hes08 ch07 184-190 S01 8/18/06 11:40 AM Page 184 ......Aquatic Ecosystems CHAPTER 7 1 Freshwater Ecosystems 2 Marine Ecosystems 184 Chapter 7 Aquatic Ecosystems PRE-READING ACTIVITY

Aquatic EcosystemsCC H A P T E R 7

1 Freshwater Ecosystems

2 Marine Ecosystems

184 Chapter 7 Aquatic Ecosystems

PRE-READING ACTIVITY

Two-PanelFlip ChartBefore youread this chap-

ter, create the FoldNote enti-tled “Two-Panel Flip Chart”described in the Reading andStudy Skills section of theAppendix. Label the flaps ofthe two-panel flip chart with“Freshwater Ecosystems” and“Marine Ecosystems.” As youread the chapter, write infor-mation you learn about eachcategory under the appropri-ate flap.

Manatees live in both freshwater andsaltwater ecosystems. Manatees areherbivores and will eat at lleast 27 kg(60 lb) of aquatic plants per day.

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The types of organisms in an aquatic ecosystem are mainly deter-mined by the water’s salinity—the amount of dissolved salts thewater contains. As a result, aquatic ecosystems are divided intofreshwater ecosystems and marine ecosystems.

Freshwater ecosystems include the sluggish waters of lakesand ponds, such as the lake shown in Figure 1, and the movingwaters of rivers and streams. They also include areas where land,known as a is periodically underwater. Marine ecosys-tems include the diverse coastal areas of marshes, swamps, andcoral reefs as well as the deep, vast oceans.

Characteristics of Aquatic EcosystemsFactors such as temperature, sunlight, oxygen, and nutrientsdetermine which organisms live in which areas of the water. Forinstance, sunlight reaches only a certain distance below the sur-face of the water, so most photosynthetic organisms live on ornear the surface.

Organisms that live in aquatic ecosystems are grouped by theirlocation and by their adaptations. Three groups of aquatic organismsinclude plankton, nekton, and benthos. are organisms thatcannot swim against currents, so they are drifters. Drifting plants,called phytoplankton, are the food base for most aquatic ecosystems.The majority of phytoplankton are microscopic. Drifting animals,which may be microscopic or as large as a jellyfish, are called zoo-plankton. are free-swimming organisms, such as fish, turtles,and whales. are bottom-dwelling organisms, such as mus-sels, worms, and barnacles. Many benthic organisms live attached tohard surfaces. Organisms called decomposers, which break downdead organisms, are also present in aquatic ecosystems.

BenthosNekton

Plankton

wetland,

Objectives

� Describe the factors that deter-mine where an organism lives inan aquatic ecosystem.

� Describe the littoral zone and thebenthic zone that make up a lakeor pond.

� Describe two environmental func-tions of wetlands.

� Describe one threat against riverecosystems.

Key Termswetlandplanktonnektonbenthoslittoral zonebenthic zoneeutrophication

S E C T I O N 1

Freshwater Ecosystems

Figure 1 � Lake Louise in Alberta,Canada, is an example of a fresh-water ecosystem.

185

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Figure 3 � In a pond or lake ecosys-tem, the most diverse and abundantlife occurs near the shore, where sunlight and nutrients are plentiful.In the open water, sunlight at andnear the surface supports driftingphytoplankton.

Lakes and PondsLakes, ponds, wetlands, rivers, and streams make up the varioustypes of freshwater ecosystems. Lakes, ponds, and wetlands canform naturally where groundwater reaches the Earth’s surface.As well, beavers can create ponds by damming up streams.Humans intentionally create artificial lakes by damming flowingrivers and streams to use them for power, irrigation, water stor-age, and recreation.

Life in a Lake Lakes and ponds can be structured into horizontaland vertical zones. In the nutrient-rich near the shore,aquatic life is diverse and abundant. Plants, such as cattails andreeds, are rooted in the mud underwater, and their upper leaves andstems emerge above the water. Plants that have floating leaves, suchas pond lilies, are rooted here also. Farther from the shore, in theopen water, there are no rooted plants. Here, phytoplankton maketheir own food by photosynthesis. As shown in Figure 3, nutrientsand sunlight influence the location and types of organisms in apond or lake ecosystem.

Some bodies of fresh water have areas so deep that there istoo little light for photosynthesis. In these deep areas, bacteriaand other decomposers live on dead plants and animals that driftdown from above. Fish adapted to cooler water also live there.Eventually, dead and decaying organisms reach thethe bottom of a pond or lake, which is inhabited by decomposers,insect larvae, and clams.

Some animals that live in lakes and ponds have interestingadaptations that help them obtain what they need to survive.Water beetles use the hairs under their bodies to trap surface airso that they can breathe during their dives for food. Whiskershelp catfish sense food as they swim over dark lake bottoms. Inregions where lakes partially freeze in winter, amphibians burrowinto the littoral mud to avoid freezing temperatures.

benthic zone,

littoral zone


Figure 2 � Amphibians, such as thisbullfrog, live in or near lakes andponds.

www.scilinks.orgTopic: Lakes and PondsCode: HE80846

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How Nutrients Affect Lakes is an increase in theamount of nutrients in an aquatic ecosystem. A lake that has a largeamount of algae and plant growth due to nutrients, as shown inFigure 4, is known as a eutrophic lake. As the amount of plantsand algae grows, the number of bacteria feeding on the decayingorganisms also grows. These bacteria use the oxygen dissolved inthe lake water. Eventually, the reduced amount of oxygen kills oxygen-loving organisms. Lakes naturally become eutrophic over along period of time. However, eutrophication can be accelerated byrunoff. Runoff is precipitation, such as rain, that can carry sewage,fertilizers, or animal wastes from land into bodies of water.

Freshwater WetlandsFreshwater wetlands are areas of land that are covered with freshwater for at least part of the year. The two main types of fresh-water wetlands are marshes and swamps. Marshes contain non-woody plants, such as cattails, while swamps are dominated bywoody plants, such as trees and shrubs.

Wetlands perform several important environmental functions, asshown in Table 1. Wetlands act as filters or sponges because theyabsorb and remove pollutants from the water that flows throughthem. Therefore, wetlands improve the water quality of lakes,rivers, and reservoirs downstream. Wetlands also control floodingby absorbing extra water when rivers overflow, which protectsfarms and urban and residential areas from damage. Many of thefreshwater game fish caught in the United States each year use thewetlands for feeding and spawning. In addition, these areas providea home for native and migratory wildlife, including the blue heronsshown in Figure 5. Wetland vegetation also traps carbon that wouldotherwise be released as carbon dioxide, which may be linked torising atmospheric temperatures.

How can wetlands reduce damage that is caused by flooding? (See the Appendix for answers to Reading Checks.)■●✓ Reading Check

Eutrophication

Section 1 Freshwater Ecosystems 187

Figure 4 � A eutrophic lake, like theone above, contains large amountsof plants as a result of high levelsof nutrients.

Figure 5 � Wetlands provide habitatfor many plants and animals, includingthe great blue herons shown below.

Environmental Functions of Wetlands

• trapping and filtering sediments, nutrients, and pollutants, whichkeep these materials from entering lakes, reservoirs, and oceans

• reducing the likelihood of a flood, protecting agriculture, roads,buildings, and human health and safety

• buffering shorelines against erosion

• providing spawning grounds and habitat for commercially importantfish and shellfish

• providing habitat for rare, threatened, and endangered species

• providing recreational areas for activities such as fishing, bird-watching, hiking, canoeing, photography, and painting

Table 1 �

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Figure 6 � This map shows the locations of large freshwater wetlandsin the United States.

Figure 7 � A marsh is a type of wet-land that contains nonwoody plants.

Marshes As shown in Figure 6, most freshwater wetlands in theUnited States are located in the Southeast. The Florida Evergladesis the largest freshwater wetland in the United States. Freshwatermarshes tend to occur on low, flat lands and have little watermovement. In shallow waters, plants such as reeds, rushes, andcattails root themselves in the rich bottom sediments. As shownin Figure 7, the leaves of these and other plants stick out abovethe surface of the water year-round.

The benthic zones of marshes are nutrient rich and containplants, numerous types of decomposers, and scavengers. Water-fowl, such as grebes and ducks, have flat beaks adapted for sift-ing through the water for fish and insects. Water birds, such asherons, have spearlike beaks that they use to grasp small fish andto probe for frogs buried in the mud. Marshes also attract manymigratory birds from temperate and tropical habitats.

The salinity of marshes varies. Some marshes have slightly salty(brackish) water, while other marshes have water that is as salty asocean water. The organisms that live in and around a marsh aregenerally adapted to the specific range of salinities of its water.

HistoryConnection to

The Florida Everglades Becauseof the work of many writers, con-servationists, and naturalists, former U.S. President Trumandedicated the Everglades NationalPark in 1947. The park was estab-lished to protect the wildlife andhabitat of the Florida Everglades.The Florida Everglades is one ofonly three sites on Earth declaredan International BiosphereReserve, a World Heritage Site,and a Wetland of InternationalImportance. The other two sitesare located in Tunisia and Bulgaria.

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MATHPRACTICEWetland ConversionFrom 1982 to 1992, approximately 1.6 million acres of wetlands on nonfederal lands inthe United States were convertedfor other uses. Fifty-seven percentof the wetlands were convertedinto land for development. Twentypercent of the wetlands were con-verted into land for agriculture.How many acres of land were con-verted into land for development?How many acres of land were con-verted into land for agriculture?

Figure 8 � The American alligatoris a common reptile that lives inmarshes and swamps.

SSwamps Swamps occur on flat, poorly drained land, often nearstreams. The species of trees and shrubs in a swamp depend onthe salinity of the water and the climate of the area. For example,mangroves are trees that grow in saltwater swamps in tropicalclimates. Freshwater swamps are the ideal habitat for manyamphibians, such as frogs and salamanders, because of the con-tinuously moist environment. Swamps also attract birds, such aswood ducks, that nest in hollow trees near or over the water.Reptiles, such as the American alligator in Figure 8, are the pred-ators of swamps and will eat almost any organism that crossestheir path.

Human Impact on Wetlands Wetlands were previously con-sidered to be wastelands that provide breeding grounds for disease-carrying insects. Therefore, many have been drained,filled, and cleared for farms or residential and commercial devel-opment, as shown in Figure 9. For example, the FloridaEverglades once covered 8 million acres of south Florida, butnow covers less than 2 million acres. The important role of wet-lands as purifiers of wastewater and in flood prevention is nowrecognized. Wetlands are vital habitats for wildlife. Law and thefederal government protect many wetlands, and most states now prohibit the destruction of certain wetlands.

Section 1 Freshwater Ecosystems 189

Figure 9 � The wetland above hasbeen drained for agricultural pur-poses. Wetlands such as the oneabove typically serve as breedingareas for ducks. The oil rig on theleft is located in a marsh along thecoast of Louisiana.

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RiversMany rivers originate from snow melt in mountains. At its head-waters, a river is usually cold and full of oxygen and runs swiftlythrough a shallow riverbed. As a river flows down a mountain,it becomes warmer, wider, and slower, containing more vegetationand less oxygen. Figure 10 compares the water flow of two sec-tions of two different rivers. A river changes with the land andthe climate through which it flows. Runoff, for example, maywash nutrients and sediment from the surrounding land into ariver. These materials affect the growth and health of the organ-isms in the river.

Life in a River Near the churning headwaters, mosses anchorthemselves to rocks by using rootlike structures called rhizoids.

Trout and minnows are adapted to the cold, oxygen-rich headwaters. Trout are powerfulswimmers and have streamlined bodies that pres-ent little resistance to the strong current.Downstream, plants such as the crowfoot setroots in the river’s rich sediment. The leaves ofsome plants, such as the arrowhead, vary in shapeaccording to the strength of the river’s current.Fish such as catfish and carp also live in thecalmer waters.

Rivers in Danger Communities and industriesaffect the health of rivers. People draw water fromrivers to use in homes and manufacturing. Peoplemay also use rivers to dispose of their sewage andgarbage. These practices have polluted rivers withtoxins. The toxins have killed river organisms and

made river fish unsuitable for eating. Today, runoff from the landputs pesticides and other poisons into rivers and coats riverbedswith toxic sediments. As well, dams alter the ecosystems in andaround a river.

What effect can runoff have on the health oforganisms that live in and around a river?■●✓ Reading Check


1. List two factors that determine where an organismlives in an aquatic ecosystem.

2. Compare the littoral zone of a lake with the benthiczone of a lake.

3. List two environmental functions that wetlands provide.How do these functions affect you?

4. Describe one threat against river ecosystems.

CRITICAL THINKING5. Identifying Relationships A piece of garbage that

is thrown into a stream may end up in a river or anocean. What effects might one piece of garbage haveon an aquatic ecosystem? What effects might 100pieces of garbage have on an aquatic ecosystem?

6. Analyzing Processes Write a short paragraph thatexplains how fertilizing your yard and applying pesti-cides can affect the health of a river ecosystem.

WRITING SKILLS

S E C T I O N 1 Review

Figure 10 � A river changes dramat-ically as it flows from a mountaintopto flat land.

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Section 2 Marine Ecosystems 191

S E C T I O N 2

Marine Ecosystems

Objectives

� Explain why an estuary is a veryproductive ecosystem.

� Compare salt marshes and man-grove swamps.

� Describe two threats to coral reefs.� Describe two threats to ocean

organisms.

Key Termsestuarysalt marshmangrove swampbarrier islandcoral reef

Figure 11 � The mixing of freshwater and salt water at the mouth ofa river creates a nutrient-rich estuary.

Marine ecosystems are ecosystems that contain salt water. Suchecosystems are found in and around the world’s oceans. In theopen water, the amount of sunlight and available nutrients varyfrom one part of an ocean to another. In coastal areas, the waterlevel and salinity usually change during the day.

Coastal WetlandsCoastal land areas that are covered by salt water for all or part ofthe time are known as coastal wetlands. Coastal wetlands providehabitat and nesting areas for many fish and wildlife. Coastal wet-lands also absorb excess rain, which protects areas from flooding,they filter out pollutants and sediments, and they provide recre-ational areas for boating, fishing, and hunting.

Estuaries Many coastal wetlands form in estuaries. An isan area in which fresh water from a river mixes with salt waterfrom the ocean. As the two bodies of water meet, currents formand cause mineral-rich mud and dissolved nutrients to fall to thebottom. Figure 11 illustrates how the waters mix in such a waythat the estuary becomes a nutrient trap. These nutrients thenbecome available to producers, and in some shallow areas, marshgrass will grow in the mud. Estuaries are very productive ecosys-tems because they constantly receive fresh nutrients from theriver. The surrounding land, such as the mainland or a peninsula,protects estuaries from the harsh force of ocean waves.

estuary

www.scilinks.orgTopic: EstuariesCode: HE80536

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Plants and Animals of Estuaries For a week each spring, horse-shoe crabs, shown in Figure 12, crawl out of the ocean and ontothe beaches of Delaware Bay. In the shallow areas along theshore, the crabs mate and lay billions of eggs. Many migratingshorebirds depend on these eggs for food.

Estuaries support many marine organisms because estuariesreceive plenty of light for photosynthesis and plenty of nutrients for plants and animals. Rivers supply nutrients that have beenwashed from the land, and because the water is shallow, sunlightcan reach all the way to the bottom of the estuary. The light andnutrients support large populations of rooted plants as well asplankton. The plankton in turn provide food for larger animals,such as fish. Dolphins, manatees, otters, and other mammals oftenfeed on fish and plants in estuaries. Oysters, barnacles, and clamslive anchored to marsh grass or rocks and feed by filtering planktonout of the water. Organisms that live in estuaries are able to toleratevariations in salinity because the salt content of the water varies asfresh water and salt water mix when tides go in and out.


Restoration of theChesapeake Bay

The Chesapeake Bay is the largestestuary in the United States. Thebay produces large amounts ofseafood each year, supports manyspecies of wildlife, and providesrecreation for millions of people.

However, the ecosystems of thebay are threatened by several envi-ronmental problems. For example,pollution builds up because thesmall tide flushes pollutants out ofthe bay very slowly. Pollution buildsup because only a very narrowopening joins the bay and theocean. By 1980, the ChesapeakeBay was severely polluted with toxicindustrial chemicals. Pesticides aswell as excess nutrients ran into thebay from housing developments,farms, and wastewater (includingsewage). Marsh grasses and plank-ton were dying, and fish, oysters,and crabs were disappearing. Birdsof prey, such as bald eagles, had

almost vanished. Therefore, fishers,environmentalists, and residentsbecame alarmed and launchedcampaigns to save the bay.

Restoring Chesapeake Bayhabitats and water quality is noteasy. Maryland and Virginia, themain bordering states of the bay,have different environmental laws.Also, the bay’s watershed coversparts of four other states. Inter-ested groups would have to worktogether if they were to restore thebay. The Chesapeake Bay Programwas set up as a partnershipbetween the EnvironmentalProtection Agency, the District ofColumbia, Maryland, Pennsylvania,Virginia, and citizen advisorygroups. Goals included reducingchemical pollution, removing damsthat prevented fish from migrating,and reforesting river banks toreduce soil erosion.

� The Chesapeake Bay forms wherethe Potomac, Rappahannock, andother rivers meet the Atlantic Ocean.

Remarkable progress has beenmade in the last 20 years. Abouthalf of the wastewater flowing into

QuickLABEstuaries

Procedure1. Place a few drops of red food

coloring in a test tube filledwith water.

2. In a separate test tube, addsalt water and a few drops ofyellow food coloring.

3. Gently place some of the freshwater solution on top of thesalt water solution.

Analysis1. How do fresh water and salt

water interact in an estuary?

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Estuaries provide protected harbors, access to the ocean,and connection to a river. As a result, many of the world’smajor ports are built on estuaries. Of the 10 largest urbanareas in the world, 6 were built on estuaries. These 6 cities are Tokyo, New York, Shanghai, Buenos Aires, Rio de Janeiro,and Bombay.

Threats to Estuaries Estuaries in populated areas were oftenused as solid waste landfills. The landfills were then developedand used as building sites. This practice occurred widely inCalifornia, which now has plans to restore some of its estuarywetlands. The pollutants that damage estuaries are the samepollutants that damage other aquatic ecosystems: sewage,industrial waste, and agricultural runoff. Most of these pollu-tants eventually break down over time, but estuaries cannotcope with the excessive amounts produced by dense humanpopulations.


Figure 12 � Horseshoe crabs go tothe Delaware Bay, an estuarybetween New Jersey and Delaware,to lay their eggs.

the bay is now biologically treatedto remove pollutants and excessnutrients. Bald eagles are back, andindustry has reduced the chemicalpollutants released into the bay bynearly 70 percent. Planting treeshas restored forested buffers toabout 60 percent of the river banks,and populations of fish, such asstriped bass, are increasing.

However, the number of peoplein the bay area is increasing and thenumber of miles these people drive

drive, trying to conserve electricityand water, planting native vegeta-tion, using only a small amount offertilizer or water on your lawn orgarden, and properly disposing ofhazardous wastes such as motor oil,antifreeze, and cleaning fluids. Youcan help by picking up trash thatothers leave behind. You can alsojoin a citizens group to help pre-serve estuaries.

each year has increased even faster.In the last 30 years, miles traveledby vehicles increased four times asfast as the population. This has ledto runoff from streets and lawnsand pollution from vehicle exhaust,all of which harm the bay. The oys-ter harvest has decreased and theforested part of the bay’s watershedis still decreasing.

You can help save your localwatershed in the following ways: byreducing the number of miles you

� This great egret lives in one of the estuaries that borders theChesapeake Bay.

CRITICAL THINKING

1. Predicting Consequences Ifthe Chesapeake Bay Program hadnever been founded, what mighthave happened to the ChesapeakeBay? Explain how one organismmay have been affected.

2. Identifying RelationshipsHow may the use of less fertilizer onplants and lawns help the Chesa-peake Bay and other estuaries?

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Salt Marshes In estuaries, where rivers deposittheir load of mineral-rich mud, form. Here, thousands of acres of salt marsh sup-port a community of clams, fish, and aquaticbirds. The marsh also acts as a nursery in whichmany species of shrimps, crabs, and fishes findprotection when they are small. As they grow tomaturity and migrate to the sea, they are eaten bylarger fish or caught by commercial fisheries. Saltmarshes, like other wetlands, absorb pollutantsand protect inland areas.

Mangrove Swamps Mangroves, such as thoseshown in Figure 13, are several species of smalltrees adapted for growing in shallow salt water.Most mangroves have wide, above-ground rootsystems for support. Dense growths of mangrovetrees in swampy areas calledare found in tropical and subtropical zones.Mangrove swamps help to protect the coastlinefrom erosion and reduce the damage from storms.

They provide habitat for about 2,000 animal species. Like saltmarshes, mangrove swamps have been filled with waste or used fordevelopment projects in many parts of the world.

Rocky and Sandy Shores Rocky shores have many more plantand animal species than sandy shores do. The rocks anchor sea-weed and the many animals that live on it, such as sea anemones,mussels, and sponges. Life on sandy shores, although less diverse,is abundant in the water and in the sand and sediments. In thewater and on land, animals are adapted to the effects of dryingand exposure at low tide. At low tide, birds poke and prod aboutfor animals that have not attached themselves firmly enough orburied themselves deeply enough to escape the tidal pull.

, such as the one in Figure 14, often run parallel tosandy shores. These islands help to protect the mainland and thecoastal wetlands from storms and ocean waves.

Name two things that a salt marsh has in commonwith a mangrove swamp. ■●✓ Reading Check

Barrier islands

mangrove swamps

salt marshes

194 Chapter 7

Figure 13 � Mangrove swamps arefound along warm, tropical coastsand are dominated by salt-tolerantmangrove trees.

Figure 14 � This barrier island islocated off the coast of Long Island,New York. Barrier islands are sepa-rated from the mainland and helpprotect the shore of the mainlandfrom erosion.

EcofactMangrove Swamps Mangrovescover 180 billion square meters oftropical coastlines around theworld. The largest single mangroveswamp is 5.7 billion square meters,located in the Sundarbans ofBangladesh. This single mangroveswamp provides habitat for theBengal tiger and helps supplyapproximately 300,000 peoplewith food, fuel, building materials,and medicines.

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Coral Reefsare limestone ridges built by tiny coral animals called

coral polyps and the algae that live inside them. Coral polypssecrete skeletons of limestone (calcium carbonate), whichslowly accumulate and form coral reefs. Thousandsof species of plants and animals live in the cracksof coral reefs, which makes coral reefs amongthe most diverse ecosystems on Earth.

Because reef-building corals live only inwarm salt water where there is enough lightfor photosynthesis, coral reefs are found inshallow, clear tropical seas. Figure 15 showsthe locations of coral reefs. Only the outerlayer of a reef contains living corals, whichbuild their rock homes with the help of the pho-tosynthetic algae that live within them. Corals,such as those shown in Figure 16, are predators thatnever chase their prey. Their stinging polyps capturesmall animals that float or swim close to the reef. Because of theirconvoluted shape, coral reefs provide a habitat for a magnificentvariety of fish, snails, clams, sponges and many other types oforganisms.

Coral Reefs in Danger Coral reefs are fragile ecosystems. If thesurrounding water is too hot or cold for too long, or if it is toomuddy, polluted, or high in nutrients, the algae that live in thecorals will leave or die. As a result, the corals turn white, a condition called coral bleaching. If coral bleaching occurs oftenor long enough, coral animals and the reefs they build will die.

Since the twentieth century, bleaching events have been occur-ring more frequently, mainly due to human activities. About 50percent of the world’s coral reefs are now in danger of destruc-tion. In addition, global warming, oil spills, and polluting runoffhave been linked to the destruction of coral reefs. As well, over-fishing upsets the balance of a reef ecosystem by devastating fishpopulations. Because coral reefs grow slowly, a reef may not beable to repair itself when parts of it are stressed or destroyed.

Coral reefs


Figure 16 � Coral reefs (bottom) arebuilt by tiny coral animals calledcoral polyps. The stinging polyps offire coral (top) capture animals bypoisoning them.

Figure 15 � Coral reefs are found inwarm, shallow waters, where there isenough light for photosynthesis.Coral reefs support a great diversityof species.

Cause-and-Effect Map

Create the Graphic Organizerentitled “Cause-and-Effect Map”described in the Appendix. Labelthe effect with “DisappearingCoral Reefs.” Then, fill in the mapwith causes of disap-pearing coral reefsand details about thecauses and effects.

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OceansBecause water absorbs light, sunlight that plants can use for photosynthesis penetrates only about 100 m (330 ft) into theocean. As a result, much of the ocean’s life is concentrated in theshallow, coastal waters. Here, sunlight penetrates to the bottomand rivers wash nutrients from the land. Seaweed and algae growanchored to rocks, and phytoplankton drift on the surface.Invertebrates and fish that feed on these plants are also concen-trated near the shore.

Plants and Animals of Oceans In the open ocean, phytoplanktongrow only in areas where there is enough light and nutrients. Asa result, the open ocean is one of the least productive of allecosystems. Phytoplankton have buoyancy devices, such as oilbubbles, that prevent them from sinking into deep water, which istoo dark for photosynthesis. The sea’s smallest herbivores are thezooplankton, which live near the surface with the phytoplanktonthey eat. The zooplankton include jellyfish, tiny shrimp, and thelarvae of fish and bottom-dwelling animals, such as oysters andlobsters. Fish feed on the plankton as do marine mammals suchas whales.

The depths of the ocean are perpetually dark, so most food at the ocean floor consists of dead organisms that fall from thesurface. Decomposers, filter feeders, and the organisms that eatthem live in the deep areas of the ocean. Figure 17 illustrates thetypes of organisms that may be found in the layers of the oceanat various depths, depending on available sunlight.

Figure 17 � The amount of sunlightavailable determines which organismscan live in each layer of the ocean.

FIELD ACTIVITYFIELD ACTIVITY Make a Miniature AquaticEcosystem Make your ownaquarium by collecting organismsfrom an aquatic ecosystem nearyour home or school. Be sure tocollect some water from theaquatic ecosystem. Bring yourcollection back to school and setup an aquarium. If necessary,research the Internet to find outthe special care that your ecosys-tem may require. Examine a fewdrops of your collected waterunder the microscope. Be sure tolook for algae or other forms oflife. Record and draw your obser-vations in your EcoLog. Observeand record the changes you seein your aquarium over the next 3weeks. What conditions areneeded to keep your miniatureecosystem healthy?


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Threats to the Oceans Although oceans arehuge, they are steadily becoming more polluted.Most ocean pollution arises from activities onland. For example, runoff from fertilized fieldsmay cause algal blooms, some of which are poi-sonous. Waste from cities and industries, fertiliz-ers, and sewage running off the land are themain sources of coastal pollution in the UnitedStates.

Overfishing and certain fishing methods arealso destroying some fish populations. Immensetrawl nets can entangle organisms that arelarger than the holes in the nets. Marine mam-mals such as dolphins, which must breathe air,can drown in the nets. Some ships illegally dis-card fishing lines into the ocean, where they canstrangle and kill animals such as the sea lion in Figure 18.

Arctic and Antarctic Ecosystems The arctic ecosystems at theNorth and South Poles are marine ecosystems because nearly allthe food comes from the ocean and seas.

The Arctic Ocean is rich in nutrients from the surroundingland masses. It supports large populations of plankton, whichfeed a rich diversity of fish in the open water and under the ice.The fish are food for ocean birds, whales, and seals. Belugawhales, shown in Figure 19, feed on nearly 100 different arcticorganisms. Fish and seals also provide food for polar bears andpeople on land.

The Antarctic is the only continent never colonized byhumans. Even during the summer, only a few plants grow atthe rocky edges of the continent. As in the Arctic, planktonform the basis of the Antarctic food web. They nourish largenumbers of fish, whales, and birds such as penguins, which can-not fly because their wings have evolved for swimming.

What are two threats to organisms that live in the ocean? ■●✓ Reading Check


1. Explain why estuaries are very productive ecosys-tems. Why are estuaries vulnerable to the effects ofpollution?

2. Compare salt marshes with mangrove swamps.

3. Describe two factors that can damage coral reefs.

4. List two ways in which animals of the oceans arethreatened.

CRITICAL THINKING5. Predicting Consequences Suppose that the sea

level suddenly rose by 100 m. What would happen tothe world’s coral reefs? Explain.

6. Analyzing Processes Read the description of estu-aries in this section, and explain why cities are oftenbuilt on estuaries. How would building a city on anestuary affect the plants and animals living in the estuary? READING SKILLS

S E C T I O N 2 Review

Figure 19 � Beluga whales inhabitthe Arctic Ocean.

Figure 18 � This sea lion was strangled by a fishing net off thecoast of California.

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HighlightsC H A P T E R 7

1 Freshwater Ecosystems

2 Marine Ecosystems

198 Chapter 7 Highlights

Key Termswetland, 185plankton, 185nekton, 185benthos, 185littoral zone, 186benthic zone, 186eutrophication, 187

Main Ideas� Aquatic ecosystems can be classified as fresh-water ecosystems or marine ecosystems. Theplants and animals in aquatic ecosystems areadapted to specific environmental conditions.

� Freshwater ecosystems include lakes, ponds,freshwater wetlands, rivers, and streams. Thetypes of freshwater ecosystems are classified bythe depth of the water, the speed of the waterflow, and the availability of minerals, sunlight,and oxygen.

� Freshwater wetlands serve many functionswithin ecosystems. They trap and filter sedi-ments and pollutants; reduce the likelihood ofa flood; and buffer shorelines against erosion.

estuary, 191salt marsh, 194mangrove swamp,

194barrier island, 194coral reef, 195

� Marine ecosystems are identified by the pres-ence of salt water and include coastal wetlands,coral reefs, oceans, and polar ecosystems.

� Estuaries are among the most productive ofecosystems because they constantly receivefresh nutrients from a river and from an ocean.Estuaries provide habitat for a multitude ofplants and animals.

� Coral reefs are susceptible to destructionbecause they must remain at tropical tem-peratures and they must receive a large amountof sunlight. Coral reefs provide habitat forapproximately one-fourth of all marine species.

� Almost every person has an impact onaquatic ecosystems. Through understandinghow we affect aquatic ecosystems, we canreduce the negative effects we haveon them.

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Review

Chapter 7 Review 199

Using Key TermsUse each of the following terms in a separate sentence.

1. wetland2. mangrove swamp3. estuary4. eutrophication5. benthos

For each pair of terms, explain how the meaningsof the terms differ.

6. littoral zone and benthic zone7. plankton and nekton8. salt marsh and barrier island9. wetland and coral reef

Understanding Key Ideas10. Wetlands are important to fisheries in the

United States becausea. wetlands are the easiest place to catch fish.b. wetlands are the breeding grounds for

insects that are eaten by fish.c. wetlands provide the most desirable species

of fish.d. many of the fish caught each year use wet-

lands for feeding and spawning.11. Animals that live in estuaries

a. tend to produce few offspring.b. are usually found in unpolluted

environments.c. must be adapted to varying levels of salinity.d. are adapted to cold-water conditions.

12. Bacteria can kill organisms in eutrophiclakes by a. feeding on decaying plants and animals.b. reducing oxygen dissolved in the water.c. Both (a) and (b)d. Neither (a) nor (b)

13. Arctic ecosystems are considered marineecosystems becausea. arctic ecosystems contain an enormous

amount of frozen sea water.b. arctic ecosystems are inhabited by few

organisms.c. sunlight is limited.d. phytoplankton form the basis of arctic

food webs.14. Which of the following statements does not

describe a function of wetlands?aa. Wetlands buffer shorelines against erosion.b. Wetlands provide spawning grounds for

commercially important fish and shellfish.c. Wetlands filter pollutants.d. Wetlands make good hazardous waste

dumpsites.15. Tiny animals, called coral polyps, that

secrete limestone createa. barrier islands.b. coral reefs.c. swamps.d. salt marshes.

16. Mangrove trees growa. along riverbanks.b. in freshwater wetlands.c. in tropical areas and in subtropical areas. d. in the benthic zones of lakes.

17. The Florida Evergladesa. is the largest freshwater marsh in the

United States.b. protects threatened and endangered

wildlife.c. provides habitat for migratory birds.d. All of the above

18. Which of the following actions is an exampleof how humans affect wetlands?a. draining a wetland to create farmlandb. clearing a wetland to build a housing

developmentcc. using a wetland as a landfilld. all of the above

C H A P T E R 7

Graph Skills Taking the following steps whenreading a graph will help you correctly interpretthe information. Be sure to read the title so thatyou understand what the graph represents. If thegraph has axes, read the titles of both the x- andthe y-axis. Examine the range of values on boththe x- and the y-axis. Finally, examine the data onthe graph, reading them from left to right, and putinto words what you think the graph represents.

STUDY TIP

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ReviewShort Answer19. How does the phrase “the best of both

worlds” relate to an estuary?20. Explain the difference between the types of

organisms that make up these classes: plank-ton, nekton, and benthos.

21. List three functions of wetlands.22. Describe what happens when a lake is con-

sidered to be eutrophic.23. What type of vegetation dominates man-

grove swamps?

Interpreting GraphicsThe pie graph below shows the percentage ofcoral reefs at risk in the world. Use the pie graphto answer questions 24–26.24. What percentage of coral reefs are still living?25. If there is a total of 255,300 km2 of coral

reefs in the world, how many square kilo-meters of coral reefs are at a high risk ofbeing destroyed?

26. How many square kilometers of coral reefsare at a medium risk of being destroyed?Assume there is a total of 255,300 km2 coralreefs in the world.

Concept Mapping27. Use the following terms to create a concept

map: lakes, estuaries, aquatic ecosystems,coral reefs, freshwater wetlands, freshwaterecosystems, rivers, oceans, marshes, marineecosystems, swamps, coastal ecosystems, andmangrove swamps.

Critical Thinking28. Analyzing Relationships Write a short para-

graph that explains the relationship betweenthe speed of a river and the oxygen contentof a river.

29. Determining Cause and Effect Explainwhat may happen if the use of fertilizer onfarms and lawns around an estuary is notcontrolled.

30. Making Comparisons Read the paragraphunder the heading “Threats to Estuaries” inthis chapter. How do these threats comparewith those described under the heading“Threats to the Oceans”?

31. Analyzing Relationships Explain why plant-ing trees along a riverbank might benefit ariver ecosystem.

Cross-Disciplinary Connection32. Demography Six out of 10 of the largest

urban areas were built on estuaries. Threeof these cities are Tokyo, New York, andRio de Janeiro. Research the population ofeach of these cities, and predict what mayhappen if population numbers continue toincrease.

Portfolio Project33. Research a Local Aquatic Ecosystem Observe

an aquatic ecosystem near your school orhome. This ecosystem can be as simple as apond or stream or as complex as a lake orestuary. Observe the color of the water andthe types of plants and animals. Record anyinteractions among the organisms that youobserve. When you have recorded all of yourdata and observations, write a one-pagereport on the aquatic ecosystem.

READING SKILLS

WRITING SKILLS

C H A P T E R 7

200 Chapter 7 Review

Status of the World’s Coral Reefs

Source: Global Coral Reef Monitoring Network

high risk24%

mediumrisk26%

destroyed20%

low risk30%

?

? ?

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Chapter 7 Review 201

MATH SKILLS

Use the graph below to answer questions 34–35.

34. Analyzing Data The graph below illustratesthe percentage of freshwater fish species thatare threatened in specific countries. Whatpercentage of freshwater fish species arethreatened in country B? in country D?

35. Evaluating Data If the number of freshwaterfish species in country C totals 599 differentspecies, how many of these species arethreatened?

WRITING SKILLS

36. Communicating Main Ideas What effectdoes overfishing have on estuaries? Whateffect does overfishing have on oceans?

37. Writing from Research Research endan-gered marine mammals of ocean and polarecosystems. Write a one-page report on thefactors that have caused these mammals tobecome endangered.

A B C

CountryD E

Perc

enta

ge

thre

aten

ed

Threatened Freshwater Fish Species

0

5

10

15

20

25

30

35

40

RRead the passage below, and then answerthe questions that follow.

In the United States during the last 200 years,over 99 percent of native prairies have beenreplaced with farmland or urban developmentand most of the old-growth forests have beencut. Loss of so many of these habitats hasresulted in losses of biodiversity.

A new discipline, called conservationbiology, seeks to identify and maintain natu-ral areas. In areas where human influence isgreater, such as agricultural areas, formerstrip mines, and drained wetlands, biologistsmay have to reverse major changes andreplace missing ecosystem components. Forexample, returning a strip-mined area tograssland may require contouring the landsurface, introducing bacteria to the soil,planting grass and shrub seedlings, and evenusing periodic fires to manage the growth ofvegetation. Restoring an area to its naturalstate is called restoration ecology.

1. Which of the following phrases describesa likely task of a restoration ecologist?a. raising funds needed to create a

national parkb. returning missing ecosystem compo-

nents to a drained wetland c. educating citizens about the need to

protect a local habitatd. both (a) and (b)

2. Based on the passage, which of the fol-lowing statements is true?a. Former strip mines tend to have a high

level of biodiversity.b. A conservation biologist would most

likely oppose the development of areasaround the Grand Canyon.

c. Periodic fires in some ecosystems donot help manage excess growth ofvegetation.

d. Most prairie ecosystems located in theUnited States have been preserved.

READING SKILLS

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C H A P T E R 7 Standardized Test Prep

202 Chapter 7 Standardized Test Prep

Understanding ConceptsDirections (1–5): For each question, write on aseparate sheet of paper the letter of the correctanswer.

1 Organisms living in coastal areas mustadapt to what changes?A. water level and degree of salinityB. water level and amount of sunlightC. temperature and availability of oxygenD. temperature and availability of nutrients

2 Bleaching events lead to the destruction ofwhat kind of marine ecosystem?F. coastal wetlandsG. coral reefsH. mangrove swampsI. salt marshes

3 Which of the following correctly lists typesof organisms in aquatic ecosystems fromshallowest to deepest?A. plankton, nekton, benthosB. plankton, benthos, nektonC. benthos, plankton, nektonD. benthos, nekton, plankton

4 What is the difference between swamps andmarshes?F. Marshes attract birds, swamps attract

amphibians.G. Marshes are freshwater, swamps are

saltwater.H. Marshes contain non-woody plants,

swamps contain woody plants.I. Marshes are mostly in the southeast U.S.,

swamps in the northeast U.S.

5 Which of the following would be considered among the most productive ecosystems?A. barrier islandB. estuaryC. riverD. salt marsh

Directions (6–7): For each question, write a shortresponse.

6 Most ocean pollution arises from activityon land. How does land pollution end up inthe oceans?

7 Aquatic ecosystems are divided into twotypes—marine and freshwater. Compareand contrast marine and freshwater ecosystems.

Reading SkillsDirections (8–10): Read the passage below. Thenanswer the questions.

Wetlands perform several important environmental functions. Some wetlands areused to produce commercially important prod-ucts, such as cranberries. But wetlands wereonce considered to be wastelands that providebreeding grounds for insects. Therefore, manyhave been drained, filled, and cleared for farmsor residential and commercial development.For example, the Florida Everglades once cov-ered 8 million acres of south Florida, but nowit covers less than 2 million acres. From 1982to 1992 alone, 57 percent of wetlands wereconverted into land for development. Wetlandsare vitally important as habitats for wildlife,and their important role in the environment isnow recognized.

8 Which of the following is not an environ-mental function of wetlands? F. Wetlands are important as habitats for

wildlife.G. Wetlands may be converted into land for

development.H. Wetlands may be used to grow commer-

cial products, such as cranberries.I. Wetlands trap and filter sediments and

pollutants.

9 By approximately how much has the areaof the Florida Everglades in south Floridadecreased?A. 40 percent B. 75 percent C. 5 percentD. 60 percent

0 What could be inferred as the cause of scientific awareness of the importance of wetlands?

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TestImagine the increas-ing levels of pollutionthat would affectwater quality andcause particularspecies to disappear.

Moderately sensitive speciesClams, cranefly larvae, crayfish,damselfly nymphs, dragonflynymphs, scuds, predacious divingbeetle larvae, sowbugs, fishfly larvae,and alderfly larvaeTolerant species Aquatic worms, blackfly larvae,leeches, midge larvae, and pouchsnails

dragonfly nymph predacious diving beetle larva

leechesmidge larva

Most sensitive species Caddisfly larvae, hellgrammites, stonefly larvae, mayfly nymphs, gilled snails, and water penny larvae caddisfly larvaemayfly nymph

Chapter 7 Standardized Test Prep 203

Interpreting GraphicsDirections (11–13): For each question below, record the correct answer ona separate sheet of paper.

The table below shows organisms whose presence or absence can be indi-cators of water quality. Use this table to answer questions 11 through 13.

Living Water Quality Indicators

q If a water sample contains mayfly nymphs, what can you concludeabout the water quality of the area where the sample was taken?F. The area has generally poor water quality.G. The area has generally good water quality.H. The water quality cannot be determined from such a sample.I. The water in the area has been chemically treated for pollutants.

w What group of organisms could be seen only in a sample taken fromunpolluted water?A. leeches and caddisfly larvaeB. leeches and dragonfly nymphsC. leeches and midge larvaeD. leeches and predacious diving beetle larvae

e Water sample A contains only leeches. Water sample B containsleeches and predacious diving beetle larvae. What comparison can bemade between samples A and B?F. Both samples A and B have the same water quality.G. Both samples came from the same water source.H. Sample A has relatively poorer water quality than sample B.I. Sample B has relatively poorer water quality than sample A.

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204 Chapter 7 Skills Practice Lab

Eutrophication: Too Much of a Good Thing?Plants depend on nutrients such as phosphates and nitrates tosurvive. However, when people release large amounts of thesenutrients into rivers and lakes, artificial eutrophication canoccur. In artificial eutrophication, nutrients cause algae andplant life to grow rapidly and then die off and decay. Whenmicroorganisms decompose the algae and plant matter, they useup oxygen in the water, which causes the death of fish and otheranimals that depend on oxygen for survival. Eutrophication iscommonly caused by phosphates, which are often found indetergents, and by nitrates, which are found in animal wastesand fertilizers. In this lab, you will observe artificial eutrophica-tion in an aquatic ecosystem.

Procedure1. Working with your team, use a wax pencil to label one jar

“Control,” a second jar “Fertilizer,” and a third jar“Excess fertilizer.”

2. Put 750 mL of distilled water in each of the three jars. Readthe label on the fertilizer container to determine the recom-mended dilution of fertilizer for watering plants. To the“Fertilizer” jar, add the amount of fertilizer recommendedfor a quart of water. To the “Excess fertilizer” jar, add 10times this amount of fertilizer. Stir the contents of each jarthoroughly to dissolve the fertilizer.

Objectives� Observe

the effects of nitrates and phos-phates on an aquatic ecosystem.

� Compare the growth of organismsin different levels of nutrients.

� Predict possible effects nitrates andphosphates would have on anaquatic ecosystem in your area.

Materialsdistilled watereyedropperfertilizer, household usefluorescent lampgraduated cylinderguide to pond life identificationjars, 1 qt (3)microscopemicroscope slides with coverslipsplastic wrappond water that contains viable

organismsstirring rodwax pencil

USING SCIENTIFIC METHODS

Skills Practice Lab: OBSERVATIONC H A P T E R 7

� Step 5 Observe a drop of pondwater under the microscope.

3. Obtain a sample of pond water. Stir itgently but thoroughly to ensure thatthe organisms in it are evenly distrib-uted. Measure 100 mL of pond waterinto each of the three jars.

4. Cover each jar loosely with plasticwrap. Place all three jars about 20cm from a fluorescent lamp. (Do notplace the jars in direct sunlight, asthis may cause them to heat up too much.)

5. Observe a drop of pond water fromyour sample, under the microscope.On a sheet of paper, draw at least fourdifferent organisms that you see.Determine whether the organisms arealgae (usually green) or consumers(usually able to move). Describe thetotal number and type of organismsthat you see.

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Chapter 7 Skills Practice Lab 205

6. Based on what you have learned about eutrophication, makea prediction about how the pond organisms will growin each of the three jars.

7. Observe the jars when you first set them up and atleast once every three days for the next 3 weeks.Make a data table to record the date, color,odor, and any other observations you makefor each jar.

8. When life-forms begin to be visible in thejars (probably after a week), use an eyedrop-per to remove a sample of organismsfrom each jar and observe the sampleunder the microscope. Record yourobservations.

9. At the end of your 3-week obser-vation period, again remove a sample from each jar andobserve it under the microscope.Draw at least four of the mostabundant organisms that yousee, and describe how the number and type of organismshave changed.

Analysis1. Describing Events After three weeks, which jar shows the

most abundant growth of algae? What may have caused thisgrowth?

2. Analyzing Data Did you observe any effects on organismsother than algae in the jar that had the most abundant algaegrowth? Explain.

Conclusions3. Applying Conclusions Did your observations match your

predictions? Explain.4. Drawing Conclusions How can artificial eutrophication be

prevented in natural water bodies?

1. Designing Experiments Modify the experiment by usinghousehold dishwashing detergent instead of household fertilizer. Are the results different?

2. Research and Communications Research the watershedsthat are located close to your area. How might activitiessuch as farming and building affect watersheds?

Extension

� Step 7 Record your observationsof the jars every 3 days for 3 weeks.

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206 Chapter 7 Maps in Action

WETLANDS IN THE UNITED STATES, 1780s Vs. 1980s

M A P S K I L L S

Use the maps of wetland loss in the United Statesto answer the questions below.

1. Using a Key Use the key to determine how manystates had a decrease in wetland distribution from 5 to 12 percent to 1 to 5 percent.

2. Analyzing Data Is there any state on the map ofwetland distribution in the 1980s that has the samepercentage of wetland distribution as it did in the1780s? If so, how many?

3. Analyzing Data Which states have had the great-est decrease in wetland distribution since the 1780s?

4. Making Inferences What might have causedFlorida’s and Louisiana’s wetlands to decrease in distribution?

5. Using a Key Use the key to determine how manystates had a decrease in wetland distribution from 25 to 50 percent to 12 to 25 percent.

6. Identifying Trends If these trends of wetland losscontinue, what might a map of wetland distributionof the United States look like circa 2040?

EARTH SCIENCE CONNECTION

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Chapter 7 Society and the Environment 207

HURRICANE KATRINA AND NEW ORLEANS

What Do You Think?Many engineers and environ-mental scientists had predict-ed that the risk of flooding inNew Orleans had been in-creased by poorly designedlevees and canals, and by mas-sive erosion. Should a largecity have been built in such an unstable environment?

The city of New Orleans was builtin the Mississippi River Delta,about 100 miles upriver from theGulf of Mexico. This city is vulnerable to flooding from theMississipppi River, Lake Pontchar-train to the north, and heavy rain-fall from tropical storms. Inaddition, hurricanes that pass overthe coast can create storm surges,waves up to 30 feet high.

To help protect New Orleansfrom flooding, engineers and cityplanners built levees and floodwalls along the river banks andlakeshore. They constructed pumpsto move floodwater from lower-lying areas through canals intoLake Pontchartrain. Despite thesemeasures, Hurricane Katrina over-whelmed the city in late August,2005. Hundreds of people werekilled, and hundreds of thousandswere left homeless. Analyses of thecatastrophe concluded that human-made changes in the natural envi-ronment were partly responsible for the damage.

Protection from the SeaThe low-lying land of the Missis-sippi River Delta is a transitionalarea between the land and theocean. It consists of saltwater andfreshwater marshes, mud flats, andcreeks, collectively known ascoastal wetlands. Winds and cur-rents move loose sediment to buildup barrier islands. These islandsshelter the coastal wetlands andmainland from the ocean. Plantsgrowing in the wetlands trap sedi-ment and help to stabilize the land.The Louisiana coast has about 40percent of all the coastal marshesin the continental United States.These coastal wetlands are animportant habitat for crustaceans,mollusks, fish, and birds. As well,they filter out pollutants from theriver, absorb floodwater, and helpto supply fresh water to aquifers.

Eroding the Barrier IslandsAs levees confined the flow of theMississippi River, sediment carriedby the river was flushed farther out

into the Gulf of Mexico. The sedi-ment was no longer deposited tobuild up more land.

Canals that were built throughthe barrier islands to handle rivertraffic increased the erosion of thecoastal wetlands. Soil dug from thecanals was piled on the banks,smothering vegetation that hadhelped to hold the banks in place.The increased speed and volume ofthe water in the canals washed awaymore soil from the barrier islands.As the barrier islands eroded, themarshes and land behind them wereleft exposed and were washed away.

The Impact of KatrinaMost of the damage from Hurri-cane Katrina was caused by risingwater that overflowed or brokethrough the levees.

A wide shipping canal funneleda storm surge from the ocean intothe city. The storm surge brokethrough the banks of the canal.Other canals, built to drain waterinto Lake Pontchartrain, had theirflow reversed as the water level in the lake rose. Eventually, thecity’s drainage system failed whenmost of the pumping stations weresubmerged.

� Over 80 percent of New Orleans was submerged by floodwater whenHurricane Katrina struck in August, 2005.

EARTH SCIENCE CONNECTION

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Documents

hes08 ch07 184-190 S01 8/18/06 11:40 AM Page 184 ......Aquatic Ecosystems CHAPTER 7 1 Freshwater Ecosystems 2 Marine Ecosystems 184 Chapter 7 Aquatic Ecosystems PRE-READING ACTIVITY