Biomes and Aquatic Ecosystems 181

TradiTionally, The biome concepT has been limited to terrestrial (land-based) ecosystems. However, about 75 percent of Earth’s surface is covered in water, not land. What characteristics deἀne aquatic environments, and how do ecologists group similar ecosystems into biome-like categories?

describing aquatic ecosystems Ecologists classify aquatic ecosystems according to criteria such

as salinity, depth, and whether the water is flowing or standing.

Biomes are described by patterns in temperature and precipitation, but there aren’t really hot summers or rainy seasons in the deep ocean. While terrestrial biomes are shaped by air temperature and precipitation, aquatic systems are characterized by factors such as water salinity, depth, and whether the water is moving or standing.

Salinity Salinity measures the amount of salts dissolved in water. A straightforward measurement of salinity is parts per thousand (ppt), meaning the number of units of salt dissolved in 1000 units of water. “Salt water” generally has a salinity between 30 and 50 ppt. The oceans have an average salinity of 35 ppt. Water is considered “fresh” if it has a salinity of a 0.5 ppt or less. The aquatic ecosystems with salinity between 0.5 and 30 ppt are called brackish.

The salinity of water has direct effects on an aquatic organism’s sur-vival. Adaptations enable organisms to maintain careful water and salt balance with their surroundings. Water moves from areas of high con-centration (low salinity) to areas of low concentration (high salinity). As such, a freshwater ἀsh placed in salt water would die from water loss, as water moves from an area of higher concentration (the ἀsh) to an area of lower concentration (the salt water). A saltwater ἀsh moved to fresh water would fare no better. The ἀsh would swell and die as water rushed from an area of higher concentration (the fresh water) to an area of lower concentration (the ἀsh).

Reading Strategy As you read the lesson, make a cluster diagram for each aquatic ecosystem. Place the ecosystem’s name in the center circle, and connect it to words, phrases, or illustra-tions that describe it.

Vocabulary salinity, photic zone, aphotic zone, benthic zone, littoral zone, limnetic zone, wetland, flood plain, estuary, upwelling

• Describe the criteria ecologists use to classify aquatic ecosystems.

• List the major categories of freshwater ecosystems.

• Explain the ecological importance of estuaries.• List the three major zones of the ocean.

Guiding Question: What conditions and organisms characterize the world’s aquatic ecosystems?

Aquatic Ecosystems

LESS

ON 3

FOCUS Show students a globe or world map. Have students note the vast amount of Earth’s surface that is covered in water. Ask students to name some of the types of water ecosystems that are found on Earth.

GUIDING QUESTION

6.3 LESSON PLAN PREVIEWInquiry Help students under-stand the effect of salinity on a freshwater organism with a simple demo.Differentiated Instruction Generate a concept map to help students organize lesson concepts.Real World Students compare estuary ecosystems to local aquatic ecosystems.

6.3 RESOURCESPaper and Pencil Activity, Mapping Kelp Forests • Lesson 6.3 Worksheets • Lesson 6.3 Assessment • Chapter 6 Overview Presentation

182 Lesson 3

Depth In most terrestrial environments, primary production is limited by temperature and precipi-tation. Under water, however, photosynthesis by aquatic plants and phytoplankton is mostly limited by available light. In aquatic ecosystems, light avail-ability is largely a function of water depth.

▶ Aquatic Layers ἀ e uppermost layer of an aquatic ecosystem, where there is enough sunlight for photosynthesis, is called the photic zone. ἀ e depth of the photic zone depends on how clear the water is. In some crystal-clear tropical seas, the photic zone may extend over 200 meters (650 feet), but in muddy streams it could be a meter or less. Below the photic zone is the aphotic zone, where no sunlight pen-etrates and photosynthesis cannot occur. ἀ e very bottom of a body of water is called the benthic zone. Depending on the depth and clarity of the water, benthic zones can be sunlit or pitch dark.

▶ Depth and Life Consumers on land rely on oxy-gen in the air to breathe. Some aquatic consumers, such as sea turtles and whales, breathe in air by peri-odically rising to the top of the water. Most aquatic consumers, however, do not breathe air. Instead, they obtain the oxygen they need to carry out cel-lular respiration from water taken in through gills. Dissolved oxygen in the water comes from aquatic photosynthetic organisms that release oxygen dur-ing photosynthesis. ἀ e photic zone, where photo-synthesis can take place, has much more dissolved oxygen than the aphotic zone. ἀ erefore, there tends to be more life—both producers and consumers—in this upper portion of any aquatic ecosystem.

▶ Depth and Temperature ἀ e presence of sunlight also causes warmer temperatures. Upper layers of aquatic ecosystems tend to be warmer than deeper layers. Temperature shifts can occur, how-ever, brought on by seasonal temperature changes or shifting currents.

Flowing and Standing Water Aquatic ecosys-tems are sometimes divided into flowing-water and standing-water categories. Flowing-water ecosys-tems contain water that is in near-constant motion, such as in a river. Standing-water ecosystems contain water that does not move, or moves slowly, such as in a pond or wetland.

Figure 5 Light and Dark The photic zone is the area of an aquatic ecosystem that receives sunlight. The depth of the photic zone differs depending on the clarity of the water.

Go Outside

Littoral zone

Benthic zone

Limnetic zone

Phot

ic Z

one

Aph

otic

Zon

e

Biomes and Aquatic Ecosystems 183

Who’s in the Water? 21 3 4 65 7 8 9 Obtain several small, clean jars

with lids.21 3 4 65 7 8 9 Collect samples of fresh water

near your school or home. Samples can come from puddles, ponds, lakes, or rivers. Be sure to label all jars carefully with the location of the sample. If possible, take samples from diἀerent water sources and depths.

21 3 4 65 7 8 9 Examine each sample with a microscope or hand lens.

21 3 4 65 7 8 9 Use reference materials to identify the organisms you see.

Analyze and Conclude1. Compare and Contrast Which

of your samples contained the most life? The least?

2. Propose a Hypothesis Explain your findings with a hypothesis that relates to abiotic conditions for life in fresh water.

Figure 6 Freshwater Zones The photic zone in a pond, lake, or inland sea is divided into the near-shore littoral zone and open limnetic zone.

Ponds, Lakes, and inland Seas

Freshwater ecosystems Standing freshwater ecosystems include ponds, lakes, inland

seas, and wetlands. Flowing freshwater ecosystems include rivers and streams.

Freshwater ecosystems have very low salinity, less than 0.5 ppt. These eco-systems vary in depth from a few meters to several hundred meters. They can be either standing or flowing.

Ponds and lakes are bodies of open standing water that collect in depressions on Earth’s surface. Typically, smaller and shallower bodies of standing water are called ponds and larger, deeper ones are called lakes—though, beyond size and depth, there is no real difference between the two. Some lakes, however, are so large that they differ substantially in their characteristics from smaller lakes. These large lakes are sometimes known as inland seas. North America’s Great Lakes are prime examples. Because they hold so much water, most of their organisms are adapted for life in open water. The range in size among these freshwater bod-ies is considerable. Backyard ponds may be only a few square meters while the world’s largest inland sea, the Caspian Sea in central Asia, has an area of 371,000 square kilometers (143,000 square miles). Lakes and ponds also vary greatly in their nutrient content. Some may have little to no nutrients at all, whereas others may be so full of nutrients that they literally fill in and become a terrestrial ecosystem through secondary succession.

As shown in Figure 6, ecologists tend to divide the photic zone of lakes and ponds based on distance from the shore. The shallow near-shore por-tion of the photic zone is named the littoral zone. Here the water is shallow enough that aquatic plants grow from the mud and reach above the water’s surface. The limnetic zone is farther from the shore, where there are no rooted plants. The nutrients and plant growth of the littoral zone make it rich in invertebrates —such as insect larvae, snails, and crayfish—on which fish, birds, turtles, and amphibians feed. ANSWERS

Go Outside1. Answers will vary.2. Answers will vary, but should show

an understanding of the connec-tion between abiotic factors and living things.

184 Lesson 3

Figure 7 Wetlands Wetlands provide many benefits, such as preventing flooding, purifying water, and serving as home for many commercially important fish species. Three of the four types of wetlands are shown: (a) a freshwater marsh, (b) a swamp, and (c) a bog.

(a)

(b)

(c)

Freshwater Marshes

Swamps

Bogs and Fens

Wetlands Systems that combine elements of fresh water and dry land are enormously rich and produc-tive. Wetlands are areas of land that are ἀooded with water at least part of the year. Water can either ἀow slowly through wetlands and into other bodies of water, or can remain year-round. There are four types of freshwater wetlands: marshes, swamps, bogs, and fens (Figure 7).

The ecological importance of Wetlands Wetlands are extremely important habitats. They help prevent ἀooding by absorbing excess water—much like a sponge. Wetlands recharge aquifers and filter pollut-ants and sediment. Recreational activities like bird-watching and photography also depend on healthy wetlands. Finally, wetlands provide habitats for many species of commercially valuable fish.

Despite these vital roles, many wetlands have been drained and filled for agriculture or development. Wetlands accumulate organic material, so they tend to be nutrient-rich—and therefore ideal for farming. It is estimated that southern Canada and the United States have lost well over half their wetlands since European colonization. In recent years, however, people have come to recognize the importance of wetlands, and many conservation groups have worked toward their restoration. Further, many artificial wetlands have been built for their ecological benefits.

Freshwater marshes are shallow-water wetlands typified by tall, grasslike plants. The shallow water allows plants, such as cattails and bul-rushes, to grow above the water surface.

Swamps also consist of shallow water rich in vegetation, but they are typified by woody shrubs and trees, not grasses. The cypress swamps of the south-eastern United States, where cypress trees grow in stand-ing water, are an example. Swamps can be created when beavers build dams across streams from trees, flooding wooded areas upstream.

Bogs are wetlands characterized by low nutrients, acidic water, and thick, floating mats of vegetation, usually a type of moss. Bogs form either in depressions where water can collect or through second-ary succession. Secondary succession can occur when ponds are filled in with nutrients or when moss covers land and traps water underneath. Fens are similar to bogs, but are connected to a source of groundwater. They tend to be less acidic and more nutrient-rich ecosystems.

Mouth

Source

Flood plainFlood plain

Oxbow lake

Meander

Oxbow lake

Meander

TributaryTributary

Biomes and Aquatic Ecosystems 185

Water from rain, snowmelt, or springs runs downhill and converges where the land dips, form-ing streams, creeks, or brooks. These watercourses merge into rivers, whose water eventually reaches either the ocean or a landlocked water body. A smaller river flowing into a larger one is called a tributary, and the area of land drained by a river and all of its tributaries is called a watershed.

A River’s Course Rivers shape the landscape through which they run, as shown in Figure 8. The source, or beginning, of most rivers is high in the moun-tains, where melting snow collects and runs downhill due to gravity. Water near a river’s source tends to be cold and full of oxygen. Few organisms can live here, however, as the water moves too swiftly. Because water moves so fast near the source, it tends to cut a relatively deep and straight path through the earth.

Downstream, where the slope is gentler, rivers slow down and are warmed by the sun. Nutrients and sedi-ment collected by the running water begin to build up in the benthic zone. Plants can then take root, providing food for consumers. Water here contains less dissolved oxygen, as organisms are constantly using it for cel-lular respiration. When water flows slowly, it creates wide, curvy paths called meanders through the earth. Eventually, a bend may become such an extreme loop that water erodes a shortcut from one end of the loop to the other. The bend is cut off and remains as an isolated, U-shaped water body called an oxbow lake.

Over thousands or millions of years, a river may shift from one course to another, back and forth over a large area, carving out a flat valley. Areas nearest a river’s course that are flooded periodically are said to be within the river’s flood plain. Frequent deposition of silt from flooding makes flood plain soils especially fertile. Rivers empty into larger bodies of water, such as an ocean, at their mouth.

River Organisms Rivers and streams host diverse ecological communities. Nearer the river’s source, organisms have adap-tations that enable them to avoid being carried away by swift currents. Mosses cling to rocks, and fish may attach themselves to a rock on the riverbed with suckerlike mouths. Fish living in the swifter parts of a river, such as trout, must have strong, sleek bodies that enable them to swim against the current.

FiguRe 8 The Path of a River Rivers and streams flow downhill, shaping landscapes as they go.

Rivers and Streams

What Doyou think?

What Doyou think?

186 Lesson 3

A developer wants to build a large marina on an estuary in your coastal town. The marina would boost the town’s economy but eliminate its salt marshes. As a homeowner living adjacent to the marshes, how would you respond?

Figure 9 Marshes and Mangroves (a) Chesapeake Bay is home to enormous stretches of salt marsh that extend along more than 300 kilometers of coastline from Maryland to Virginia. (b) Mangrove trees, such as these in Florida’s Everglades National Park, show specialized adaptations for growing in changeable coastal conditions. The trees provide habitats for many types of fishes, birds, crabs, and other animals.

(a) (b)

Salt Marshes

Mangrove Forests

estuaries Estuaries are home to diverse ecosystems that prevent soil ero-

sion and flooding.

Estuaries are bodies of water that occur where rivers ἀow into an ocean or inland sea. Organisms living in coastal estuaries, where fresh and salt water mix to form brackish ecosystems, must be able to tolerate a wide range of temperature and salinity conditions. For fishes such as salmon that spawn in fresh water and mature in salt water, estuaries provide a transitional zone where young fish make the passage from fresh water to salt water. Years later, when the fish return to their hatching grounds to spawn, estuaries provide the transitional zone in the opposite direction—from salt to fresh water. Some estuaries involve only fresh water, for example, where rivers empty into inland seas such as the Great Lakes. Although there is no change in salinity in these freshwater estuaries, they tend to be diverse ecosystems with a mix of river and lake organisms.

Because river water constantly supplies nutrients to estuaries, they tend to be extremely productive. In fact, coastal estuaries are often home to two of the most productive ecosystems on Earth: salt marshes and mangrove forests.

Along many of the world’s coasts at temperate latitudes, salt marshes occur. Salt marshes are ecosystems characterized by salt-tolerant grasses. Salt marshes boast very high primary productivity and provide critical habitat for shorebirds, waterfowl, and the adults and young of many commercially important fish and shellfish species. They also filter out pollution and stabilize shorelines against storm surges. Chesapeake Bay, off the coasts of Maryland and Virginia, contains some of the most extensive areas of salt marsh in the United States.

In tropical and subtropical latitudes, mangrove forests grow along gently sloping sandy and silty coasts. Mangroves are specialized trees and shrubs with roots that curve upward, out of the ground, to attain oxygen lacking in the mud or that curve downward like stilts to sup-port the tree in changing water levels. Fish, shellfish, crabs, snakes, and other organisms thrive among the root networks, and birds feed and nest in the dense foliage of these coastal forests. Besides serving as nurseries for fish and shellfish that people harvest, mangroves also provide materials that people use for food, medicine, tools, and construction. The largest mangrove forests in America are in Florida’s Everglades region.

ANSWERS

What Do You Think? Answers will vary, but should reflect knowledge of the ecological value of salt marshes.

ASIA

AUSTRALIA

NORTHAMERICA

AFRICA

EUROPE

ANTARCTICA

SOUTHAMERICA

PACIFICO CEAN

ARCTIC O CEAN

ATL ANTICO CEAN

INDIANO CEAN

Cold currentWarm current

SOUTHERN OCEAN

GU

LF S

TREAM

Biomes and Aquatic Ecosystems 187

The Ecological Importance of Estuaries Estuary ecosystems provide many beneἀts. Like freshwater wetlands, salt marshes and man-grove forests help prevent soil erosion and flooding. They also act as a protective barrier between the sea and land. However, people want to live along coasts, and coastal sites are desirable for commerce. As a result, vast expanses of coastal estuaries worldwide have been destroyed to make way for coastal development.

Unfortunately, we have suffered for their loss. When Hurricane Katrina struck the Gulf Coast in 2005, for instance, the flooding was made worse because vast areas of salt marshes had been destroyed due to development. Similarly, when a tsunami struck areas along the Indian Ocean in 2004, it devastated coasts where mangrove forests had been removed but caused less damage where the forests were intact.

The Oceans The ocean can be divided into three zones based on their

distance from shore: intertidal, neritic, and open ocean.

We generally speak of the world’s oceans in the plural, giving each major basin a name: Paciἀc, Atlantic, Indian, Arctic, and Southern. However, all of these oceans are connected, forming a single, vast body of water. This one “world ocean” covers 71% of Earth’s surface and contains 97.5% of its water. The world’s oceans touch and are touched by virtually every environmental system and every human endeavor.

Ocean Structure Salts in ocean water are carried from land by wind and water. If we were able to evaporate all the water from the oceans, the empty basins would be covered with a layer of salt roughly 60 meters (200 feet) thick. Salt content and tempera-ture have an effect on water density. Water density increases as salinity rises and as tem-perature falls. These relationships give rise to different layers of water. Heavier (colder and saltier) water sinks, and lighter (warmer and less salty) water remains nearer the surface.

▶ Ocean Currents Earth’s ocean is com-posed of many riverlike flows driven by heating and cooling, gravity, wind, and dif-ferences in water density. These currents, shown in Figure 10, flow hori-zontally and for great distances. Some currents are very slow. Others, like the Gulf Stream, are rapid and powerful. From the Gulf of Mexico, the Gulf Stream moves up along the Atlantic coast at a rate of 160 kilometers per day (over 4.1 miles per hour). Averaging 70 kilometers (43 miles) across, the Gulf Stream continues across the North Atlantic, bringing warm water to Europe and moderating that continent’s climate, which otherwise would be much colder.

FIgurE 10 Ocean Currents The upper waters of the oceans flow in currents, which are long-lasting and predictable global patterns of water movement. Warm- and cold-water currents interact with the planet’s climate system, and people have used them for centuries to navigate the oceans.

Continental shelf

IntertidalZone

NeriticZone Open-Ocean Zone

Phot

ic Z

one

Aph

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Zon

e

10,000 m

200 m

1000 m

4000 m

Benthic Zone

188 Lesson 3

▶ Upwelling and Downwelling Surface winds and heating create vertical currents in seawater that move nutrients and oxygen through the ocean’s layers. Upwelling, the vertical ἀow of cold, nutrient-rich water toward the surface, occurs where horizontal currents diverge, or ἀow away from one another. In areas where surface currents converge, or come together, surface water sinks in a process called downwelling. Downwelling transports warm water rich in dissolved gases from the surface into the ocean depths.

▶ Ocean Zones Ecologists typically break the ocean into zones defined by water depth and distance from shore. As shown in Figure 11, the ocean is divided vertically into the sunlit photic zone, the vast and dark aphotic zone, and the mysterious benthic zone along the ocean ἀoor. In most places, the photic zone extends about 200 meters (660 feet) down. The photic zone is further divided into three major zones: the intertidal zone, the neritic zone, and the open-ocean zone.

Ocean Ecosystems Most ocean ecosystems are powered by solar energy, with sunlight driving photosynthesis by phytoplankton in the photic zone. Yet, even the darkest ocean depths host life.

Figure 11 Ocean Zones Ecologists divide the ocean into zones based on depth and distance from the shore. By far, the largest zone is the aphotic zone. The most productive zones, however, are shallower and closer to shore.

How does the environment af-fect where and how an organism lives?Application Have pairs of students consider why depth and distance from the shore are important envi-ronmental factors for ocean organ-isms. Use students responses to start a class discussion on how the ocean environment affects where and how ocean organisms live.

BIG QUESTION

Mussels

Intertidal zone

Subtidal zone

Level of low tide

Level of high tide

Crab

Mussels

Barnacles

Anemone

Chiton

Sea star

Limpet

Urchin

Algae

Barnacles

Anemone

Chiton

Sea star

Limpet

Urchin

Algae

Continental shelf

IntertidalZone

NeriticZone Open-Ocean Zone

Phot

ic Z

one

Aph

otic

Zon

e

10,000 m

200 m

1000 m

4000 m

Benthic Zone

Biomes and Aquatic Ecosystems 189

Figure 12 The intertidal Zone The rocky intertidal zone is rich in biodiversity, typically containing large invertebrates such as sea stars, barnacles, crabs, sea anemones, limpets, chitons, mussels, and sea urchins. Areas higher on the shoreline are exposed to the air more frequently and for longer periods, so organisms that can tolerate exposure best specialize in the upper intertidal zone. The lower intertidal zone is exposed less frequently and for shorter periods, so organisms less able to tolerate exposure thrive in this zone.

intertidal ecosystems Where ocean meets land, intertidal ecosys-tems spread between the uppermost reach of the high tide and the lowest limit of the low tide. Tides are the periodic rising and falling of the ocean’s height at a given location, caused by the gravitational pull of the moon and sun. In most places, high and low tides occur roughly 6 hours apart, so intertidal organisms spend part of each day submerged in water, part of the day exposed to the air and sun, and part of the day being lashed by waves. Subject to tremendous extremes in temperature, moisture, sun exposure, and salinity, these creatures must also protect themselves from marine predators at high tide and terrestrial predators at low tide.

The intertidal environment is a tough place to make a living, but it is home to a remarkable diversity of organisms. Nutrient content is generally high, and ample sunlight fuels a variety of primary producers. On a rocky shore, animals such as anemones, mussels, and barnacles live attached to rocks, filter-feeding on plankton in the water that washes over them. Urchins, chitons, and limpets eat intertidal algae or scrape food from the rocks. Sea stars creep slowly along, preying on the filter feeders and herbivores at high tide. Crabs clamber around the rocks, scavenging detritus. Sandy intertidal areas, such as those of Cape Cod in Massachusetts, host less biodiversity, yet plenty of organisms burrow into the sand at low tide to await the return of high tide, when they emerge to feed.

190 Lesson 3

Figure 14 Coral reefs Coral reefs provide food and shelter for a tremendous diversity of fish and other creatures. However, these reefs face multiple environmental stresses from human activities, and many corals have died as a result.

Figure 13 Kelp Forests “Forests” of tall brown algae known as kelp grow from the floor of the continental shelf. Numerous fish and other creatures eat kelp or find refuge among it.

Kelp Forests

Coral Reefs

Neritic Ecosystems The ocean’s neritic zone extends out from the low-tide mark to the edge of the continental shelf. Continental shelves underlie the shallow waters bordering the continents. Generally, the depth of the continental shelf is less than 66 meters (217 feet), so the neritic zone is entirely sunlit, enabling great productivity. In fact, two of the world’s most productive ecosystems exist here: kelp forests and coral reefs.

In some oceans, a type of large brown algae called kelp grows from the floor of continental shelves, reaching upward toward the sunlit surface. Kelp can be 60 meters (200 feet) in height and can grow 45 centimeters (18 inches) in a single day. Kelp forests supply shelter and food for invertebrates and fish, which in turn provide food for predators, such as seals and sharks. Recall that sea otters are considered keystone species that control sea urchin popula-tions. When otters disappear, urchins overgraze the kelp, destroying the forests. Kelp forests absorb wave energy and protect shorelines from erosion.

In shallow subtropical and tropical waters, coral reefs occur. A coral reef is a mass of calcium carbonate composed of the skeletons of marine organisms known as corals. Corals are tiny invertebrate animals related to sea anemones and jellyfish. They remain attached to rock, existing reef, or the ocean bottom and capture passing food with sting-ing tentacles. Corals also derive nourishment from symbiotic algae, known as zooxanthellae, which inhabit their bodies and produce food through photosynthesis. Most corals are colonial, and the colorful surface of a coral reef consists of millions of densely packed individuals. When corals die, their skeletons remain part of the reef. New corals grow on top of the skel-etons, increasing the reef’s size.

Like kelp forests, coral reefs protect shorelines by absorb-ing wave energy. They also host tremendous biodiversity. Coral reefs are experiencing worldwide declines, however. Many have undergone “coral bleaching,” a process that occurs when zoox-anthellae leave the coral, depriving it of nutrition. Corals lack-ing zooxanthellae lose color and frequently die, leaving behind ghostly white patches in the reef. Coral bleaching is thought to result from increased sea surface temperatures, from the influx of pollutants, from unknown natural causes, or from some com-bination of these factors.

Biomes and Aquatic Ecosystems 191

1. Apply Concepts How do ecologists classify aquatic ecosystems?

2. Compare and Contrast What are the similarities and dif-ferences among a lake, wetland, and river?

3. Predict Dams are obstructions placed in a river or stream to block its ἀow. In addition to water, sediment builds up behind the dam. How might a dam placed upriver affect the productivity of an estuary at the river’s mouth?

4. Apply Concepts What kinds of conditions do organisms need to be adapted for in the intertidal zone of the ocean?

5. Scientists use deep-sea submarines to explore the ocean’s benthic zone. However, it is extremely diffi-cult to bring deep-sea organisms to the sur-face alive without expensive and specialized equipment. Use what you know about the conditions in the deepest ocean to explain why deep-sea organisms cannot survive in conditions closer to the ocean’s surface.

3

The open ocean begins at the edge of the continental shelf. It contains the majority of Earth’s ocean water—over 90 percent of it! However, it is among the least productive ecosystems because most of it is dark and incapable of productivity by photosynthesis. Even in the sunlit photic zone, photosynthesis is often limited because not enough nutrients make their way into the open ocean from land. As a result, primary productivity and animal life near the surface are concentrated in regions of nutrient-rich upwelling.

Microscopic phytoplankton make up the base of food chains in the open ocean, where water is too deep for rooted plants. Phytoplankton are a source of food for zoo-plankton, which in turn become food for fish, jellyfish, whales, and other free-swimming animals. Predators at higher trophic levels include larger fish, sea turtles, sharks, and fish-eating birds that nest on islands and coastlines.

In the aphotic open-ocean zone, animals have adapta-tions that enable them to live in the dark without food from plants. Some of these often bizarre-looking creatures scavenge carcasses or organic detritus that falls from above. Others are predators, and still others obtain food from symbiotic mutualistic bacteria. Some species carry bacteria that produce light chemi-cally with bioluminescence. Anglerfish, like the one in Figure 15, use bioluminescence to attract prey.

Benthic ecosystems that occur around hydrothermal vents, where heated water spurts from the seafloor, host some of the stranger forms of ocean life. Tubeworms, shrimp, and other creatures in these recently discovered systems use symbiotic bacteria to derive their energy from chemicals in the heated water rather than from sunlight. They manage to thrive here, kilometers below the surface, despite enormous pressure caused by the weight of the water above.

Figure 15 Creatures From the Deep Life is scarce in the dark depths of the deep ocean, but the creatures that do live there may have a bizarre appearance. The anglerfish lures prey toward its mouth with a bioluminescent organ that protrudes from the front of its head.

Open-Ocean Ecosystems

ANSWERS

Lesson 3 Assessment For answers to the Lesson 3 Assessment, see page A–9 at the back of the book.