Describe the ecosystems along continental margins and human influences on those ecosystems.

Life in the oceans is not grouped into biomes, as it is on land. Marine biological interactions are instead referred to as ecosystems, and more locally, as communities of interacting organisms.

Our tour of life in the oceans begins at the margins of continents, where people interact intensively with marine ecosystems. Marine ecosystems on the margins of continents are readily accessible to be seen, enjoyed, used, and influenced by people. These ecosystems, in the order in which they are covered in this chapter, include coral reefs, mangrove forests, seagrass meadows, estuaries, kelp forests, and beaches and rocky shores.

Coral reefs, often called the rainforests of the sea, are one of nature’s greatest displays of life. Coral reefs are the most biodiverse marine ecosystems. There are two broad types of corals: cold-water corals and warm-water corals. Cold-water corals live at all latitudes and at great depths in the ocean, but compared with warm-water corals, less is known about them.

This section will focus on warm-water corals. Warm-water corals live only near the sea surface in coastal regions at latitudes less than 30° north and south (Figure 10.12).

Coral reefs occupy only 0.1% of the area near the ocean’s surface, about 286,000 km² (110,000 mi²), but they are used by approximately 4,000 species of marine fishes. Scientists are still learning about coral reefs, and perhaps less than 10% of coral reef species have been identified.

Corals look like plants, but they are animals called zooxanthellae, which are related to jellyfishes. There are about 800 known species of hard corals. Most corals share an obligate mutualism with algae (see Section 7.2): The corals filter nutrients from seawater and make them available to photosynthetic algae living within the corals’ tissues. The algae, in turn, provide the corals with carbohydrates and fats.

Corals require clean, well-lighted, warm water between 18°C and 30°C (64°F and 86°F). They are intolerant of low salinity and low light levels. Therefore, they are absent at the mouths of large rivers, such the Amazon River or the great rivers of Southeast Asia, where the water is too fresh and suspended sediments diminish the light. Furthermore, corals cannot survive exposure to air above the low-tide line.

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Reef-building corals create hard limestone shells that form rocklike reefs. Only the topmost layer of the reef is alive; each generation of corals builds on top of the preceding generation’s shells. The hard skeletons build the reef through time.

There are three kinds of coral reefs: fringing reefs, barrier reefs, and atolls. A fringing reef forms near and parallel to a coastline. A barrier reef runs parallel to a coastline and forms a deep-water lagoon behind it. An atoll is a ring of coral reefs with an interior lagoon that forms around a sinking volcano. Lagoons are fully or partly enclosed stretches of salt water formed by a coral reef or sand spit. Charles Darwin (see Section 7.1) was the first to recognize that atolls develop as volcanoes gradually sink beneath the ocean’s surface. All three reef types can develop during the sequence of atoll formation, as illustrated in Figure 10.13.

Threats to Coral Reefs

Worldwide, coral reefs are in decline, and their decline points to deeper, global-scale changes occurring in Earth’s physical systems. Scientists estimate that 90% of coral reefs will be threatened by 2030, and by 2050, all reefs could be threatened. As Figure 10.14 shows, fewer than half the world’s coral reefs are currently in a healthy state.

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The first sign of trouble for a reef is coral bleaching. Coral bleaching is the loss of coloration in corals that occurs when they have been stressed or have died. When corals become stressed, they expel the algae living within them. The corals then starve because they are unable to produce enough food. Most of the color of corals comes from their algal partners. Once the algae are gone, the corals appear white, as Figure 10.15 shows.

Coral bleaching events are increasing for a number of different reasons. Warm water associated with strong El Niños (see Section 5.5) brings widespread coral bleaching events. Many other factors also stress corals and trigger bleaching:

In April 2013, research published in the journal Science reported that many coral species have the ability to survive bleaching events and recover more quickly than previously known. Following bleaching events, corals and reef biodiversity are able to rebound within about a decade. This finding is good news because it suggests that, once stressors are reduced, the decline of the world’s coral reefs can be slowed or possibly reversed.

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The Value of Coral Reefs

About a half billion people use coral reefs directly or rely on them for their food and economic livelihood. Compounds to fight cancer, HIV, and malaria have been discovered in coral reef species. International tourism in many tropical regions, such as the Caribbean Sea and Hawai‘i, is centered on coral beaches and coral reefs. In the Caribbean Sea alone, coral reefs are estimated to provide roughly $5 billion yearly in the form of tourism, fishing, and food security. Picture This explores the Great Barrier Reef of Australia in the context of tourism revenue.

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Mangrove forests are ecosystems dominated by saltwater-tolerant coastal shrubs and trees. They are found in the tropics and subtropics. These unique halophytic (salt-tolerant) plants, called mangroves, occupy, or once occupied, nearly all tropical coastal waters. Where the water is warm enough, their range extends far into the subtropics of North America, Brazil, and eastern Australia (Figure 10.16).

The mud in which mangrove roots grow is almost completely devoid of oxygen. In response, mangrove trees have evolved pneumatophore (air-breathing) roots (Figure 10.17). These roots allow the plant to take in air through small tubes, called lenticels, when they are exposed at low tide.

Mangrove forests are important ecosystems because they function as nurseries for coral reef fishes as well as many commercial fish species. Marine invertebrates such as oysters, clams, mussels, barnacles, and anemones affix themselves to the roots of mangrove trees. The roots of mangroves trap and hold loose sediments that would otherwise be swept away by currents. The resulting mud in which mangroves grow provides habitat for crabs and sea worms, which in turn attract other organisms, such as predatory fish and birds.

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As Figure 10.18 details, there are many threats to mangrove forests. The single most important force of degradation for mangrove forests is shrimp aquaculture (farming). Between 1980 and 2000, more than 30% of the world’s mangrove forests were converted to farms growing shrimp for export to North America, Europe, and Asia.

Half the world’s original mangrove forests are now gone, and these ecosystems are estimated to have dropped from 32 million to 15 million ha (80 million to 37 million acres) as of 2007. About 1% of the remaining mangrove forest area, or 142,000 ha (350,000 acres), is lost each year. At this rate, mangrove forests outside of protected reserves will be gone by the end of the century.

Mangroves are a renewable economic resource for people, and they provide a defense against dangerous surges of water caused by hurricanes or tsunamis (see The Human Sphere in Chapter 14). Coastlines with healthy mangrove forests experience considerably less flooding and erosion compared with regions where the mangrove forests are gone. Protecting mangroves goes hand-in-hand with protecting the livelihoods of people and the habitats of species.

Seagrass meadows, like mangrove forests, are an often-overlooked marine ecosystem. Seagrass meadows are shallow coastal ecosystems dominated by flowering plants that resemble grasses. They are important natural fish nurseries. Although they are most common in tropical waters, seagrass meadows are found in temperate waters as well (Figure 10.19).

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The plant species we call seagrasses are not true grasses. The seagrasses comprise about 60 different species within four families of flowering plants, all of which look like grasses, with flat and narrow leaf blades. Some common seagrasses are eelgrass (Zostera spp.), turtle grass (Thalassia testudinum), and manatee grass (Syringodium filiforme).

Seagrass meadows provide critical habitat for many species of marine life in their juvenile stages of development. Some animals, such as green sea turtles, as well as manatees (Trichechus spp.) and their relatives, dugongs (Dugong dugong), graze on seagrasses. The plants trap sediment, which reduces suspended sediments and improves coastal water quality. Their roots stabilize sediments in coastal areas, reducing coastal erosion.

Several anthropogenic forces, summarized in Table 10.2, are causing losses of seagrass meadows globally. Like coral reefs and mangrove forests, seagrass meadows contribute most to local economies when they are preserved rather than converted to another use. In the Mediterranean Sea and elsewhere, effective efforts to save and restore seagrass meadows have focused on stemming pollution from stream runoff in coastal areas (particularly phosphorus and nitrogen from agricultural fields), establishing protected areas, and replanting seagrass meadows that have diminished or have been lost.

An estuary is a brackish-water ecosystem at the mouth of a river that is influenced by tides. Brackish water forms where fresh river water and ocean water mix to produce water that is salty, but less salty than ocean water. Salinity in estuaries varies considerably, depending on river discharge and tidal levels.

Many large estuaries are about 10,000 years old. They were formed as the great continental ice sheets in the Northern Hemisphere (see Section 6.2) began melting 15,000 years ago. The meltwater drained into the oceans and caused sea levels to rise about 85 m (280 ft). Coastal areas were flooded and river-carved coastlines were drowned, forming many estuaries. These partially flooded coastal rivers valleys are called rias. San Francisco Bay, Rio de la Plata, the mouth of the Amazon River, Puget Sound in Washington State, and the Chesapeake Bay are some of the largest estuaries (Figure 10.20).

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In the tropics, mangrove forests colonized these drowned river environments. At middle and high latitudes, a variety of salt-tolerant herbaceous plants dominate estuarine environments. Common estuary plants include cordgrass (Spartina alterniflora), pick-leweed (Salicornia spp.), and needlerush (Juncus roemerianus).

As rivers flow into estuaries, they bring in organic-rich clays and silts. These sediments are deposited and accumulate into mudflats, which are exposed twice daily during low tide. All estuaries have deep deposits of mud that are rich in organic material. Primary productivity is high in estuaries, as plant growth is fueled by the nutrient-rich sediments.

Estuaries are important habitats for many kinds of organisms, particularly for the juvenile stages of many fish species. Some 75% of the commercial fish catch in the United States consists of fish that depend on estuaries for at least part of their life cycle. Many bird and mammal species also rely on estuary habitat. Migratory birds use estuaries as rest and feeding stops on their long-distance migrations.

Because estuaries are flat and have calm waters, they make ideal locations for human settlement. Seventy percent of the world’s largest cities, including London, Shanghai, Buenos Aires, Hong Kong, Boston, and New York City, are located on estuaries (or what were once estuaries). People expand settlements onto estuaries by first filling them in with debris, such as soil or even garbage, then expanding the cities onto the newly created surface.

Because of the heavy human influence on estuaries, they are the world’s most endangered marine ecosystems. Table 10.3 summarizes the threats to estuaries.

Estuaries provide many economic benefits to people. They not only provide shelter for juvenile commercial fish species, but also enhance tourism, recreation (including recreational fishing), and production of shellfish (such as oysters and abalone). Estuaries also absorb wave energy from large storms, such as hurricanes, significantly reducing coastal erosion and property loss. Furthermore, they filter pollutants from rivers before they enter the sea, significantly improving coastal water quality.

Kelp forests are marine ecosystems found in temperate and polar waters where the water temperature does not exceed 20°C (68°F) (Figure 10.21). They are dominated by large algae called kelp. There are three groups of kelp: green kelp (Chlorophyta), tan kelp (Phaeophyta), and red kelp (Rhodophyta). Only tan kelp form kelp forests.

Limited to zones with adequate light, kelp forests grow only in water no deeper than 50 m (165 ft). The kelp rely on holdfasts, which resemble roots, but function only to anchor the algae to the rocks. Many kelp grow quickly—some tan kelp can grow 1 m (3.3 ft) per day.

Communities of fish, sea urchins, anemones, sea stars, mollusks, and lobsters inhabit kelp forests. Kelp employ toxins to deter many grazers. Sea urchins, however, can eat kelp, and if their populations increase, kelp forests often suffer declines. Figure 10.22 explores the Pacific sea otter’s role as a keystone species in kelp forests.

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Like other coastal ecosystems, kelp forests are vulnerable to threats posed by human activities (Table 10.4). Fortunately, the rapid growth rates of kelp allow them to recover rapidly if degradation pressure stops.

The coastal ecosystems most familiar to people may be beaches and rocky shores. Most of us have strolled along a beach barefoot or gazed at the horizon from a rocky shore. Because they are exposed to the force of crashing waves and wind, beaches and rocky shores are high-energy environments. As a result, many organisms in these environments are either anchored to the rocks (such as barnacles, sea urchins, and sea stars) or hidden within the sand (such as clams, periwinkles, and crabs). Rocky coastlines sometimes harbor communities that live in tide pools, still pools that form at low tide (Figure 10.23).

In general, biodiversity is not particularly high in beach and rocky shore environments, but many organisms live only in these habitats or rely on them during at least part of their life cycle. Shorebirds such as gulls, sandpipers, terns, and pelicans are common on beaches and rocky shores. Marine mammals such as sea lions, seals, and sea otters are found in these environments as well. Sea turtles nest only on warm beaches in the tropics. Human settlement, pollution runoff, and erosion of sand are the most significant threats to beach and rocky shoreline ecosystems (Table 10.5).

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