MT
Main content
Chapter Introduction
Coral reef in Egypt’s Red Sea
Core Case StudyWhy Should We Care about Coral Reefs?
Learning Objectives
· LO 8.1List three important ecological or economic services provided by coral reefs.
· LO 8.2Describe three major threats to coral reefs.
Coral reefs form in clear, warm coastal waters in tropical areas. These stunningly beautiful natural wonders (see chapter-opening photo) are among the world’s oldest, most diverse, and most productive ecosystems.
Coral reefs are formed by massive colonies of tiny animals called polyps (close relatives of jellyfish). They slowly build reefs by secreting a protective crust of limestone (calcium carbonate) around their soft bodies. When the polyps die, their empty crusts remain behind as part of a platform for more reef growth. The resulting elaborate network of crevices, ledges, and holes serves as calcium carbonate “condominiums” for a variety of marine animals.
Coral reefs are the result of a mutually beneficial relationship between polyps and tiny single-celled algae called zooxanthellae (“zoh-ZAN-thel-ee”) that live in the tissues of the polyps. In this example of mutualism, the algae provide the polyps with food and oxygen through photosynthesis and help the corals produce calcium carbonate. Algae also give the reefs their stunning coloration. The polyps, in turn, provide the algae with a well-protected home and some of their nutrients.
Although shallow and deep-water coral reefs occupy only about 2% of the ocean floor, they provide important ecosystem and economic services. They act as natural barriers that help to protect 15% of the world’s coastlines from flooding and erosion caused by battering waves and storms. They also provide habitats, food, and spawning grounds for one-quarter to one-third of the organisms that live in the ocean, and they produce about one-tenth of the global fish catch. Through tourism and fishing, they provide goods and services worth about $40 billion a year.
Coral reefs are vulnerable to damage because they grow slowly and are disrupted easily. Runoff of soil and other materials from the land can cloud the water and block the sunlight that the algae in shallow reefs need for photosynthesis. In addition, the water in which shallow reefs live must have a temperature of and cannot be too acidic. This explains why a major long-term threat to shallow coral reefs is climate change, which could raise the water temperature above tolerable limits in most shallow reef areas. The closely related problem of ocean acidification could make it harder for polyps to build reefs and could even dissolve some of their calcium carbonate formations. (We discuss this problem further in this and later chapters.)
One result of stresses such as pollution and rising ocean water temperatures is coral bleaching (Figure 8.1), which can cause the colorful algae, upon which corals depend for food, to die off. Without food, the coral polyps die, leaving behind a white skeleton of calcium carbonate. Studies by the Global Coral Reef Monitoring Network and other scientific groups estimate that since the 1950s, some 45% to 53% of the world’s shallow coral reefs have been destroyed or degraded. Another 25% to 33% could be lost within 20 to 40 years. These centers of marine biodiversity are by far the most threatened marine ecosystems. Their disappearance would affect the survival of the one-fourth to one-half of the ocean species that depend on them.
Figure 8.1
This bleached coral has lost most of its algae because of changes in the environment such as warming of the waters and deposition of sediments.
iStock.com/Rainer von Brandis
In this chapter, we explore the nature of marine and freshwater aquatic ecosystems, and begin to examine the effects of human activities on these vital forms of natural capital.
Change font size
8.1Aquatic Systems
· LO 8.1AName and give three examples of each of the two major aquatic life zones.
· LO 8.1BDescribe the three groups of plankton found in aquatic life zones.
· LO 8.1CDescribe the three other major groups of organisms (besides plankton) found in aquatic life zones.
· LO 8.1DList the four key factors that determine the types and numbers of organisms found in different areas of the ocean.
· LO 8.1EExplain how algal blooms create problems for shallow coral reefs.
Change font size
8.1aWater Covers Most of the Planet
Saltwater covers about 71% of the earth’s surface, and freshwater occupies roughly another 2%. Although the global ocean is a single and continuous body of water, geographers divide it into five large areas—the Atlantic, Pacific, Indian, Arctic, and Southern Oceans—separated by the continents. The largest ocean is the Pacific, which contains more than half of the earth’s water and covers one-third of the earth’s surface. Together, the oceans hold almost 98% of the earth’s water. Each of us is connected to, and utterly dependent on, the earth’s global ocean through the water cycle (see Figure 3.19).
73%
Percentage of the earth that is covered with water
The aquatic equivalents of biomes are called aquatic life zones —saltwater and freshwater portions of the biosphere that can support life. The distribution of many aquatic organisms is determined largely by the water’s salinity —the amounts of various salts such as sodium chloride (NaCl) dissolved in a given volume of water. As a result, aquatic life zones are classified into two major types: saltwater or marine life zones (oceans and their bays, estuaries, coastal wetlands, shorelines, coral reefs, and mangrove forests) and freshwater life zones (lakes, rivers, streams, and inland wetlands). Some systems such as estuaries are a mix of saltwater and freshwater, but scientists classify them as marine systems for purposes of discussion.
Change font size
8.1bAquatic Species Drift, Swim, Crawl, and Cling
Saltwater and freshwater life zones contain several major types of organisms. One type consists of plankton , which can be divided into three groups. The first group consists of drifting organisms called phytoplankton, which includes many types of algae. These tiny aquatic plants and even smaller ultraplankton—the second group of plankton—are the producers that make up the base of most aquatic food chains and webs (see Figure 3.16). Through photosynthesis, they produce about half of the earth’s oxygen, on which we depend for survival.
The third group is made up of drifting animals called zooplankton, which feed on phytoplankton and on other zooplankton (see Figure 3.16). The members of this group range in size from single-celled protozoa to large invertebrates such as jellyfish (Figure 8.2).
Figure 8.2
Jellyfish are drifting zooplankton that use their long tentacles with stinging cells to stun or kill their prey.
A second major type of aquatic organism is nekton , strongly swimming consumers such as fish, turtles, and whales. The third type, benthos , consists of bottom-dwellers such as oysters and sea stars (Figure 8.3), which anchor themselves to ocean-bottom structures; clams and worms, which burrow into the sand or mud; and lobsters and crabs, which walk about on the sea floor. A fourth major type is decomposers (mostly bacteria), which break down organic compounds in the dead bodies and wastes of aquatic organisms into nutrients that aquatic primary producers can use.
Figure 8.3
Starfish on a coral reef. The starfish is also called a sea star because it is not a fish.
Key factors determining the types and numbers of organisms found in different areas of the ocean are temperature, dissolved oxygen content, availability of food, and availability of light and nutrients required for photosynthesis, such as carbon (as dissolved gas), nitrogen (as ), and phosphorus (mostly as ).
In deep aquatic systems, photosynthesis is largely confined to the upper layer—the euphotic or photic zone—through which sunlight can penetrate. The depth of the euphotic zone in oceans and deep lakes is reduced when the water is clouded by excessive growth of algae. This is called an algal bloom and it results from nutrient overloads. This cloudiness is called turbidity . It is also caused by soil and other sediments being carried by wind, rain and melting snow from cleared land into adjoining bodies of water. This is one of the problems affecting shallow coral reefs (Core Case Study).
In shallow systems such as small open streams, lake edges, and ocean shorelines, ample supplies of nutrients for primary producers are usually available, which tends to make these areas high in biodiversity. By contrast, in most areas of the open ocean, nitrates, phosphates, iron, and other nutrients are often in short supply, and this limits net primary productivity (NPP) (see Figure 3.18) and the diversity of species.
Change font size
8.2Importance of Marine Aquatic Systems
· LO 8.2AList five ecosystem services and five economic services provided by marine ecosystems.
· LO 8.2BDescribe the three major life zones in an ocean in terms of the ecosystems and types of organisms found in each.
· LO 8.2CDescribe five ecosystems that are key components of ocean coastal zones.
· LO 8.2DExplain how ocean acidification is threatening coral reefs.
· LO 8.2EDescribe the three vertical zones of the open ocean in terms of sunlight and aquatic organisms found in each.
· LO 8.2FExplain how an upwelling occurs and how it affects populations of aquatic organisms.
Change font size
8.2aOceans Provide Vital Ecosystem and Economic Services
Oceans dominate the planet and provide enormously valuable ecosystem and economic services (Figure 8.4) that help keep us and other species alive and support our economies. They provide us with seafood, produce more than half of the oxygen we breathe and, as a vital part of the water cycle, provide most of the rain that sustains our water supply.
Figure 8.4
Marine systems provide a number of important ecosystem and economic services.
Critical Thinking:
1. Which two ecosystem services and which two economic services do you think are the most important? Why?
Top: Willyam Bradberry/ Shutterstock.com. Bottom: James A. Harris/ Shutterstock.com.
As land dwellers, we have a distorted and limited view of the oceans that cover most of the earth’s surface. We know more about the surface of the moon than we know about the earth’s oceans. According to aquatic scientists, the scientific investigation of poorly understood marine and freshwater aquatic systems could yield immense ecological and economic benefits.
Learning from Nature
Engineers are learning how whales use sound waves to communicate over long distances underwater to improve our underwater communication technologies.
The oceans are also enormous reservoirs of biodiversity. Marine life is found in three major life zones: the coastal zone, the open sea, and the ocean bottom (Figure 8.5).
Figure 8.5
Major life zones and vertical zones (not drawn to scale) in an ocean. Actual depths of zones may vary. Available light determines the euphotic, bathyal, and abyssal zones. Temperature zones also vary with depth, shown here by the red line.
Critical Thinking:
1. How is an ocean similar to a rain forest? (Hint: See Figure 7.20.)
The coastal zone is the area of warm, nutrient-rich, shallow coastal waters that occupies less than 10% of the world’s ocean area. It contains 90% of all marine species and is the site of most large commercial fisheries. This zone’s aquatic systems include estuaries , the partially enclosed bodies of water where rivers meet the sea (Figure 8.6), and coastal wetlands, or land areas covered by coastal waters during all or part of the year. The latter include coastal marshes (Figure 8.7) and mangrove forests (Figure 8.8). Other important systems are sea-grass beds (Figure 8.9) and coral reefs (see chapter-opening photo, Core Case Study, and Case Study that follows).
Figure 8.6
Satellite photo of an estuary. The Mississippi River carries sediment and plant nutrients from fertilizer runoff into the Gulf of Mexico. The excess plant nutrients create an algal bloom (green area) that upsets marine life by depleting dissolved oxygen near the Gulf’s bottom.
NASA/Landsat/Phil Degginger/Alamy Stock Photo
Figure 8.7
Coastal marsh in the U.S. state of South Carolina.
Figure 8.8
Mangrove forest on the coast of Thailand. Mangroves have roots that curve up from the mud and water to obtain oxygen from the air.
Manit Larpluechai/ Dreamstime.com
Figure 8.9
Seagrass beds, such as this one near the coast of San Clemente Island, California support a variety of marine species.
James Forte/National Geographic Image Collection
Revisiting Coral Reefs—Amazing Centers of Biodiversity
Coral reefs (Core Case Study and chapter-opening photo) are some of the world’s oldest and most diverse and productive ecosystems. They are the marine equivalents of tropical rain forests, with complex interactions among their diverse populations of species.
Worldwide, coral reefs are being damaged and destroyed at an alarming rate by a variety of human activities. The newest growing threat is ocean acidification —the rising levels of acidity in ocean waters. This is occurring because the oceans absorb about 25% of the emitted into the atmosphere by human activities, primarily from the burning of fossil fuels. The reacts with ocean water to form a weak acid (carbonic acid, ). This reaction decreases the levels of carbonate ions necessary for the formation of coral reefs and the shells and skeletons of many marine organisms. This makes it harder for these species to thrive and reproduce. At some point, this rising acidity could slowly dissolve corals and the shells and skeletons of some marine species.
Ocean acidification and other forms of degradation could have devastating effects on the biodiversity and food webs of coral reefs. This will in turn degrade the ecosystem services that reefs provide. It will also have a severe impact on the approximately 500 million people who depend on coral reefs for food or for income from fishing and tourism. We discuss the threat from ocean acidification in more detail in Chapter 11.
Pioneering underwater photographer Richard Vevers and teams of marine scientists have launched the XL Catlin Seaview Survey and the Underwater Earth Project. These researchers are using sophisticated underwater cameras to create three-dimensional digital images of the world’s major coral reefs. These projects aim to provide a baseline of the health of the world’s coral reefs and to identify areas that need emergency protection to keep the reefs from dying (Core Case Study).
These coastal aquatic systems provide important ecosystem and economic services. They help to maintain water quality in tropical coastal zones by filtering toxic pollutants, excess plant nutrients, and sediments, and by absorbing other pollutants. They provide food, habitats, and nursery sites for a variety of aquatic and terrestrial species. Coastal wetlands also reduce storm damage and coastal erosion by absorbing waves and storing excess water produced by storms and tsunamis.
Change font size
8.2bRocky and Sandy Shores
The gravitational pull of the moon and sun causes tides , or periodic flows of water onto and off the shore, to rise and fall about every 6 hours in most coastal areas. The area of shoreline between low and high tides is called the intertidal zone . Organisms living in this zone must be able to avoid being swept away or crushed by waves. They need to survive when immersed during high tides and left high and dry (and much hotter) at low tides. They must also survive changing levels of salinity when heavy rains dilute saltwater. To deal with such stresses, most intertidal organisms hide in protective shells, dig in, or hold on tight to something.
Learning from Nature
The blue mussel produces a nontoxic, biodegradable glue to cling to underwater rocks in oceans. Scientists have mimicked this process to produce nontoxic glues that can be used under and above water.
On some coasts, steep rocky shores are pounded by waves (Figure 8.10, top). The numerous pools and other habitats in these intertidal zones contain a great variety of species. Each occupies a different niche to deal with daily and seasonal changes in environmental conditions such as temperature, water flows, and salinity.
Figure 8.10
Living between the tides: Some organisms with specialized niches are found in various zones on rocky shore beaches (top) and barrier or sandy beaches (bottom). Organisms are not drawn to scale.
Galyna Andrushko/ Shutterstock.com; Marketa Mark/ Shutterstock.com
Other coasts have gently sloping barrier beaches, or sandy shores, that support other types of marine organisms (Figure 8.10, bottom). Most of them stay hidden from view and survive by burrowing, digging, and tunneling in the sand. These beaches and their adjoining coastal wetlands are also home to a variety of shorebirds that have evolved in specialized niches to feed on crustaceans, insects, and other organisms (see Figure 4.10).
Many of these same species also live on barrier islands—low, narrow, sandy islands that form offshore, parallel to coastlines. Undisturbed barrier beaches generally have one or more rows of sand dunes in which the sand is held in place by the roots of grasses and other plants. These dunes are the first line of defense against the ravages of the sea. Real estate developers frequently remove the protective dunes or cover them with buildings and roads. Large storms can then flood and even sweep away seaside construction and severely erode the sandy beaches.
8.2cOpen Sea and Ocean Floor
The sharp increase in water depth at the edge of the continental shelf separates the coastal zone from the vast volume of the ocean called the open sea . This aquatic life zone is divided into three vertical zones (Figure 8.5), or layers, primarily based on the degree of penetration of sunlight. Temperatures also change with depth (Figure 8.5, red line) and scientists use them to define zones of varying species diversity in these layers.
The euphotic zone is the brightly lit upper zone, where drifting phytoplankton carry out about 40% of the world’s photosynthetic activity. Large, fast-swimming predatory fishes such as swordfish, sharks, and bluefin tuna populate the euphotic zone.
Nutrient levels are low and levels of dissolved oxygen are high in the euphotic zone. The exception to this is areas called upwelling zones. An upwelling , or upward movement of ocean water, brings cool and nutrient-rich water from the bottom of the ocean to the warmer surface. There it supports large populations of phytoplankton, zooplankton, fish, and fish-eating seabirds. Strong upwellings occur along the steep western coasts of some continents when winds blowing along the coasts push surface water away from the land. This draws water up from the ocean bottom (Figure 8.11 and Figure 7.3). Figure 7.7 shows the oceans’ major upwelling zones.
Figure 8.11
A shore upwelling occurs when deep, cool, nutrient-rich waters are drawn up to replace surface water moved away from a steep coast by wind flowing along the coast toward the equator.
The bathyal zone is the dimly lit middle zone that receives little sunlight and therefore does not contain photosynthesizing producers. Zooplankton and smaller fishes, many of which migrate to feed on the surface at night, are found in this zone.
The deepest open sea zone, called the abyssal zone, is dark and cold. There is no sunlight to support photosynthesis, and this water has little dissolved oxygen. Nevertheless, the deep ocean floor is teeming with life because it contains enough nutrients to support a large number of species. Most of this zone’s organisms get their food from showers of dead and decaying organisms—called marine snow—drifting down from the upper zones. Some abyssal-zone organisms, including many types of worms, are deposit feeders, which take mud into their guts and extract nutrients from it. Others such as oysters, clams, and sponges are filter feeders, which pass water through or over their bodies and extract nutrients from it.
Net primary productivity (NPP) is quite low in the open sea, except in upwelling areas. However, because the open sea covers so much of the earth’s surface, it makes the largest contribution to the earth’s overall NPP. In fact, scientists have learned that the open sea contains more biodiversity than they thought a few years ago (Science Focus 8.1).
Science Focus 8.1
We Are Still Learning about the Ocean’s Biodiversity
Scientists have long assumed that open-ocean waters contained few microbial life forms. However, recent research has challenged that assumption and greatly increased our knowledge of the ocean’s genetic diversity.
A team of scientists led by J. Craig Venter took 2 years to conduct a census (an estimated count based on sampling) of ocean microbes. They sailed around the world, stopping every 320 kilometers (200 miles) to pump seawater through extremely fine filters, from which they gathered data on bacteria, viruses, and other microbes. It was the most thorough of such censuses ever conducted.
Using a supercomputer, they counted genetic coding for 6 million new proteins—double the number that had previously been known. They also reported that they were discovering new genes and proteins at the same rate at the end of their voyage as they had at the start of it. This indicated that there is still much more of this biodiversity to discover.
This means that the ocean contains a much higher diversity of microbial life than had previously been thought. Ocean-water microbes play an important role in the absorption of carbon by the ocean, as well as in the ocean food web. Venter has led more expeditions to continue the sampling in other areas.
Critical Thinking
1. Why was the rate of discovery of new genes and proteins important to Venter and his colleagues? Explain.
Change font size
8.3Effects of Human Activities on Marine Ecosystems
· LO 8.3AList five harmful impacts of human activities on marine ecosystems.
· LO 8.3BList six harmful impacts of human activities on coral reefs.
· LO 8.3CExplain why climate change is considered one of the most serious threats to marine ecosystems.
· LO 8.3DExplain why ocean acidification is considered one of the most serious threats to marine ecosystems.
· LO 8.3EList five steps taken through integrated coastal management of the Chesapeake Bay to reduce the serious pollution of the bay.
Change font size
8.3aHuman Activities Are Disrupting and Degrading Marine Ecosystems
Certain human activities are disrupting and degrading many of the ecosystem and economic services provided by marine aquatic systems, especially coastal marshes, shorelines, mangrove forests, and coral reefs (Core Case Study), as summarized in Figure 8.12.
Figure 8.12
Human activities have major harmful impacts on all marine ecosystems (left) and particularly on coral reefs (right).
Critical Thinking:
1. Which two of the threats to marine ecosystems do you think are the most serious? Why? Which two of the threats to coral reefs do you think are the most serious? Why?
Top left: travelview/ Shutterstock.com. Top right: Rich Carey/ Shutterstock.com. Bottom left: B-D-S Piotr Marcinski/ Shutterstock.com. Bottom right: Rostislav Ageev/ Shutterstock.com.
According to the World Wildlife Fund (WWF), more than 35% of the world’s original mangrove forest area (Figure 8.8) had been lost to agricultural and urban expansion, marinas, roadways, and other forms of coastal development. According to the International Union for the Conservation of Nature (IUCN), more than one of every six species of mangrove is in danger of extinction. In addition, since 1980 about 29% of the world’s seagrass beds (Figure 8.9) have been lost to pollution and other disturbances.
The U.S. National Center for Ecological Analysis and Synthesis (NCEAS) used computer models to analyze and provide the first-ever comprehensive map of the effects of 17 different types of human activities on the world’s oceans. In this 4-year study, an international team of scientists found that human activities have heavily affected 41% of the world’s ocean area that covers 71% of the earth’s surface.
A serious threat to marine systems, according to many marine scientists, is climate change. The earth’s average atmospheric temperature and the average temperature of the oceans have increased since 1980. This is raising the world’s average sea level. One reason is that warmer ocean water expands. A second reason is that as the atmosphere has warmed, land-based glaciers in Greenland and other parts of the world are slowly melting and adding large quantities of water to the oceans. The rise in sea levels projected for this century will likely destroy shallow coral reefs and will flood coastal marshes, other coastal ecosystems, and many coastal cities. (We discuss these effects in Chapter 19).
A second serious threat to the oceans, which some scientists view as more serious than the threat of climate change, is ocean acidification. It is especially threatening to coral reefs (Core Case Study) and to phytoplankton and many shellfish that form their shells from calcium carbonate, as we discuss in more detail in Chapter 11.
Other major threats to marine systems from human activities include the following:
· Coastal development, which destroys or degrades coastal habitats
· Runoff of pollutants such as fertilizers, pesticides, and livestock wastes (see Case Study that follows) and pollution from cruise ships and oil tanker spills
· Overfishing and depletion of commercial fish species populations
· Destruction of ocean bottom habitats by fishing trawlers dragging weighted nets
· Invasive species that deplete populations of native species.
Case Study
The Chesapeake Bay—An Estuary in Trouble
Since 1960, the Chesapeake Bay watershed (Figure 8.13)—the largest estuary in the United States—has been in trouble, mostly because of human activities. One problem is population growth. Between 1940 and 2017, the number of people living in the Chesapeake Bay area grew from 3.7 million to 18.2 million, and is projected to reach 20 million by 2030, according to estimates by the Chesapeake Bay Program.
Figure 8.13
Chesapeake Bay watershed.
(Data from Chesapeake Bay Program)
The estuary receives wastes from point and nonpoint sources scattered throughout its huge drainage basin, which lies in parts of six states and the District of Columbia (Figure 8.13). The shallow bay has become a huge pollution sink because only 1% of the waste entering it is flushed into the Atlantic Ocean. Phosphate and nitrate levels have been high in many parts of the bay, causing algal blooms that deplete the oxygen dissolved in the waters making them unsuitable for most forms of aquatic life. Commercial harvests of the bay’s once-abundant oysters and crabs, as well as several important fish species, fell sharply after the 1960s because of a combination of pollution, overfishing, and disease.
Point sources, primarily sewage treatment plants and industrial plants, add large amounts of phosphates to the bay. The bay also receives nonpoint sources of phosphates, nitrates, and sediments in the runoff of fertilizer and animal wastes from agricultural land. The bay receives inputs of pesticides from direct spraying over water and as runoff from nearby croplands. In addition, discharges from sewage treatment plants into the bay often contain pharmaceutical chemicals that have entered the sewage treatment system after use by humans. Most treatment systems do not remove many of these chemicals, some of which can harm aquatic species. Finally, runoff of sediment, mostly from soil erosion, has harmed the bay’s seagrasses on which crabs and young fish depend.
A century ago, oysters were so abundant that they filtered and cleaned the Chesapeake’s entire volume of water every 3 days. This important form of natural capital provided by these keystone species helped reduce excess nutrients and algal blooms. Now the oyster population has been reduced to the point where this filtration process takes a year. Since 2007, the oyster population has grown but in 2018, was still low compared to historic levels.
In 1983, the United States implemented the Chesapeake Bay Program. In this ambitious attempt at integrated coastal management, citizens’ groups, communities, and state and federal governments worked together to reduce pollution in the bay. One strategy was to set land-use regulations to reduce agricultural and urban runoff in the bay’s drainage area. Other strategies included banning phosphate detergents, upgrading sewage treatment plants, and monitoring industrial discharges more closely. Watershed management plans have been established and sensitive land areas have been protected through donations or purchases by conservation organizations. Some adjoining wetlands have been restored and large areas of the bay were replanted with seagrasses to help filter out excessive nutrients and other pollutants.
Scientists are working to learn more about the little-understood marine ecosystems, our effects on them, and the ways in which we can seek to preserve them (Individuals Matter 8.1). We examine human impacts on aquatic life zones more closely in Chapters 11 and 19.
Individuals Matter 8.1
Enric Sala: Working to Protect Ocean Ecosystems
Enric Sala/National Geographic Image Collection
Marine biologist Enric Sala has made a career of working to protect undisturbed marine ecosystems. He travels to remote areas with the goal of learning what marine ecosystems were like before human activities disrupted them. In 2008, he launched National Geographic’s Pristine Seas project to find, survey, and help protect the last wild places in the ocean. Pristine Seas aims for an ocean where representative examples of all major ecosystems are protected, so that they can be healthier, more productive, and more resilient to the impacts of ocean warming and acidification.
After exploring the Southern Line Islands undisturbed coral reef system in the South Pacific, Sala reported that “all the scientific data confirm that humans are the most important factor in determining the health of coral reefs.” He says that reefs are killed by “a combination of the local impact of human activities such as fishing and pollution and the global impact of human-induced climate change.”
Sala suggests that in order to allow coral reefs and other systems to survive and function, we need to “take out less and throw in less.” One way to accomplish this is to establish large areas of protected ocean habitat, free of human activities of any kind. Sala was instrumental in establishing such marine protected areas (MPAs). As of 2019, Pristine Seas had inspired the creation of 21 MPAs, covering more than 5 million square kilometers (2 million square miles)—more than ten times the area of Spain. Every year, Sala and his team of scientists and filmmakers spend weeks at sea and thousands of hours underwater seeking out the least understood places in hopes of creating more MPAs. For his outstanding scientific work, Sala has been named a National Geographic Explorer.
Critical Thinking
1. How might the loss of most of the world’s remaining shallow tropical coral reefs (Core Case Study) affect your life and the lives of any children or grandchildren you might have? What are two things you could do to help reduce this loss?
Change font size
8.4Importance of Freshwater Ecosystems
· LO 8.4AList six ecosystem services and six economic services provided by freshwater ecosystems.
· LO 8.4BDescribe the four distinct life zones found in deep lakes in terms of sunlight and types of organisms found in each.
· LO 8.4CExplain how a lake can become eutrophic and include the effects of human inputs in your explanation.
· LO 8.4DDescribe the three zones through which a typical stream flows.
· LO 8.4EList six ecosystem services provided by inland wetlands.
Change font size
8.4aWater Stands in Some Freshwater Systems and Flows in Others
Precipitation that does not sink into the ground or evaporate becomes surface water —freshwater that flows or is stored in bodies of water on the earth’s surface. Freshwater aquatic life zones include standing (lentic) bodies of freshwater such as lakes, ponds, and inland wetlands, and flowing (lotic) systems such as streams and rivers.
Surface water that flows into such bodies of water is called runoff . A watershed , or drainage basin , is the land area that delivers runoff, sediment, and dissolved substances to a stream, lake, bay (Figure 8.13), or wetland. Although freshwater systems cover less than 2.5% of the earth’s surface, they provide a number of important ecosystem and economic services (Figure 8.14).
Figure 8.14
Freshwater systems provide many important ecosystem and economic services.
Critical Thinking:
1. Which two ecosystem services and which two economic services do you think are the most important? Why?
Top: Galyna Andrushko/ Shutterstock.com. Bottom: Kletr/ Shutterstock.com.
Lakes are large natural bodies of standing freshwater formed when precipitation, runoff, streams, rivers, and groundwater seepage fill depressions in the earth’s surface. Causes of such depressions include glaciation, displacement of the earth’s crust, and volcanic activity. A lake’s watershed supplies it with water from rainfall, melting snow, and streams.
Freshwater lakes vary in size, depth, and nutrient content. Deep lakes normally consist of four distinct life zones that are defined by their depth and distance from shore (Figure 8.15). The top layer, called the littoral zone, is near the shore and consists of the shallow sunlit waters to the depth at which rooted plants stop growing. It has a high level of biodiversity because of ample sunlight and inputs of nutrients from the surrounding land. Species living in the littoral zone include many rooted plants; animals such as turtles, frogs, and crayfish; and fish such as bass, perch, and carp.
Figure 8.15
A typical deep temperate-zone lake has distinct zones of life.
Critical Thinking:
1. How are deep lakes similar to tropical rain forests? (Hint: See Figure 7.20)
Lake Zonation
Watch this animation to learn about the different parts of a lake.
Volume 90%
Copyright © Cengage Learning. All Rights Reserved.
The next layer is the limnetic zone, the open, sunlit surface layer away from the shore that extends to the depth penetrated by sunlight. This is the main photosynthetic zone of the lake, the layer that produces the food and oxygen that support most of the lake’s consumers. Its most abundant organisms are phytoplankton and zooplankton. Some large species of fish spend most of their time in this zone, with occasional visits to the littoral zone to feed and reproduce.
The profundal zone is the volume of deeper water lying between the limnetic zone and the lake bottom. It is too dark for photosynthesis. Without sunlight and plants, oxygen levels are often low. Fishes adapted to the lake’s cooler and darker water, such as perch, are found in this zone.
The bottom of the lake is called the benthic zone, inhabited mostly by decomposers, detritus feeders, and some bottom-feeding species of fish such as catfish. The benthic zone is nourished mainly by dead matter that falls from the littoral and limnetic zones and by sediment washing into the lake.
8.4bSome Lakes Have More Nutrients than Others
Ecologists classify lakes according to their nutrient content and primary productivity. Lakes that have a small supply of plant nutrients are called oligotrophic lakes . This type of lake (Figure 8.16) is often deep and can have steep banks. Glaciers and mountain streams supply water to many of these lakes, which usually have crystal-clear water and small populations of phytoplankton and fish species, such as smallmouth bass and trout. Because of their low levels of nutrients, these lakes have a low net primary productivity (NPP).
Figure 8.16
Trillium Lake in the U.S. state of Oregon with a view of Mount Hood.
Over time, sediments, organic material, and inorganic nutrients wash into most oligotrophic lakes, and plants grow and decompose to form bottom sediments. The process by which lakes gain nutrients is called eutrophication . A lake with a large supply of nutrients is called a eutrophic lake (Figure 8.17). Such lakes typically are shallow and have murky brown or green water. Because of their high levels of nutrients, these lakes have a high NPP. Most lakes fall somewhere between the two extremes of nutrient enrichment.
Figure 8.17
This eutrophic lake has received large flows of plant nutrients. As a result, its surface is covered with mats of algae.
Nicholas Rjabow/ Dreamstime.com
Human inputs of nutrients through the atmosphere and from urban and agricultural areas within a lake’s watershed can accelerate the eutrophication of the lake. This process, called cultural eutrophication , often puts excessive nutrients into lakes.
Change font size
8.4cFreshwater Streams and Rivers
In a watershed, water accumulates in small streams that join to form rivers. Collectively, streams and rivers carry huge amounts of water from highlands to lakes and oceans. Typically, a stream flows through three zones (Figure 8.18): the source zone, which contains headwater streams found in highlands and mountains; the transition zone, which contains wider, lower-elevation streams; and the floodplain zone, which contains rivers that empty into larger rivers or into the ocean. Rivers and streams can differ from this generalized model.
Figure 8.18
Three zones in the downhill flow of water: the source zone (inset photo), the transition zone, and the floodplain zone.
Critical Thinking:
1. How might the building of many dams and reservoirs along a river’s path to the ocean affect its sediment input into the ocean and change the river’s delta?
In the narrow source zone (Figure 8.18, left), headwater streams are usually shallow, cold, clear, and swiftly flowing (Figure 8.18, inset photo). As this water tumbles over rocks, waterfalls, and rapids, it dissolves large amounts of oxygen from the air. Most of these streams are not very productive because of a lack of nutrients and primary producers. Their nutrients come primarily from organic matter, mostly leaves, branches, and the bodies of living and dead insects that fall into the stream from nearby land.
The source zone is populated by cold-water fish species that need lots of dissolved oxygen. Fishes in this habitat, such as trout and minnows, tend to have streamlined and muscular bodies that allow them to swim in the rapid, strong currents. Other animals such as riffle beetles have compact, hard, or flattened bodies that allow them to live among or under stones in fast-flowing headwater streams. Most of the plants in this zone are algae and mosses attached to rocks and other surfaces underwater.
In the transition zone (Figure 8.18, center), headwater streams merge to form wider, deeper, and warmer streams that flow down gentler slopes with fewer obstacles. They can be more turbid (containing suspended sediments) and slower flowing than headwater streams, and they tend to have less dissolved oxygen. The warmer water and other conditions in this zone support more producers, as well as cool-water and warm-water fish species (such as black bass) with slightly lower oxygen requirements.
As streams flow downhill, they shape the land through which they pass. Over millions of years, the friction of moving water has leveled mountains and cut deep canyons. Streams and rivers carry sand, gravel, and soil and deposit them as sediments in low-lying areas. In these floodplain zones (Figure 8.18, right), streams join into wider and deeper rivers that flow across broad, flat valleys. Water in this zone usually has higher temperatures and less dissolved oxygen than water in the two higher zones. The slow-moving rivers sometimes support large populations of producers such as algae and cyanobacteria, as well as rooted aquatic plants along the shores.
Because of increased erosion and runoff over a larger area, water in the floodplain zone often is muddy and contains high concentrations of silt. These murky waters support distinctive varieties of fishes, including carp and catfish. At its mouth, a river may divide into many channels as it flows through its delta —an area at the mouth of a river built up by deposited sediment, usually containing coastal wetlands and estuaries (Figure 8.6).
Connections
Stream Water Quality and Watershed Land
Streams receive most of their nutrients from bordering land ecosystems. Such nutrients come from falling leaves, animal feces, insects, and other forms of biomass washed into streams during heavy rainstorms or by melting snow. Chemicals and other substances flowing off the land can also pollute streams. Thus, the levels and types of nutrients and pollutants in a stream depend on what is happening in the stream’s watershed.
Change font size
8.4dFreshwater Inland Wetlands Are Vital Sponges
Inland wetlands are lands located away from coastal areas that are covered with freshwater all or part of the time—excluding lakes, reservoirs, and streams. They include marshes (Figure 8.19, left), swamps (Figure 8.19, right), and prairie potholes (depressions created by glaciers). Other examples are floodplains, which receive excess water from streams or rivers during heavy rains and floods.
Figure 8.19
This great white egret lives in an inland marsh in the Florida Everglades (left). This cypress swamp (right) is located in the U.S. state of South Carolina.
FloridaStock/ Shutterstock.com; Jayne Chapman/ Shutterstock.com
Some wetlands are covered with water year-round. Others, called seasonal wetlands, remain under water or are soggy for only a short time each year. The latter include prairie potholes, floodplain wetlands, and arctic tundra (see Figure 7.16, bottom). Some can stay dry for years before water covers them again. In such cases, scientists must use the composition of the soil or the presence of certain plants (such as cattails and bulrushes) to determine that a particular area is a wetland. Wetland plants are highly productive because of an abundance of nutrients available to them. Wetlands are important habitats for muskrats, otters, beavers, migratory waterfowl, and other bird species.
Inland wetlands provide a number of free ecosystem and economic services. They
· filter and degrade toxic wastes and pollutants;
· reduce flooding and erosion by absorbing storm water and releasing it slowly and by absorbing overflows from streams and lakes;
· sustain stream flows during dry periods;
· recharge groundwater aquifers;
· maintain biodiversity by providing habitats for a variety of species;
· supply valuable products such as fishes and shellfish, blueberries, cranberries, and wild rice; and
· provide recreation for birdwatchers, nature photographers, boaters, anglers, and waterfowl hunters.
8.5Effects of Human Activities on Freshwater Ecosystems
· LO 8.5ADescribe four major ways in which human activities are disrupting and degrading ecosystem and economic services provided by freshwater ecosystems.
· LO 8.5BExplain how human activities, especially the engineering of the flow of the Mississippi River, have affected the river’s delta.
· LO 8.5CExplain why the disastrous flooding of New Orleans in 2005 was caused largely by human activities.
Change font size
8.5aHuman Activities and Freshwater Systems
Human activities are disrupting and degrading many of the ecosystem and economic services provided by freshwater rivers, lakes, and wetlands in four major ways. First, dams and canals restrict the flows of about 40% of the world’s 237 largest rivers. This alters or destroys terrestrial and aquatic wildlife habitats along these rivers and in their coastal deltas and estuaries by reducing the water flow and the flow of sediments to river deltas. This can lead to degraded coastal wetlands and greater damage from coastal storms (see Case Study that follows).
Case Study
River Deltas and Coastal Wetlands—Threatened Components of Natural Capital
Coastal river deltas, mangrove forests, and coastal wetlands provide considerable natural protection against flood and wave damage from coastal storms, hurricanes, typhoons, and tsunamis. They weaken the force of waves and absorb excess storm water like sponges.
When we remove or degrade these ecosystems, any damage from a natural disaster such as a hurricane is intensified. As a result, flooding in places such as New Orleans, Louisiana (USA), and Venice, Italy, is a largely self-inflicted unnatural disaster. For example, Louisiana, which contains about 40% of all coastal wetlands in the lower 48 states, has lost more than a fifth of such wetlands since 1950 to oil and gas wells and other forms of coastal development.
Dams, levees, and hydroelectric power plants have been built on many of the world’s rivers to control water flows and to generate electricity. This helps to reduce flooding along rivers, but it also reduces flood protection provided by the coastal deltas and wetlands. Because river sediments are deposited in the reservoirs behind dams, the river deltas do not get their normal inputs of sediment to build them back up, and they subside, or sink into the sea.
For example, the Mississippi River once delivered huge amounts of sediments to its delta each year. However, the multiple dams, levees, and canals built in this river system funnel much of this sediment load through the wetlands and out into the Gulf of Mexico. Instead of building up delta lands, this causes them to subside. As many of the river delta’s freshwater wetlands have been lost to this subsidence, saltwater from the Gulf has intruded and killed many plants that depended on the river water, further degrading this coastal aquatic system.
Subsidence helps to explain why the city of New Orleans, Louisiana (Figure 8.20), has long been 3 meters (10 feet) below sea level. Dams and levees were built to help protect the city from flooding. However, in 2005 the powerful winds and waves from Hurricane Katrina overwhelmed these defenses. They have been rebuilt, but subsidence will put New Orleans further below sea level in the future. Add to this the reduced protection from degraded coastal wetlands, and you have a recipe for a major and possibly more damaging disaster if the area is struck by another major hurricane.
Figure 8.20
Much of the U.S. city of New Orleans, Louisiana, was flooded by the storm surge that accompanied Hurricane Katrina, which made landfall just east of the city on August 29, 2005.
National Oceanic and Atmospheric Administration (NOAA)
To make matters worse, global sea levels have risen almost 0.3 meters (1 foot) since 1900 and are projected to rise another 0.3–0.9 meter (1–3 feet) by the end of this century. This is because climate change is warming the ocean, causing its waters to expand. In addition, the melting of glaciers and other land-based ice is adding to the ocean’s volume. Such a rise in sea level would put many of the world’s coastal areas, including New Orleans and most of Louisiana’s present-day coast, under water (Figure 8.21).
The areas in red represent projected coastal flooding that would result from a 1-meter (3-foot) rise in sea level due to projected climate change by the end of this century.
Used by permission from Jonathan Overpeck and Jeremy Weiss, University of Arizona
Second, flood control levees and dikes built along rivers disconnect the rivers from their floodplains, destroy aquatic habitats, and alter or degrade the functions of adjoining wetlands.
Third, cities and farms add pollutants and excess plant nutrients to nearby streams, rivers, and lakes. For example, runoff of nutrients into a lake (Figure 8.17) causes explosions in the populations of algae and cyanobacteria, which deplete the lake’s dissolved oxygen. Fishes and other species may then die off, which can mean a major loss in biodiversity.
Fourth, many inland wetlands have been drained or filled to grow crops or have been covered with concrete, asphalt, and buildings. More than half of the inland wetlands estimated to have existed in the continental United States during the 1600s are gone. About 80% of these lost wetlands were drained to grow crops. The rest were lost to mining, logging, oil and gas extraction, highway construction, and urban development. The heavily farmed U.S. state of Iowa has lost about 99% of its original inland wetlands.
This loss of natural capital has been an important factor in increasing flood damage in parts of the United States. Many other countries have suffered similar losses. For example, 80% of all inland wetlands in Germany and France have been destroyed.
When we look further into human impacts on aquatic systems in Chapter 11, we will also explore possible solutions to environmental problems that result from these impacts, as well as ways to help sustain aquatic biodiversity. This area of study will offer great opportunities to young scientists and other professionals in the years to come.
Big Ideas
· Saltwater and freshwater aquatic life zones cover almost three-fourths of the earth’s surface, and oceans dominate the planet.
· The earth’s aquatic systems provide important ecosystem and economic services.
· Certain human activities threaten biodiversity and disrupt ecosystem and economic services provided by aquatic systems.
Change font size
Tying It All TogetherCoral Reefs and Sustainability
This chapter’s Core Case Study pointed out the ecological and economic importance of the world’s incredibly diverse coral reefs. They are living examples of the three scientific principles of sustainability in action. They thrive on solar energy, play key roles in the cycling of carbon and other nutrients, and sustain a great deal of aquatic biodiversity.
In this chapter, we have seen that coral reefs and other aquatic systems are being severely stressed by a variety of human activities. Research shows that when such harmful human activities are reduced, some coral reefs and other endangered aquatic systems can recover quickly.
As with terrestrial systems, scientists have made a start in understanding the ecology of the world’s aquatic systems and how humans are degrading and disrupting the ecosystem and economic services they provide. However, we still know far too little about these vital parts of the earth’s life-support system. Scientists argue that we urgently need more research on the components of aquatic life zones, on how they are interconnected, and on which systems are in the greatest danger of being disrupted.
We can take the lessons on how life in aquatic ecosystems has sustained itself for billions of years and use these scientific principles of sustainability to help sustain our own systems and the ecosystems on which we depend. By relying more on solar energy and less on fossil fuels, we could drastically cut pollution of aquatic systems and emissions that are causing ocean warming and ocean acidification. By reusing and recycling more of the materials and chemicals we use, we could further reduce pollution and disruption of the chemical cycling within aquatic systems. Moreover, by learning more about aquatic biodiversity and its importance, we could go a long way toward preserving it and sustaining its valuable ecosystem services.
Change font size
Chapter Review
Critical Thinking
1. What are three steps that governments and private interests could take to protect the world’s remaining coral reefs (Core Case Study)?
3. You are a defense attorney arguing in court for protecting a coral reef (Core Case Study) from harmful human activities. Give your three most important arguments for the defense of this ecosystem.
4. How would you respond to someone who argues that we should use the deep portions of the world’s oceans to deposit our radioactive and other hazardous wastes because the deep oceans are vast and are located far away from human habitats? Give reasons for your response.
5. From the list of threats to marine ecosystems listed in Figure 8.12, pick the three that you think are the most serious. For each of them, if it continues to degrade ocean ecosystems during your lifetime, how might this affect you? Can you think of ways in which you might be contributing to each problem? List three things you could do to reduce your impact?
6. Suppose a developer builds a housing complex overlooking a coastal marsh (Figure 8.7) and the result is pollution and degradation of the marsh. Describe the effects of such a development on the wildlife in the marsh, assuming at least one species is eliminated as a result.
7. Suppose you have a friend who owns property that includes a freshwater wetland and the friend tells you she is planning to fill the wetland to make more room for her lawn and garden. What would you say to this friend?
8. Congratulations! You are in charge of the world. What are the three most important features of your plan to help sustain the earth’s aquatic biodiversity?
Change font size
Chapter Review
Doing Environmental Science
1. Find an aquatic ecosystem near where you live or go to school, such as a lake or wetland. Study and write a description of the system, including its dominant vegetation and any animal life that you are aware of. Also, note how any human disturbances have changed the system. Return to the system after a month or two and note any changes, based on your earlier notes. Compare your notes with those of your classmates.
Change font size
Chapter Review
Data Analysis
Some 45–53% of the world’s shallow coral reefs have been destroyed or severely damaged (Core Case Study). A number of factors have played a role in this serious loss of aquatic biodiversity, including ocean warming, sediment from coastal soil erosion, excessive algal growth from fertilizer runoff, coral bleaching, rising sea levels, ocean acidification, overfishing, and damage from hurricanes.
In 2005, scientists Nadia Bood, Melanie McField, and Rich Aronson conducted research to evaluate the recovery of coral reefs in Belize from the combined effects of mass bleaching and Hurricane Mitch in 1998. Some of these reefs are in protected waters where no fishing is allowed. The researchers speculated that reefs in waters where no fishing is allowed should recover faster than reefs in waters where fishing is allowed. The graph to the left shows some of the data they collected from three highly protected (unfished) sites and three unprotected (fished) sites to evaluate their hypothesis. Study this graph and then answer the following questions.
1. By about what percentage did the mean coral cover drop in the protected (unfished) reefs between 1997 and 1999?
2. By about what percentage did the mean coral cover drop in the protected (unfished) reefs between 1997 and 2005?
3. By about what percentage did the coral cover drop in the unprotected (fished) reefs between 1997 and 1999?
4. By about what percentage did the coral cover change in the unprotected (fished) reefs between 1997 and 2005?
5. Do these data support the hypothesis that coral reef recovery should occur faster in areas where fishing is prohibited? Explain.