainwater and run-off into rivers and streams and from there into the ocean. Ash from volcanic eruptions has also been shown to fertilise the ocean and lead to an increase in primary productivity. When the ash lands in the water, mineral Figure 3.25: Marine snow in the water. ions such as phosphate, iron and magnesium dissolve and add to the reservoir. This continuous fall of organic matter provides food for many organisms that live deeper in the ocean. Some of it Processes that remove nutrients from is fed on by zooplankton and fish as it falls, some is eaten the surface layer by filter feeders much deeper down. Much of it is not The main way in which nutrients are removed from eaten at all and forms part of the sediment at the bottom the surface layer is through uptake and assimilation of the ocean. Some of the nutrients in the sediment are by producers. They fix the inorganicDRAFT ions into useable released into the water by processes such as erosion and organic compounds that consumers feed on. In this way, dissolving, others remain in the sediment for many years. the nutrients are able to move through the food chain. For example, phytoplankton take up nitrate ions and Harvesting use them to produce amino acids. These are then built Harvesting refers to the removal of marine species by up into proteins that form part of the phytoplankton humans. According to the United Nation’s most recent structure. Zooplankton eat the phytoplankton and figures, the total capture of fish in 2016 was 90.9 million

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

tonnes. Other animal species are also included in this The algae and photosynthetic bacteria that make up figure despite not being fish. These include crustaceans, the phytoplankton are able to take in dissolved forms such as crabs and lobsters, and molluscs, such as mussels of carbon dioxide and use it in photosynthesis. It is and squid. Macro-algae such as seaweeds can also fixed into glucose, which can then be used to form be harvested both for use as foods and in industrial other compounds needed by the phytoplankton. When processes such as the manufacture of gels and fertilisers. the phytoplankton are eaten by zooplankton, the All the nutrients present in these species are removed carbon-containing compounds are broken down during when they are harvested from the ocean. However, digestion. The zooplankton then assimilate them into many of the nutrients eventually find their way back to their own biomass. This process is repeated when the the ocean through the normal cycling of nutrients. For zooplankton are eaten by other consumers. example, fish may be eaten and digested by humans and At each stage, the organisms are respiring so they release some of the nitrogen-containing compounds are then carbon dioxide back into the water. From here it can lost in urine, which ends up in sewage. In many areas, diffuse back into the atmosphere. When the organisms sewage is released into rivers and oceans after only being die, some of the organic matter is broken down by partially treated. In some areas, raw sewage is released. decomposers and returns to the water as dissolved In this way, the nitrogen-containing compounds present inorganic carbon. Some of the organic matter falls to in the original fish return to the ocean. the ocean floor as marine snow, where it may remain for long periods of time. The carbon cycle The flux of carbon between the ocean and the atmosphere is around 90 gigatonnes year per year. This Carbon is needed by living things because it is the basis mean that, the same amount of carbon dioxide dissolves of all organic materials. Carbohydrates such as glucose into the ocean as diffuses back into the atmosphere. and starch, lipids, proteins, and nucleic acids such as However, there are also approximately 2 gigatonnes of DNA are all based on chains of carbon molecules. carbon each year added to the ocean through human Carbon enters the biotic phase of the cycle through activities such as the combustion of fossil fuels. This the fixation of carbon dioxide in photosynthesis. makes the oceans a very important carbon sink in terms Carbon dioxide is then released through respiration of reducing atmospheric carbon dioxide. But the risk is by all living things. that the ocean will become more acidic because of the The main way carbon enters the ocean is by dissolving extra carbonic acid formed. It has been estimated that of carbon dioxide gas from the atmosphere. Carbon since the 18th century, the pH of the ocean has decreased dioxide dissolves in water to form carbonic acid by 30%. This can have negative effects on the ecosystem. For example, a low pH triggers chemical reactions that (H2CO3). This then dissociates into hydrogencarbonate – + decrease the concentration of carbonate ions; this makes ions (HCO3 ) and hydrogen ions (H ) in a reversible reaction. Hydrogencarbonate dissociates further into it more difficult for corals to produce their calcium 2– + carbonate skeleton. This can also affect other species carbonate ions (CO3 ) and hydrogen ions (H ). So, in solution, there is a dynamic equilibrium between with calcified shells, including oysters and clams. If the carbon dioxide, hydrogencarbonate and carbonic acid. water becomes even more acidic, it can dissolve the coral In seawater, 89% of the dissolved inorganic carbon is skeletons and the shells of other organisms, making them found as hydrogencarbonate, 10% is carbonate and 1% weaker and more vulnerable to damage. is dissolved carbon dioxide. Some scientists have suggested that artificially fertilising The reactions in this equilibrium are: the ocean with iron would increase the productivity of the phytoplankton and mean that more carbon carbon dioxide + water carbonic acid dioxide could be absorbed. This has been put forward CO + H O H CO 2 2 DRAFT2 3 carbonic acid hydrogencarbonate ion + KEY WORD hydrogen ion dissociation (dissociates): a reversible chemical – + H2CO3 HCO3 + H change where the molecules of a single compound separate into two or more other hydrogencarbonate ion carbonate ion + hydrogen ion substances – 2– + HCO3 CO3 + H

3 Interactions in marine ecosystems

as a possible way to reduce the amount of carbon dioxide in the atmosphere. The theory is that, since iron KEY WORDS is often a limiting factor for phytoplankton growth, sedimentary rock: rock formed by the adding more will cause increased growth rates and deposition of particles on the ocean floor thus increased use of carbon dioxide. This process is known as ocean seeding or iron fertilisation. Trials have marine uplift: a process by which the floor of the shown that ocean seeding does increase the growth of ocean rises, possibly to the extent that it is no phytoplankton but there are risks to this procedure. longer beneath the water If the productivity increases too much, a harmful algal bloom could take place. The long-term effects fossil fuels: buried organic materials from dead of altering the ecosystem in this way are not clearly plants and animals which have been converted understood. If more carbon dioxide is absorbed, the into oil, coal or natural gas by exposure to heat pH of the water could decrease further, causing harm to and pressure in the Earth’s crust many different species. Respiration and photosynthesis are part of the short- plants whereas oil and gas are formed from the organic term carbon cycle. This cycle can take as little as a few matter which falls to the bottom of the ocean and days in the case of an algal bloom, or hundreds of years form a sediment. Sometimes the sediment is buried in in the case of an oak forest. mud, as the layer of mud gets thicker it is subjected to heat and pressure from the Earth’s core. This changes However, there are also much longer term processes the chemical structure and forms the fossil fuels. The which affect the carbon cycle. The organic matter process takes up to 650 million years and removes which falls to the bottom of the ocean as marine snow carbon from the cycle. The carbon is returned when contains carbonate ions, for example, in corals and the the fossil fuels are burned. Combustion of course is shells of sea animals. As the sediment builds up, the much quicker than the formation of fossil fuels and so layers at the bottom are compacted by the pressure of can lead to an imbalance in the carbon dioxide in the the layers above. Over long periods of time this can atmosphere. This is discussed in detail in Chapter 9. A them form sedimentary rocks such as limestone on the summary of the carbon cycle is shown in Figure 3.26. ocean floor. This process removes carbon from the carbon cycle. Eventually sedimentary rock from the ocean may end up being part of a landmass through a process called marine uplift. This can be a slow process Test yourself through the gradual movement of tectonic plates or 9 a Describe what is meant by the words biotic and a much quicker process during an earthquake. Once abiotic with reference to nutrient cycles. the rock is exposed to the rain and waves it will be b Explain how nutrients move from the abiotic subjected to erosion and weathering. The carbon stored to the biotic part of a nutrient cycle. in the rocks will be released as carbon dioxide as part 10 a Describe how nutrients move within the biotic of the chemical weathering process. For example, when part of the cycle. sulfur dioxide in the atmosphere dissolves in the rain it forms weak sulfuric acid. This reacts with the calcium b Name two places where you would find carbonate in limestone rocks to form calcium sulfate, nutrient ions within the abiotic part of a water and carbon dioxide which returns to the carbon nutrient cycle. cycle. The sediments in the oceanic crust can also be 11 Explain why harvesting is important in marine subjected to subduction at convergent plate boundaries nutrient cycles and explain whether you think it is as described in Chapter 2. In this case the carbon stored beneficial or harmful. in the rocks can later be returned to the cycle through 12 Draw simple diagrams to show the structure of a volcanic eruptions. DRAFTlipid, a named carbohydrate and a protein. Another part of the long-term carbon cycle is the formation of fossil fuels. Coal is formed from dead

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

atmospheric carbon dioxide

diffusion dissolving

reservoir of dissolved hydrogencarbonate ions, carbonate ions and carbon dioxide

photosynthesis respiration

producers respiration

weathering combustion death feeding

consumers

decomposers death

decay dead organic matter

sedimentary rock fossil fuels Figure 3.26: Summary of theDRAFT main processes in the marine carbon cycle.

3 Interactions in marine ecosystems

CASE STUDY 3.1: THE IMPORTANCE OF SALMON TO THE GROWTH OF TREES

Harvesting by humans is an important way in which It has become clear that not only do the salmon nutrients are removed from the marine environment. need the trees, but the trees need the salmon. As However, nutrients are also removed by migrations millions of salmon move through the waters of the of marine organisms such as salmon to fresh-water Pacific Northwest coast of the USA, they provide areas where they are eaten by predators like bears huge amounts of food for bears and eagles (Figure and eagles. 3.28). It has been estimated that each bear fishing in British Columbia, for example, can catch 700 salmon during the spawning period. Although the bears kill the salmon in the water, they move away from the water to eat. Roughly half of each salmon carcass is consumed by the bear, with the rest feeding scavenger species and insects. The compounds from the decomposing salmon eventually find their way back into the soil as part of the normal cycling of nutrients. From here they can be absorbed through the roots of the trees. For example, nitrates are often a limiting factor for plant growth. The fish provide up to 120 kg nitrogen per hectare of forest, which enables the trees to grow Figure 3.27: Spawning Pacific salmon moving upstream. up to three times faster than they would without the added nitrates. In this way, a positive feedback loop is formed. The more salmon that are deposited, the better the trees grow, and the better the trees grow, the better the conditions in the stream for the spawning of salmon. This has important implications for conservation of both salmon and forests, as each helps the other to survive. Since the 1990s there have been sharp declines in the numbers of Pacific salmon. In order to conserve the salmon populations, the forests must be protected. And in order to conserve the forests, there need to be enough salmon spawning each year.

Figure 3.28: Bear catching salmon. Questions 1 a Explain why the growth of trees is important Each year salmon return to the fresh-water streams for the survival of salmon. and lakes where they were born in order to breed b Explain how the salmon increase the growth (Figure 3.27). For successful reproduction, the streams of the trees. need to be shaded by trees so that the water is not too warm. Warm water contains less oxygen, so fewer 2 Suggest a type of organism, other than the eggs are able to survive. The trees help to prevent trees, that would benefit from the nutrients soil erosion, stopping sedimentDRAFT from entering the within the salmon. streams and keeping the water clear for the salmon. Large populations of insects live in the trees and 3 Imagine that you are working in conservation. provide food for the young salmon once they hatch. Explain whether you would start by conserving The trees are, therefore, important for the survival of the forests or the salmon and justify your the salmon. answer.

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

MATHS SKILLS

3.1 INTERPRETING ENERGY DIAGRAMS AND DRAWING PYRAMIDS OF ENERGY

Energy diagrams are another way of showing the flow So, the energy lost in respiration, X, must be of energy through an ecosystem (Figure 3.29). They equal to 598 – 67 = 531 kJ m–2 year–1. show the energy entering the ecosystem from the Sun, You can check your answer by making sure the moving through producers and being lost as a result of arrows add up: respiration and excretion. The main point to remember The ‘in arrow’ is 598 and the ‘out arrows’ are 531 is that all the energy has to go somewhere, so the values and 67. on the arrows going into a box must always equal the sum of the values on those coming out. The units 531 + 67 = 598, so the answer is correct. can be given as kJ m–2 year–1 or simply arbitrary units. 2 What percentage of the energy from the sun is Arbitrary units are relative units of measurement to transferred to the producers? allow comparison. For example, if there is twice as It is useful to calculate a percentage as it allows much energy in producers as in primary consumers the you to compare different ecosystems where the arbitrary units could be 5 and 10 or 50 and 100, it does initial energy inputs might be different. The not matter. percentage is the energy which is transferred divided by the energy that was in the previous Worked examples trophic level and then multiplied by 100. energy from sunlight The energy transferred to the producers 1.6 × 106 was 72 567 kJ m–2 year–1 and the energy in 43 137 energy in 23 009 the previous trophic level (the Sun) was producers 72 567 1.6 × 106 kJ m–2 year–1.

6421 So, the percentage transferred is: 72567 Y energy in primary 2389 × 100 = 4.54% consumers 1.6 × 106 You may also be given similar information in the 598 form of a pyramid of energy. In this case, just use the figures in the pyramid in your percentage X energy in secondary 67 consumers calculation. Always divide the figure in any particular trophic level by the figure for the respiration decomposers previous level and then multiply by 100. For example: Figure 3.29: Worked example of an energy flow diagram –2 –1 energy in primary consumers (units are kJ m year ). × 100 energy in producers 1 How much energy is being lost in respiration by the Draw a pyramid of energy which shows the secondary consumers shown in Figure 3.29? 3 energy in producers, primary and secondary Remember that the energy entering the secondary consumers. consumers must equal the energy leaving. You will not have to draw an energy flow diagram From the diagram, you can see that 598 kJ m–2 year–1 DRAFTbut you may have to draw a pyramid of energy. enters the secondary consumers (the ‘in arrow’) If you are asked to draw it to scale, consider each and 67 kJ m–2 year–1 ends up being passed to the bar of the pyramid to be like the bar on a bar decomposers. chart. In this case, the data will often be given in The only other arrow is the respiration arrow arbitrary units to make it easier to fit into one scale. (labelled X).

3 Interactions in marine ecosystems

MATHS SKILLS: CONTINUED

If you are asked to sketch a pyramid of energy (or 2 a Sketch a pyramid of biomass for the following numbers or biomass) it does not have to be drawn data: Fish 86 arbitrary units, zooplankton 912 to scale, the bars must simply be larger or smaller arbitrary units, phytoplankton 8000 arbitrary depending on the numbers you are given. The producers units. are always drawn as the lowest bar, followed by the b Use the data from your pyramid to calculate primary consumers and then secondary consumers until the percentage of energy which is transferred you reach the top of the food chain. Label each bar with from the zooplankton to the fish. the name of the organism from the food chain you are c The average percentage of energy transferred drawing. between producers and primary consumers is 10%. Suggest why the transfer in this Questions food chain between phytoplankton and 1 a Calculate the amount of energy used in zooplankton is higher than this. respiration (Y) by the primary consumers in Figure 3.29. KEY WORD b Calculate the percentage of energy transferred sketch: xxxxxxxxxxxxxxxxxx between producers and primary consumers.

PROJECT: MARINE RELATIONSHIPS CARTOON

Work in small groups to produce a cartoon which • Is it obvious which organism benefits or is illustrates the relationship between two marine harmed and why? organisms. The type of relationship should be one • Is it interesting to look at? studied in this chapter (for example, mutualism) but you must do research to find your own example. Give the other group a mark out of five for each Your cartoon should show the type of relationship category and total their final mark out of 20. riteW and how it affects both organisms, preferably in an down one thing you really liked about it and one interesting or creative way. piece of advice for making it even better.

Thinking about your project Each group should look at one other cartoon and think about the following questions: • Does it explain what the relationship is? • Would someone who has not studied this chapter understand DRAFTwhat was happening?

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

EXAM-STYLE QUESTIONS 1 a i Describe what is meant by the term productivity. [3] ii Give three factors that can affect productivity. [3] b i Explain the difference between gross primary production and net primary production. [3] ii The gross primary production in an ecosystem is 78 935 kJ m–2 year–1 and the energy lost in respiration is 23 674 kJ m–2 year–1. Calculate the net primary production. [2] c Explain why measurements of productivity in kJ m–2 year–1 are a more accurate representation of what is happening in the ecosystem than measurements of biomass. [2] Total mark = 13 2 a Describe the process of photosynthesis [4] b Why does photosynthesis not occur at hydrothermal vents on the ocean floor? [2] c The solar energy falling on the ocean is 1.7 × 106 kJ m–2 year–1 and the phytoplankton are able to use 18 754 kJ m–2 year–1 of this. i Calculate the percentage of the Sun’s energy that is used by phytoplankton. Show your working. [2] ii Explain why 100% of the energy is not used. [3] Total mark = 11 3 Study the graph below.

10 –3 8 µg dm

/ 6

chlorophyll 2

0 J FMAM JJASOND month

Figure 3.30

a Describe the shape of the graph. [3] b Explain whyDRAFT the amount of chlorophyll increases in March. [5] c Suggest and explain what will happen to the population of zooplankton in March and April. [2]

3 Interactions in marine ecosystems

CONTINUED d Within this ecosystem, herring feed on the zooplankton and mackerel feed on the herring. There are 809 phytoplankton, 37 zooplankton, 11 herring and one mackerel. Draw the pyramid of numbers for this food chain. [3] Total mark = 13 4 a i Describe how nutrient-rich water from deep in the ocean enters the reservoir of nutrients at the surface. [2] ii Describe two other ways in which nutrients enter surface waters. [2] b i Explain the benefit of increased nutrients in surface aters.w [2] ii Suggest how increased nutrients in surface waters could be harmful. [2] c Give an example of an essential element needed by living COMMAND WORD organisms and state what it is used for. [2] Total mark = 10 Give: Produce 5 a With reference to named examples, explain the meaning of an answer from a each of the following terms: given source or i Parasitism [3] recall/memory. This ii Commensalism [3] normally means a b Contrast producers and consumers. [3] simple answer which you are expected to Total mark = 9 remember. 6 A student wanted to investigate the effect of light on the rate of photosynthesis in pondweed by counting the bubbles coming from the stem. a Describe how this experiment should be set up to obtain accurate results. [5] b Give two variables which must be controlled. [2] c Contrast producers and consumers. [3] Total mark = 10 7 Study the graph below.

phytoplankton zooplankton biomass

J FMAMJJDRAFTASOND month

Figure 3.31

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

CONTINUED a Sketch a pyramid of biomass to show the phytoplankton and zooplankton in March. [2] b Explain why there is normally more biomass in the producers than in the consumers. [3] c Describe how the pyramid of biomass would be different in July. [2] d Explain why the pyramid of biomass would be different in July. [1] Total mark = 8 8 Figure 3.32 shows a food chain from a marine ecosystem. The figures show the energy in each trophic level in winter in arbitrary units.

phytoplankton zooplankton sardine tuna 5000 589 61 4

Figure 3.32

a Calculate the percentage of the energy transferred between the phytoplankton and the zooplankton. Show your working. [2] b Explain why there is less energy in the consumers than in the producers.. [3] c i Suggest what would happen to the energy in each level during summer. [3] ii Suggest what might happen to the food chain if fertilisers from coastal farmland drain as run-off into the water. [3] Total mark = 11 9 a Complete Table 3.6 to show the uses of different nutrients. [3]

Nutrient Biological use nitrogen calcium phosphorus

Table 3.6

b i Describe the process of run-off. [3] ii Describe the effect of run-off of nitrogen fertilisers on producers.DRAFT [3] iii Explain how this will affect the consumers in the food chain. [2] Total mark = 11

3 Interactions in marine ecosystems

CONTINUED 10 Salmon were sampled at a site in the Atlantic Ocean once per year and checked for sea lice. Table 3.7 shows the results.

Number of salmon Proportion of salmon Year caught at sampling site infested with sea lice / % 2007 356 1.7 2008 302 4.3 COMMAND WORD 2009 340 4.4 Evaluate: Judge or calculate the 2010 253 4.6 quality, importance, amount, or value of 2011 307 6 something. 2012 105 7.4 2013 68 5.3 2014 35 8.2 2015 32 8.9 2016 15 11.4

Table 3.7

a Plot a graph to show the changes in the number of salmon each year. [4] b Describe the pattern of the graph. [2] c Evaluate the evidence that sea lice affect the health of the salmon. [4] Total mark = 10 DRAFT

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

SELF-EVALUATION

Needs Almost Ready to I can: more work there move on describe the three main types of symbiotic relationship and describe examples of each one represent feeding relationships as food chains or food webs and be able to describe the organisms in these relationships in terms of their trophic level explain how photosynthesis provides energy to the food chain and summarise the process as a word equation give the word equation for respiration define productivity and explain how high productivity can affect food chains explain why energy is lost at each stage of the food chain draw pyramids of number, energy and biomass and be able to explain and interpret their shapes explain what a nutrient is and be able to give the biological roles of nitrogen, carbon, magnesium, calcium and phosphorus draw simple diagrams to show how carbohydrates, proteins and lipids are made up from smaller molecules explain how nutrients in the ocean are depleted and replenished describe the different stages in the carbon cycle including combustion, photosynthesis, decomposition, fossil fuels and the formation and weathering of rocks DRAFT

3 Interactions in marine ecosystems

EXTENDED CASE STUDY:

New Mexico Mississippi South Carolina Alabama Texas Georgia Louisiana Houston Florida Gulf of Mexico Mexico

Mexico city Cuba

Figure 3.33: The Gulf of Mexico and surrounding Guatemala Dominican Honduras Republic countries.

The Gulf of Mexico dead zone Formation of dead zones The Gulf of Mexico is an ocean basin surrounded by Dead zones form when excess nutrients such as the United States, Mexico and Cuba (Figure 3.33). It phosphates and nitrates enter the water. These is approximately 1500 km wide and is connected to primarily come from chemical fertilisers and waste both the Atlantic Ocean and the Caribbean Sea. such as sewage. Dead zones can also form naturally if changes in wind and currents alter the upwelling of A dead zone is an area of water where oxygen nutrients from deep water. levels have become very low (hypoxic). This means that there is insufficient oxygen for respiration, and When the excess nutrients enter the water, they organisms either die or move to a different area massively increase the growth of algae, leading to where the dissolved oxygen levels are higher. Dead an algal bloom. Phosphates and nitrates tend to zones occur near coastlines where there are high increase the growth of blue-green cyanobacteria, levels of nutrients washing off agricultural land. which are not eaten by many zooplankton. This The first dead zone was reported in the early 20th means that their numbers can build up unchecked. century and since then the numbers have increased When the organisms die they sink to the bottom, every year. (Figure 3.34). It has been estimated that where they provide a food source for the bacteria, between 1950 and 2018 the overall level of oxygen which break them down in the process of in the oceans dropped by 77 billion tonnes. decomposition. The numbers of bacteria rapidly increase and their respiration uses up the majority 400 of the oxygen dissolved in the water. The water,

350 therefore, becomes hypoxic and other aquatic organisms die. 300

250 The Gulf of Mexico

200 The Gulf of Mexico dead zone is interesting for two reasons. First, it is the second largest dead 150 zone in the world. Second, it is seasonal and its size 100 fluctuates depending on the weather conditions cumulative number of systems cumulative each year. The Mississippi River drains into the Gulf 50 and has the largest drainage basin of any river in 0 DRAFTNorth America. The levels of nutrients it washes into 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 <1910 the Gulf are correspondingly large. Twelve million year people live in areas that border the Mississippi and that discharge treated sewage into the water. The Figure 3.34: Number of dead zones by decade. majority of the land near the Mississippi is farmland

CAMBRIDGE INTERNATIONAL AS & A LEVEL MARINE SCIENCE: COURSEBOOK

CONTINUED

so rainwater constantly washes fertilisers into the (NOAA) has estimated that the dead zone costs the water. About 1.7 million tonnes of nutrients are tourism and fishing industries $US 82 million per released into the Gulf of Mexico from the Mississippi year. When fish move out of the dead zone because every year. In the spring and summer this causes of the lack of oxygen, fishers have to travel further to algal blooms and the development of the dead catch them. This costs both time and money. Shrimp zone. The size of the dead zone varies; on average are often unable to escape the dead zone and it is around 13 000 km2. The largest recorded dead instead are simply killed, reducing the population zone here was in 2017 when it reached 22 730 km2. and making them harder to catch in the future. Interestingly, a year later, during the summer of 2018, the dead zone returned to an average size. Reducing the size of the dead zone During times of heavy flooding the dead zone tends The main way to reduce the size of any dead zone to be very large. In the late summer of 1998 the is to reduce the level of nutrients entering the water. dead zone disappeared because there was a severe In 1997 the Gulf of Mexico Watershed Nutrient drought and the amount of water entering the Gulf Task Force was formed with the aim of reducing decreased significantly. Figure 3.35 shows the size of the average size of the dead zone to 5000 km2. the dead zone in an average year. Strategies that can be used include reducing the Baton Lake Pont TEXAS Mississippi river use of inorganic fertilisers on farms, as well as Lake Lafayette Rouge 10 altering the timing of their use to avoid leaching by 10 Charles 10 New Orleans rainwater. Management of flood plains is important 90 because an increased area of flood plains means that less floodwater makes its way into the Gulf and nutrient-rich sediment is captured. Farmers Gulf of Mexico normal lowest are being encouraged not to drain wetlands but 50 miles oxygen oxygen to leave them in their natural state to improve 2 levels levels 13 084 km soil quality and reduce erosion. Waste treatment key: processes are being improved to reduce the Figure 3.35: An average sized dead zone in 2018. discharge of nutrients into the water and to avoid animal waste entering waterways at all.

The fresh water flowing into the Gulf from the Questions Mississippi is less dense than the seawater and so forms a layer on the top. This means that the deeper 1 Describe how a dead zone forms. water, where the hypoxia occurs, is cut off from a 2 Suggest why the number of dead zones has resupply of oxygen from the atmosphere. The dead increased since they were first discovered. zone, therefore, persists until the water mixes again, either because of a hurricane or when cold fronts 3 Explain why the Gulf of Mexico dead zone form in the autumn and winter. varies in size each year.

The effects of the dead zone 4 Explain why the Gulf of Mexico dead zone is seasonal. The seafood industry in the Gulf of Mexico is very important. The Gulf provides the United States with 5 Summarise the control measures being taken to the majority of its farmed oysters and shrimp, as reduce the size of the Gulf of Mexico dead zone well as being a source for several types of fish. The and explain how each measure works. National Oceanic and AtmosphericDRAFT Administration

Original material © Cambridge University Press 2019. This material is not final and is subject to further changes prior to publication.