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.

Aquatic ecosystems contain several types of organisms thatare grouped by their location and by their adaptations. Threegroups of aquatic organisms include plankton, nekton, and ben-thos. are the organisms that float near the surface of thewater. Two types of plankton are microscopic plants called phy-toplankton, and microscopic animals called zooplankton. Phyto-plankton produce most of the food for an aquatic ecosystem.

are free-swimming organisms, such as fish, turtles, andwhales. are bottom-dwelling organisms, such as mussels,worms, and barnacles. Many benthic organisms live attached tohard surfaces. Decomposers, organisms that break down deadorganisms, are also a type of aquatic organism.

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.

173Copyright© by Holt, Rinehart and Winston. All rights reserved.

Figure 3 � A pond or lake ecosys-tem is structured according to howmuch light is available. Tiny plantscalled phytoplankton and tiny animalscalled zooplankton live in open water,where more sunlight is available.

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.Beavers can also create ponds by damming up streams. Humansintentionally create artificial lakes by damming flowing riversand streams to use them for power, irrigation, water storage,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 out from the shore, inthe open water, plants, algae, and some bacteria capture solarenergy to make their own food during photosynthesis. As shown inFigure 3, the types of organisms present in a pond or lake ecosys-tem depend on the amount of sunlight available.

Some bodies of fresh water have areas so deep that there istoo little light for photosynthesis. Bacteria live in the deep areasof the fresh water to decompose dead plants and animals thatdrift down from the land and water above. Fish adapted tocooler, darker water also live there. Eventually, dead and decay-ing organisms reach the the bottom of a pond orlake, which is inhabited by decomposers, insect larvae, and clams.

Animals that live in lakes and ponds have adaptations thathelp them obtain what they need to survive. Water beetles use thehairs under their bodies to trap surface air so that they canbreathe during their dives for food. Whiskers help catfish sensefood as they swim over dark lake bottoms. In regions where lakespartially freeze in winter, amphibians burrow into the littoralmud to avoid freezing temperatures.

benthic zone,

littoral zone

174 Chapter 7 Aquatic Ecosystems

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

www.scilinks.orgTopic: Lakes and PondsSciLinks code: HE4058

Copyright© by Holt, Rinehart and Winston. All rights reserved.

How Nutrients Affect Lakes is an increase in theamount of nutrients in an aquatic ecosystem. A lake that has a largeamount of plant growth due to nutrients, as shown in Figure 4, isknown as a eutrophic lake. As the amount of plants and algaegrows, the number of bacteria feeding on the decaying organismsalso grows. These bacteria use the oxygen dissolved in the lake’swaters. Eventually, the reduced amount of oxygen kills oxygen-loving organisms. Lakes naturally become eutrophic over a longperiod 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. Some wetlands are used to pro-duce many commercially important products, such as cranberries.

Eutrophication

Section 1 Freshwater Ecosystems 175

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, includ-ing the 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 plantsand animals

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

Table 1 �

Copyright© by Holt, Rinehart and Winston. All rights reserved.

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 arelocated in the southeastern United States. 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.

There are several kinds of marshes, each of which is character-ized by its salinity. Brackish marshes have slightly salty water,while salt marshes contain saltier water. In each marsh type, organ-isms are adapted to live within the ecosystem’s range of salinity.

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.

176Copyright© by Holt, Rinehart and Winston. All rights reserved.

Swamps Swamps occur on flat, poorly drained land, often nearstreams. Swamps are dominated by woody shrubs or water-lovingtrees, depending on the latitude and climate in which the swampsare located. Mangrove swamps occur in warm climates near theocean, so their water is salty. Freshwater swamps are the idealhabitat for many amphibians, such as frogs and salamanders,because of the continuously moist environment. Swamps alsoattract birds, such as wood ducks, that nest in hollow trees nearor over the water. Reptiles, such as the American alligator inFigure 8, are the predators of swamps and will eat almost anyorganism that crosses their path.

Human Impact on Wetlands Wetlands were previously con-sidered to be wastelands that provide breeding grounds forinsects. Therefore, many have been drained, filled, and cleared forfarms or residential and commercial development, as shown inFigure 9. For example, the Florida Everglades once covered 8 mil-lion acres of south Florida but it now covers less than 2 millionacres. The important role of wetlands as purifiers of wastewaterand in flood prevention is now recognized. Wetlands arevitally important as habitats for wildlife. Law and the fed-eral government protect many wetlands, and most statesnow prohibit the destruction of certain wetlands.

Section 1 Freshwater Ecosystems 177

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.

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 off thecoast of Louisiana.

Copyright© by Holt, Rinehart and Winston. All rights reserved.

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,a river may broaden, become warmer, wider, and slower, anddecrease in oxygen. Figure 10 compares the water flow of twosections 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.

Plankton do not live in the headwaters becausethe current is too strong for them to float.However, trout and minnows are adapted to thecold, oxygen-rich headwaters. Trout are powerfulswimmers and have streamlined bodies that pres-ent little resistance to the strong current. Fartherdownstream, plankton can float in the warmer,calmer waters. Other plants, such as the crowfoot,set roots down in the river’s rich sediment. Theleaves of some plants, such as the arrowhead, willvary in shape according to the strength of a river’scurrent. Fish such as catfish and carp also live inthe calmer waters.

Rivers in Danger Industries use river water inmanufacturing processes and as receptacles forwaste. For many years, people have used rivers to

dispose of their sewage and garbage. These practices have pol-luted rivers with toxins, which have killed river organisms andmade river fish inedible. Today, runoff from the land puts pesti-cides and other poisons into rivers and coats riverbeds with toxicsediments. Dams also alter the ecosystems in and around a river.

178 Chapter 7 Aquatic Ecosystems

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 � The water flow of ariver slows and the habitat changesas narrow headwaters give way towide channels downstream.

Copyright© by Holt, Rinehart and Winston. All rights reserved.