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4.2 Nutrient Cycles

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4.2 Nutrient Cycles Learning outcomes By the end of this chapter, you should be able to: describe the genera l processes t hat take place within a nutrient cycle explain what is meant by a reservoir within a nutrient cycle describe and explain the processes that add nutrients to the surface water of the ocean describe the processes that remove nutrients from the surface water of the ocean summarise the ni trogen, carbon, magnesium, ca lcium and phosphorus cycles as a simple diagram state the uses of each of the above nutrients in living organisms plot and interpret accurate graphs of experimental resu lts apply what you have learnt to new, unfami liar contexts . .,. ~D--·;'r , , II(P .. :J,. .¥ \~...·' ~. >' ( •, '· ,, ·~'· '-4~· I "" .. , .. .,. .~·. ~~-:· // " ~ "· ~ .liP"":;' .. , ... ... ·' ... •, i • -- . '"' ""' ··· ~- 1' . ' • ' ·:r • 0 ... ~0 ~ .,.1"" ... _., 1/ ;, ; ' • "'~ , ...;. ,,, 0 ~ " ;( ' .... ~ ~ }' • !lil)t, . .. \ . '· o' f l 0 , • t'· ' 'f' I 1 I> ' ~ ~ I ~ ' / ~ · ~ I , " . f', , ~ ' -~• -~ .0 • • ' . " ~ ~"' .•• .• " ...... ./':; . ' ..... - ' _ '_ , ... _ t ,' ~ \ ~' ..... ~.. •'"'\ ~ I ~ ~ ... '.~ Chapter 4: Nutrient cycles in rriarine ecosystems are too many nutrients the productivi ty can in crease too fast and the ecosystem is damaged. Recently, it has been suggested that artificially altering the nutrient Nutrient cycles are some of the most important balance in the oceans could increase productivity processes that occur in any ecosystem. They show the and therefore increase the amount of carbon dioxide movement of nutrients that are essential for life, such used in photosynthesis. This has been proposed as a as nitrogen, carbon and phosphorus. These nutrients solution to the increasing levels of ca rbon dioxide in the are used by living organisms and are moved through the atmosphere. However, this solution may have unintended food chain by feeding. When organisms die the nutrients consequences, such as decreasing the pH of the water and are recycled by decomposers and return to inorganic damaging animals with shells. It could also lead to harmful forms. The inorganic forms remain in the environment, algal blooms as discussed in Chapters 3 and 8. sometimes for millions of years, before being converted back into organic forms to be used once again, thus continuing the cycle. The ocean is an important reservoir for these elements, Nutrient cycles: the movement and exchange of elements that are essential to life, from inorganic molecules, through which means that they may be held there for long fixation and then into living organisms, before being periods of time. Microorganisms are able to fix inorganic decomposed back into inorganic molecules substances into organic molecules, which enables them Nutrient: a chemical that provides wh3t is needed for to be used by other organisms. In this way the nutrients organisms to live and grow are moved from the abiotic part of the cycle to the Decomposers: bacteria and fungi that break down dead biotic. The nutrients may then be removed temporarily organic matter and release the nutrients back into the from the cycle if they sink to the ocean floor as faeces, or environment after the organism has died. Some wi ll be incorporated Reservoir: part of the abiotic phase of the nutrient cycle into coral reefs and others will be removed from the where nutrients can remain for long periods of time ocean altogether by harvesting. Inorganic molecules are Abiotic: the environment's geological, physical and chemical I returned to the ocean by various processes, including features, the non-living part of an ecosystem dissolving directly into the water, run-off from the land Biotic: the living parts of an ecosystem, which includes the and upwelling. organisms and their effects on each other Run -off: the flow of water from land c2used by precipitation Chapter 3 discussed the effect of nutrient concentration Upwelling: the movement of cold, nu t rient-rich water from in the ocean. Up to a certain point, the more nu trients deep in the ocean to the surface present, the more productive the environment. When there 4.2 Nutrient cycles Fo r example carbon dioxide (an inorgan ic molecu le and therefore part of the abiotic cycle) is fi xed du ring Nutrient cycles are the essential movement and recycling photosynthesis into glucose. This ca later be converted of the elements that are necessary for organisms to live into the other molecules needed by he producer, for an d grow. Globally, the ca rbon and nitrogen cyc les are example starch. It has been ass imi la ed and is now part probably the best known and most clea rly understood, of the biotic cycle. During the biotic phase nutrients are but there are many other elements that are important. moved from one organism to the next by feeding. So These include phosphorus, calcium and magnesium. In nutrients move along the food chai r from the producers this chapter you look at why each of these is necessary for to the consumers. Some will be lost from each organis m life, as we ll as the mec han isms that add them or remove them from the oceans. All nutrient cycles have a biotic and an abiotic phase Assimilation: the conversion of a nut'ient into a useable (Figure 4.1). A nutrient moves from the abiotic to the biotic form that can be incorporated into thE tissues of an organism phase when it is absorbed an d assimilated by producers. Cambridge lnterna Figure 4.1. A generalised nutrient cycle showing the movement from the biotic to the abiotic phases. by egestion and excretion and the rest will remain within nutrient J average residence time I years organic compounds until the organism dies. After death, organisms must be br?ken down by decomposers, which phosphate 20 000-100 000 I results in nutrients returning to their inorganic form and (p hosphorus)* therefore the abiotic pbrt ofthe cycle. During this part of magnesium 17 000 000 the cycle nutrients can be found dissolved in water, as gases in the atmosphJre, or forming sediment that can hyd rogenca rbonate** 100 000 later become rocks. (carbon)* nitrogen 2 000 Reservoirs in nut1ent cycles calcium 1000 000 A reservoir is part of t1,e abiotic phase of the nutrient cycle *Where the nutrient is found as an ion, the element is gi ven in where elements can remain for long periods of time. The parentheses ocean is an important reservoir for many elements. The **Sometimes called bicarbonate residence time is the average time a particle spends in Table 4.1. Approximate residence times for different a system. Average times for nutrient ions in the resi ~ ence nutrients in the ocean . ocean tend to be very ~ ong because some of them fall to the bottom in faeces or dead organisms. They can rema in li ving there. This surface reservoir is of particular in sediment on the ocTan floor for thousands or even importance because it enables the high productivity millions of yea rs (Table 4.1) of phytoplankton. Nutrient availability is often the The time the same nu ~ rients spend in just the surface main limiting factor after light intensity for growth of layer of the ocean is much shorter because the nutrients producers. are constantly being ysed and recycled by the organisms Phytoplankton are found in the su rface layer of the ocean where there is plenty of light. It is therefore the concentration of nutrients that determines the rate of growth. The higher the rate of growth of phytoplankton, Residence time: the average time that a particle spends in a particular system the higher the rate of photosynthesis and therefore the higher the productivity. The productivity of the Chapter 4: Nutrient cycles in inarine ecosystems phytoplankton determines how much energy can be The relative importance of these processes depends on transferred to the next trophic level (see Chapter 3). In each nutrient. For nutrients present in high concentrations general, the amounts of nitrogen and phosphorus limit in the atmosphere, dissolving will add more to the the rate of growth because they are found in the lowest reservoir than run-off, for example. concentrations in the water. This means that there is usually slightly less than is needed by the producers. If Dissolving of atmospheric gases the concentrations increase, the productivity in creases. Nitrogen and carbon are both preser1t in the Earth's The average concentrations of ions dissolved in the water atmosphere and are therefore both ble to dissolve at the ocean surface are shown in Table 4.2. directly into the water. Nitrogen is present in the form of nitrogen gas, N2, and carbon as carbon dioxide gas, C02. ion I average concentration in The amount of gas that can dissolve in the water depends seawater I ppm on several factors. These include the: chloride 19 345.00 • temperature of the water sodium 10 752.00 • atmospheric concentration of each gas su lfate 2701.00 • amount of mixing of water at the surface. ma gnesium 1295.00 In some areas there will be more gas dissolving in the calcium 416.00 water than there is diffusing back in o the atmosphere. These areas are known as sinks. hydrogencarbonate 145.00 In other areas it will the other way around, and more nitrate 0.50 gas will diffuse into the atmosphere than is dissolving phosphate 0.07 into the water. These areas are callem sources. Generally the overall concentration tends to r~ main at an Table 4.2. Average concentrations of some of the ions found I eq uilibrium, with the same amount disso lving into the dissolved in seawater. ocean as is removed by diffusion ba[ k into the atmosphere (Figure 4.2). ASSESSMENT QUESTIONS 1 a Describe what is meant by the words biotic and abiotic with.reference to nutrient cycles.
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