Energy is crucial to an ecosystem. But all organisms need more

than energy to survive. They also need water, minerals, and

other life-sustaining compounds. In most organisms, more than

95 percent of the body is made up of just four elements: oxygen,

carbon, hydrogen, and nitrogen. Although these four elements

are common on Earth, organisms cannot use them unless the

elements are in a chemical form that cells can take up.

Recycling in the BiosphereEnergy and matter move through the biosphere very differently.

Unlike the one-way flow of energy, matter is recycledwithin and between ecosystems. Elements, chemical com-

pounds, and other forms of matter are passed from one organ-

ism to another and from one part of the biosphere to another

through As the long word suggests,

biogeochemical cycles connect biological, geological, and chemi-

cal aspects of the biosphere.

Matter can cycle through the biosphere because biological

systems do not use up matter, they transform it. The matter is

assembled into living tissue or passed out of the body as waste

products. Imagine, for a moment, that you are a carbon atom in a

molecule of carbon dioxide floating in the air of a wetland like

the one in Figure 3–10. The leaf of a blueberry bush absorbs you

during photosynthesis. You become part of a carbohydrate

molecule and are used to make fruit. The fruit is eaten by a

caribou, and within a few hours, you are passed out of the ani-

mal’s body. You are soon swallowed by a dung beetle, then com-

bined into the body tissue of a hungry shrew, which is then eaten

by an owl. Finally, you are released into the atmosphere once

again when the owl exhales. Then, the cycle starts again.

Simply put, biogeochemical cycles pass the same molecules

around again and again within the biosphere. Just think—with

every breath you take, you inhale hundreds of thousands of

oxygen atoms that might have been inhaled by dinosaurs

millions of years ago!

biogeochemical cycles.

3–3 Cycles of Matter

Key Concepts• How does matter move

among the living and nonliv-ing parts of an ecosystem?

• How are nutrients important inliving systems?

Vocabularybiogeochemical cycleevaporationtranspirationnutrientnitrogen fixationdenitrificationprimary productivitylimiting nutrientalgal bloom

Reading Strategy:Using Visuals Before youread, preview the cycles shownin Figures 3–11, 3–13, 3–14,and 3–15. Notice how eachdiagram is similar to or differentfrom the others. As you read,take notes on how each chemicalmoves through the biosphere.

� Figure 3–10 Matter movesthrough an ecosystem in biogeo-chemical cycles. In this Alaskanwetland, matter is recycled throughthe air, the shrubs, the pond, and thecaribou—as it is used, transformed,moved, and reused.

74 Chapter 3

1 FOCUSObjectives3.3.1 Describe how matter cycles

among the living and nonliv-ing parts of an ecosystem.

3.3.2 Explain why nutrients areimportant in living systems.

3.3.3 Describe how the availabilityof nutrients affects the pro-ductivity of ecosystems.

Vocabulary PreviewFigure 3–11, page 75, introducesseven terms. Two of the terms, evap-oration and transpiration, areexplicitly defined in the text. Themeanings of the remaining fiveterms—condensation, precipitation,runoff, seepage, and uptake—can beinferred from their context. As stu-dents read about the water cycle onpage 75, have them look for sen-tences that relate to the terms andcopy them on a sheet of paper.Finally, using the sentences theycopied as a basis, students canextrapolate a “formal” definition foreach term. Have students share theirdefinitions in a class discussion.

Reading StrategyHave students make their own simpli-fied cycle diagrams of the watercycle, carbon cycle, nitrogen cycle,and phosphorus cycle.

2 INSTRUCT

Recycling in theBiosphereBuild Science SkillsInferring After students have readRecycling in the Biosphere, point outthe sentence that begins You are soonswallowed by a dung beetle . . . Ask:How can a molecule that’s swal-lowed by a dung beetle “combineinto”—or become part of—thebody tissue of a tree shrew andthen an owl? (The tree shrew takes inthe molecule when it eats the dungbeetle, and then an owl takes in themolecule when it eats the tree shrew.)

Section 3–3

SECTION RESOURCES

Print:

• Laboratory Manual A, Chapter 3 Lab• Laboratory Manual B, Chapter 3 Lab• Teaching Resources, Lesson Plan 3–3,

Adapted Section Summary 3–3, AdaptedWorksheets 3–3, Section Summary 3–3,Worksheets 3–3, Section Review 3–3,Enrichment

• Reading and Study Workbook A, Section 3–3

• Adapted Reading and Study Workbook B,Section 3–3

• Biotechnology Manual, Issue 4• Lab Worksheets, Chapter 3 Exploration

Technology:

• BioDetectives DVD, Pfiesteria: A Killer in theWater

• iText, Section 3–3• Transparencies Plus, Section 3–3

Tim

eSaver

0062_0080_bi_c07_te 3/7/06 2:12 PM Page 74

The Biosphere 75

CondensationPrecipitation

Runoff

Seepage

Lake

Ocean

Evaporation Transpiration

Ground water

Root

Uptake

� Figure 3–11 This diagramshows the main processes involvedin the water cycle. Scientists esti-mate that it can take a single watermolecule as long as 4000 years tocomplete one cycle. InterpretingGraphics What happens to thewater that evaporates from oceansand lakes?

The Water CycleAll living things require water to survive. Where does all thiswater come from? It moves between the ocean, atmosphere, andland. As Figure 3–11 shows, water molecules enter the atmos-phere as water vapor, a gas, when they evaporate from the oceanor other bodies of water. The process by which water changesfrom liquid form to an atmospheric gas is called (ee-vap-uh-RAY-shun). Water can also enter the atmosphere by evaporating from the leaves of plants in the process of

(tran-spuh-RAY-shun).During the day, the sun heats the atmosphere. As the warm,

moist air rises, it cools. Eventually, the water vapor condensesinto tiny droplets that form clouds. When the droplets becomelarge enough, the water returns to Earth’s surface in the form ofprecipitation—rain, snow, sleet, or hail.

On land, much of the precipitation runs along the surface ofthe ground until it enters a river or stream that carries the runoffback to an ocean or lake. Rain also seeps into the soil, some of itdeeply enough to become ground water. Water in the soil entersplants through the roots, and the water cycle begins anew.

How are evaporation and transpiration related?

transpiration

evaporation

For: Water Cycle activityVisit: PHSchool.comWeb Code: cbp-2033

Answers to . . . Evaporation is part of the

process of transpiration.

Figure 3–11 The water vapor risesinto the atmosphere and then coolsand condenses to form clouds.

Less Proficient ReadersSome students might be unfamiliar with theterms used in Figure 3–11. Call on students atrandom to read aloud each sentence or set ofsentences in the text that describes one of theprocesses in the water cycle. For example, thefirst two sentences in the second paragraph onpage 75 describe condensation. After the textexplanation for each process is read aloud, havestudents find that step in the cycle in Figure 3–11and describe it in their own words.

Advanced LearnersEncourage students who need an extra chal-lenge to investigate further one of thebiogeochemical cycles discussed in this section.You might assign one student to do furtherresearch on each of the four cycles discussed.Students can find more information on thesecycles in higher-level biology texts as well as inearth science texts. Have students prepare apresentation to the class, complete with visualaids.

The Water CycleUse VisualsFigure 3–11 After students havestudied the diagram and read thecaption, have them recall what theylearned about water in Chapter 2.Remind them that water is the singlemost abundant compound in mostliving things. Then, ask: What aretwo ways that water can enter theatmosphere? (Evaporation and tran-spiration) What process moveswater through the cycle from theair to the ground? (Precipitation)What are two routes by whichwater might make its way to theocean? (Through runoff and throughseepage into ground water and eventu-al flow into the ocean)

Build Science SkillsComparing and ContrastingEmphasize that transpiration byplants releases water vapor, a gas,into the air, not liquid water. Ask:What process in humans and othermammals also releases water vaporinto the air? (Respiration) How aretranspiration in plants and respira-tion in mammals different? (Sampleanswer: Mammals have specializedorgans that are involved in respira-tion—the lungs, diaphragm, bronchialtubes, and so forth—but plants do nothave “breathing” organs.)

For: Water Cycle activityVisit: PHSchool.comWeb Code: cbe-2033Students can examine howwater moves through the water cycle.

UNIVERSAL ACCESS

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Nutrient CyclesThe food you eat provides energy and chemicals that

keep you alive. All the chemical substances that an

organism needs to sustain life are its Think

of them as the body’s chemical “building blocks.”

Primary producers, such as plants, usually obtain

nutrients in simple inorganic forms from their environ-

ment. Consumers, such as the monkey in Figure 3–12,obtain nutrients by eating other organisms. Everyliving organism needs nutrients to build tissuesand carry out essential life functions. Like water,nutrients are passed between organisms and theenvironment through biogeochemical cycles.

The carbon cycle, nitrogen cycle, and phosphorus

cycle are especially important. Note also that oxygen

participates in all these cycles by combining with these

elements and cycling with them during various parts of

their journey.

What is a nutrient?

The Carbon Cycle Carbon plays many roles. Carbon is a key

ingredient of living tissue. In the form of calcium carbonate

(CaCO3), carbon is an important component of animal skeletons

and is found in several kinds of rocks. Carbon and oxygen form

carbon dioxide gas (CO2), an important component of the atmos-

phere. Carbon dioxide is taken in by plants during photosynthe-

sis and is given off by both plants and animals during respiration.

Four main types of processes move carbon through its cycle:

• Biological processes, such as photosynthesis, respiration, and

decomposition, take up and release carbon and oxygen.

• Geochemical processes, such as erosion and volcanic activity,

release carbon dioxide to the atmosphere and oceans.

• Mixed biogeochemical processes, such as the burial and

decomposition of dead organisms and their conversion under

pressure into coal and petroleum (fossil fuels), store carbon

underground.

• Human activities, such as mining, cutting and burning forests,

and burning fossil fuels, release carbon dioxide into the

atmosphere.

Scientists identified these processes decades ago, but they are still

actively investigating them. For example, how much carbon moves

through each part of the cycle? How do other parts of the carbon

cycle respond to changes in atmospheric carbon dioxide? How much

carbon dioxide can the ocean absorb? Later in this unit, you will

learn why answers to these questions are so important.

nutrients.

� Figure 3–12 Like all livingorganisms, the owl monkey needsnutrients to grow and carry outessential life functions. Thismonkey, which is found in Centraland South America, obtains most ofits nutrients by eating plants.

For: Links on cycles of matter

Visit: www.SciLinks.orgWeb Code: cbn-2033

NSTA

76 Chapter 3

Nutrient CyclesMake ConnectionsHealth Science For each pair ofstudents, provide an empty vitamincontainer with its nutrition labelintact. Try to provide a mix of vita-mins for adults, for young children,and for infants. You may want to askstudents in advance to bring in con-tainers from home. Ask: What typesof information are given on thenutrition label? (The serving size, thetotal number of servings in the contain-er, the specific nutrients in the pills ordrops, the amount of each nutrient inone serving, and the percentage ofdaily value each amount represents.)What do you think a “daily value”is? (How much of a nutrient a personshould take in each day) What does“percentage of daily value” mean?(How much of the daily value is in oneserving of the vitamin) Ask students ifthey recognize the names of any ofthe nutrients listed on the label andwhether they know the nutrients’common dietary sources and theirfunctions in maintaining goodhealth. Depending on the extent ofstudents’ knowledge, you may wantto suggest that they research thisinformation and share their findingsin posters or brief oral presentations.

The rain in Spain, and elsewhereHuge quantities of water cycle between Earth’ssurface and atmosphere. Hydrologists estimatethat about 390,000 cubic kilometers of waterevaporate from Earth’s surface and enter theatmosphere each year.

Considering all forms of precipitation, about 77percent falls on oceans and about 23 percent on

land. However, this same proportion does nothold true for water that evaporates from Earth’ssurface: 84 percent of the water in the atmos-phere comes from oceans and only 16 percentfrom land. The reason for the difference in pro-portions is simple: about a third of theprecipitation that falls on land runs off intostreams and rivers and is carried to oceans.

FACTS AND FIGURES

3–3 (continued)

NSTA

Download a worksheeton cycles of matter for students tocomplete, and find additional teachersupport from NSTA SciLinks.

0062_0080_bi_c07_te 3/7/06 2:12 PM Page 76

Use VisualsFigure 3–13 Call on different stu-dents in turn to “translate” thediagram’s pictures, labels, and arrowsinto complete, descriptive sentences.For example, the circled picture oftrees, the arrows, and thePhotosynthesis label on the left side ofthe diagram can be expressed as,“During photosynthesis, plants takein carbon dioxide from the atmos-phere and release oxygen.” Thecircled picture of the elk with twoarrows can be translated as, “Duringrespiration, animals take in oxygengiven off by plants and release car-bon dioxide into the atmosphere,”and, “When animals die and decom-pose, carbon is released into thesoil.” Continue until all the processesin both pathways have beendescribed this way.

The Biosphere 77

Carbonaterocks

Photosynthesis

Photosynthesis

Deposition

Decomposition

Deposition

Uplift

Volcanicactivity

Humanactivity

CO2 in Atmosphere

CO2 in Ocean

Respiration

Respiration

Feeding

Feeding

Fossilfuels

Erosion

� Figure 3–13 Carbon is foundin several large reservoirs in thebiosphere. In the atmosphere, it isfound as carbon dioxide gas; in theoceans as dissolved carbon dioxide;on land in organisms, rocks, andsoil; and underground as coal,petroleum, and calcium carbonaterock. Interpreting GraphicsWhat are the main sources of carbondioxide in the ocean?

Figure 3–13 shows how these processes move carbon through

the biosphere. In the atmosphere, carbon is present as carbon

dioxide gas. Carbon dioxide is released into the atmosphere by

volcanic activity, by respiration, by human activities such as the

burning of fossil fuels and vegetation, and by the decomposition

of organic matter. Plants take in carbon dioxide and use the

carbon to build carbohydrates during photosynthesis. The

carbohydrates are passed along food webs to animals and other

consumers. In the ocean, carbon is also found, along with cal-

cium and oxygen, in calcium carbonate, which is formed by

many marine organisms. Calcium carbonate can also be formed

chemically in certain marine environments. This chalky, carbon-

based compound accumulates in marine sediments and in the

bones and shells of organisms. Eventually these compounds

break down and the carbon returns to the atmosphere.

Answers to . . . Any chemical substance

that an organism needs to sustain life

Figure 3–13 Respiration by oceananimals, precipitation containing dis-solved carbon dioxide, erosion ofcarbonate rocks formed from the skele-tons of ocean organisms such as corals

The carbon poolScientists estimate the biosphere’s total carbonpool to be approximately 49,000 metric gigatons.(1 metric gigaton equals 109 metric tons.) Of thattotal, 71 percent is contained in Earth’s oceans,mainly in the form of carbonate and bicarbonateions. Fossil carbon comprises 22 percent of thetotal pool. An additional 3 percent is contained indead organic matter and phytoplankton, andanother 3 percent is held in terrestrial ecosystems.

The remaining 1 percent is held in the atmos-phere, circulated, and used in photosynthesis.

The carbon that is contained in organic molecules—such as the wood in trees—may not be recycled back to the abiotic environmentfor several hundred years or even longer.Carbon compounds found in coal that formedfrom ancient trees, for example, are the prod-ucts of photosynthesis that occurred millions ofyears ago.

FACTS AND FIGURES

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78 Chapter 3

Make ConnectionsChemistry Write the chemical for-mulas for atmospheric nitrogen (N2),ammonia (NH3), the nitrate ion(NO3

�), and the nitrite ion (NO2�)

on the board. Ask students: Whichelement is symbolized by each let-ter in these formulas? (N is thesymbol for nitrogen; H for hydrogen;and O for oxygen.) What do thesmall numbers mean? (The smallnumbers tell how many atoms of theelement are in one molecule or ion ofthe substance.) What atoms makeup one molecule of atmosphericnitrogen? (Two atoms of nitrogen)One molecule of ammonia? (Oneatom of nitrogen and three atoms ofhydrogen) The nitrate ion? (Oneatom of nitrogen and three atoms ofoxygen) The nitrite ion? (One atomof nitrogen and two atoms of oxygen)

Build Science SkillsUsing Models To reinforce students’understanding of the nitrogen cycle,have them make models of the fourchemical formulas of the differentforms of nitrogen. Provide a varietyof materials, and let each studentchoose the type of model to make.For example, two-dimensional mod-els could be made with circles cutfrom colored construction paper andglued to a larger sheet. Three-dimen-sional models could be made withclay balls of different colors heldtogether with toothpicks. Studentscan use their models as they studythe nitrogen cycle in Figure 3–14.

Use VisualsFigure 3–14 The “translation” pro-cedure described for use with Figure3–13 would also work well with thisdiagram. For example, the part of thediagram that illustrates nitrogen fixa-tion by bacteria could be describedin the following way: “In nitrogen fix-ation, bacteria in the soil and onplant roots change atmosphericnitrogen into ammonia.”

The Nitrogen Cycle All organisms require nitrogen to make

amino acids, which in turn are used to build proteins. Many

different forms of nitrogen occur naturally in the biosphere.

Nitrogen gas (N2) makes up 78 percent of Earth’s atmosphere.

Nitrogen-containing substances such as ammonia (NH3), nitrate

ions (NO3-), and nitrite ions (NO2

-) are found in the wastes pro-

duced by many organisms and in dead and decaying organic

matter. Nitrogen also exists in several forms in the ocean and other

large water bodies. Human activity adds nitrogen to the biosphere

in the form of nitrate—a major component of plant fertilizers.

Figure 3–14 shows how the different forms of nitrogen cycle

through the biosphere. Although nitrogen gas is the most abun-

dant form of nitrogen on Earth, only certain types of bacteria

can use this form directly. Such bacteria, which live in the soil

and on the roots of plants called legumes, convert nitrogen gas

into ammonia in a process known as Other

bacteria in the soil convert ammonia into nitrates and nitrites.

Once these products are available, producers can use them to

make proteins. Consumers then eat the producers and reuse the

nitrogen to make their own proteins.

When organisms die, decomposers return nitrogen to the

soil as ammonia. The ammonia may be taken up again by

producers. Other soil bacteria convert nitrates into nitrogen gas

in a process called This process releases

nitrogen into the atmosphere once again.

denitrification.

nitrogen fixation.

Bacterial nitrogen fixation

Uptakeby producers

Reuseby consumers

Uptakeby producers

Reuseby consumers

Denitrification

Deposition,excretion

Atmospheric nitrogen fixation

N2 in Atmosphere

NO3–

and NO2–

NH3

Decomposition,excretion

Synthetic fertilizermanufacture

� Figure 3–14 The atmosphereis the main reservoir of nitrogen inthe biosphere. Nitrogen also cyclesthrough the soil and through thetissues of living organisms.Interpreting Graphics What arethe main nitrogen-containingnutrients in the biosphere?

The scarcity of nitrogenAlthough about 80 percent of the air surroundingEarth is nitrogen gas (N2), usable forms of the ele-ment are scarce in ecosystems. The reason for thisis that the two atoms in atmospheric nitrogen areheld together by triple covalent bonds that onlylightning, volcanic action, and certain bacteriacan break. In addition, the ammonia, nitrite, andnitrate formed by nitrifying bacteria are very sus-ceptible to leaching and runoff, which carry awaynitrogen dissolved in the water.

Nitrifying bacteria use nitrogenase, anenzyme, to break the covalent bonds in N2 mol-ecules. Nitrogenase functions only when it isisolated from oxygen. On land, nitrogen-fixingbacteria accomplish this by living inside oxygen-excluding nodules or layers of insulating slimeon plant roots. In aquatic ecosystems,cyanobacteria—the primary nitrogen-fixers—have specialized cells called heterocysts thatexclude oxygen.

FACTS AND FIGURES

3–3 (continued)

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Build Science SkillsComparing and ContrastingPoint out the terms inorganic andorganic in the description of thephosphorus cycle. Encourage stu-dents to consult science dictionariesand chemistry textbooks to deter-mine the difference between organicand inorganic compounds and reporttheir findings to the rest of the class.

The Biosphere 79

Answers to . . . In rock and soil minerals

and in ocean sediments

Figure 3–14 Ammonia, nitrate, andnitrite

Figure 3–15 Phosphorus forms partof life-sustaining molecules such asRNA and DNA.

What makes a compound organic?Organic compounds are often defined simply ascompounds that contain carbon. However, not allcarbon-containing compounds are consideredorganic. Carbon dioxide and hydrocarbons suchas methane and propane are prime examples ofcarbon compounds that are not organic. A moreprecise definition of an organic compound is: acompound that contains carbon and is construct-ed in living cells.

A carbon atom has four electrons in its outershell—but the shell is capable of holding eightelectrons. For this reason, a carbon atom canform covalent bonds with as many as four atomsof other elements. Carbon atoms are usuallyjoined together in a ring or chain that forms a sta-ble “backbone” for building molecules. Suchstructures are not present in simple inorganiccompounds such as carbon dioxide.

FACTS AND FIGURES

Help students understand the experi-ment procedure by asking: Why wasthe fifth field left bare? (It was thecontrol in the experiment. If the corn in the fifth field grew as well as or betterthan the corn in any of the other fields,the researchers would know that theprevious year’s plantings in those other fields did not increase corn pro-ductivity.)Answers1. Students could plot plant nameson the vertical axis and yields on thehorizontal axis; use different incre-ments for the yield axis; and/orarrange the crops in sequence fromhighest to lowest yield or from lowestto highest.2. Growing legumes the previousyear significantly increased the cropyields. Growing rye did not increaseyields. In fact, the bare field’s yieldwas slightly higher than the fieldplanted with rye.3. The legumes—particularly theAustrian peas—produced the highestyields. Rye produced the lowest yield.4. The corn plants benefited greatlywhen the soil was enriched withnitrogen fixed by legumes the previ-ous year.

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Farming in the RyeSometimes, farmers grow crops of rye and othergrasses and then plow them under the soil to decay.This practice helps to increase crop yields of otherplants. Farmers may also plow under legumes such aspeas, vetch, and lentils. Legumes are plants that havecolonies of nitrogen-fixing bacteria living in nodules onthe plant roots.

In an effort to determine which practice producesthe best crop yields, scientists performed an experi-ment in Georgia. They grew corn on land that hadpreviously received one of five treatments. Three fieldshad previously been planted with three differentlegumes. A fourth field had been planted with rye. The fifth field was left bare before the corn wasplanted. None of the fields received fertilizer while thecorn was growing. The table shows how much cornwas produced per hectare of land (kg/ha) in each field.One hectare is equivalent to 10,000 square meters.

1. Using Tables and Graphs Use the data in thetable to create a bar graph.

2. Comparing and Contrasting Compare theeffect of growing legumes to that of growing grasson the yield of corn. How do the yields differ fromthe yield on the field that had received no priortreatment?

3. Using Tables and Graphs Which treatmentproduced the best yield of corn? The worst yield?

4. Applying Concepts Based on your knowledge of the nitrogen cycle, how can you explain theseresults?

The Phosphorus Cycle Phosphorus is essential to living

organisms because it forms part of important life-sustaining

molecules such as DNA and RNA. Although phosphorus is of

great biological importance, it is not very common in the bio-

sphere. Unlike carbon, oxygen, and nitrogen, phosphorus does

not enter the atmosphere. Instead, phosphorus remains mostly

on land in rock and soil minerals, and in ocean sediments.

There, phosphorus exists in the form of inorganic phosphate. As

the rocks and sediments gradually wear down, phosphate is

released. On land, some of the phosphate washes into rivers and

streams, where it dissolves. The phosphate eventually makes its

way to the oceans, where it is used by marine organisms.

As Figure 3–15 shows, some phosphate stays on land and

cycles between organisms and the soil. When plants absorb

phosphate from the soil or from water, the plants bind the

phosphate into organic compounds. Organic phosphate moves

through the food web, from producers to consumers, and to the

rest of the ecosystem.

Where is most of the phosphorus stored in the biosphere?

� Figure 3–15 Phosphorus in the biosphere cyclesamong the land, ocean sediments, and living organ-isms. Interpreting Graphics How is phosphorusimportant to living organisms?

Previous Crop Average Yield of Corn (kg/ha)

Monantha vetch

Hairy vetch

Austrian peas

Rye

None

2876

2870

3159

1922

1959

Corn Production

0062_0080_bi_c07_te 3/7/06 2:12 PM Page 79

80 Chapter 3

Nutrient LimitationBuild Science SkillsMaking Judgments Call attentionto the paragraph about farmers’ useof fertilizers. Ask: How might the useof fertilizers harm ecosystemsnearby? (Runoff from the fields couldcarry fertilizers to bodies of water andcause algal blooms there.) Have stu-dents discuss the pros and cons offertilizer use. Then, let two teamsdebate the issue.

3 ASSESSEvaluate UnderstandingMake one photocopy of Figure 3–11(the water cycle), Figure 3–13 (thecarbon cycle), and Figure 3–14 (thenitrogen cycle). Cover the diagrams’labels with white tape, and then usethese “masters” to make a set ofcopies for students to add labels.

ReteachHave each student write a briefdescription of each cycle discussed inthe section referring to Figures 3–11,3–13, 3–14, and 3–15. Let studentsshare their work in a class discussionand correct any errors or omissions inone another’s descriptions.

Nutrient LimitationEcologists are often interested in the

of an ecosystem, which is the rate at

which organic matter is created by producers. One

factor that controls the primary productivity of an

ecosystem is the amount of available nutrients. If a

nutrient is in short supply, it will limit an organism’s

growth. When an ecosystem is limited by a single

nutrient that is scarce or cycles very slowly, this

substance is called a

Because they are well aware of this phenomenon,

farmers apply fertilizers to their crops to boost their

productivity. Fertilizers usually contain three impor-

tant nutrients—nitrogen, phosphorus, and potassium.

These nutrients help plants grow larger and more

quickly than they would in unfertilized soil.

The open oceans of the world can be considered

nutrient-poor environments compared to the land. Sea-

water contains at most only 0.00005 percent nitrogen,

or 1/10,000 of the amount typically found in soil. In the

ocean and other saltwater environments, nitrogen is

often the limiting nutrient. In some areas of the

ocean, however, silica or even iron can be the limiting

nutrient. In streams, lakes, and freshwater environments,

phosphorus is typically the limiting nutrient.

When an aquatic ecosystem receives a large input of a limit-

ing nutrient—for example, runoff from heavily fertilized fields—

the result is often an immediate increase in the amount of algae

and other producers. This result is called an Why

do algal blooms occur? There are more nutrients available, so the

producers can grow and reproduce more quickly. If there are not

enough consumers to eat the excess algae, conditions can become

so favorable for growth that algae cover the surface of the water.

Algal blooms, like the one shown in Figure 3–16, can sometimes

disrupt the equilibrium of an ecosystem.

algal bloom.

limiting nutrient.

productivityprimary

� Figure 3–16 When an aquaticecosystem receives a large input of alimiting nutrient, the result is often anincrease in the number of producers.Here, an extensive algal bloom coversthe shoreline of Tule Lake in California.Using Analogies How is this situa-tion similar to the one that occurs in afish tank in which the fish have beenoverfed?

1. Key Concept How doesthe way that matter flowsthrough an ecosystem differ fromthe way that energy flows?

2. Key Concept Why doliving organisms need nutrients?

3. Describe the path of nitrogenthrough its biogeochemical cycle.

4. Explain how a nutrient can be alimiting factor in an ecosystem.

5. Critical Thinking PredictingBased on your knowledge of thecarbon cycle, what do you thinkmight happen if vast areas offorests are cleared?

6. Critical Thinking ApplyingConcepts Summarize the roleof algal blooms in disrupting theequilibrium in an aquatic ecosystem.

3–3 Section Assessment

Making a Flowchart

Use a flowchart to trace theflow of energy in the carboncycle. Hint: You may wish torefer to Figure 3–13,

especially to the labelsPhotosynthesis, Feeding,Respiration, and Decom-position. Also, you may wantto refer to Figure 3–7 inSection 3–2 for a description ofenergy flow in an ecosystem.

Answer to . . . Figure 3–16 Bacteria in a lake con-sume dead algae and deplete oxygen inthe lake just as excess food in a fishtank is consumed by bacteria thatdeplete oxygen in the water.

3–3 (continued)

If your class subscribes to theiText, use it to review the KeyConcepts in Section 3–3.

3–3 Section Assessment1. Unlike the one-way flow of energy, matter is

recycled within and between ecosystems.2. To build tissues and carry out life functions.3. Students should summarize the steps in the

nitrogen cycle as shown in Figure 3–14. Agood response should describe the differentforms of nitrogen as well as explain bacterialnitrogen fixation and denitrification.

4. If a nutrient is in short supply, it will limit anorganism’s growth.

5. If vast areas of forest were cleared, less car-bon dioxide would be removed from theatmosphere by plants.

6. When an aquatic ecosystem receives a largeinput of a limiting nutrient, the result isoften an algal bloom. Algal blooms cansometimes disrupt the equilibrium of anecosystem by producing more algae thanconsumers can eat.

Students’ flowcharts may vary.Each flowchart, though, shouldmention and describe the process-es included in Figure 3–13. Agood flowchart might incorporatethe flow of energy, and carbon,through a food chain.

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