15.2: Coral Reefs

Although some types of corals are present in all parts of the oceans, including Arctic seas, species of corals that build coral reefs, called hermatypic corals, grow only in areas where the water temperature never falls below about 18°C. Coral reefs are thus restricted to a broad band of mostly tropical waters between about 30°N and 30°S (Fig. 15-3). The range of coral reef occurrence extends into somewhat higher latitudes on the western side of each ocean because warm western boundary currents flow poleward.

Figure 15-3. The area of the oceans with average sea surface temperatures above 20°C extends to about 30° north and south of the equator in the Pacific and Indian oceans, and somewhat less in the Atlantic Ocean. The temperature never falls below 18°C in most of this area. Hermatypic corals grow only in these areas and only where other environmental conditions are suitable within these areas. Although many small tropical islands have extensive coral reefs, they are too small to show in this figure.

Environmental Requirements for Coral Reef Formation

Reef-building corals require an appropriate substrate on which to attach, and they have a symbiotic relationship with zooxanthellae, a nonmotile form of dinoflagellate that lives within the coral’s tissues. Because zooxanthellae need light to photosynthesize (CC14), living coral reefs are present only in waters where the seafloor is within the photic zone. In clear waters, corals can grow to depths of about 150 m, but in high-turbidity waters coral growth is reduced or prevented by two mechanisms: First, the higher turbidity reduces light penetration and limits the depth at which zooxanthellae can photosynthesize and, therefore, also limits the depth at which corals can grow. Second, large quantities of suspended sediment that cause high turbidity may smother the corals. Corals can clear away a certain amount of deposited sediment material, but they must use energy to do so, and they lose additional energy because they must also stop feeding in order to do so. When these energy costs are too high, the corals cannot survive. Therefore, coral reefs do not grow in coastal areas near river mouths or near other sources of large amounts of suspended matter, such as dredging projects. Corals also grow poorly in water of low or variable salinity.

The physical conditions necessary for coral growth (clear, warm, shallow waters with relatively invariable normal ocean salinity) are present primarily between about 23.5°N and 23.5°S. This is a region that we might expect to be a biological desert because surface waters are isolated by a steep permanent thermocline throughout most of the tropical oceans. In contrast, instead of biological deserts, coral reefs are areas of high productivity with an amazing diversity (CC17) of fish and invertebrate species. In what may seem to be a paradox, waters surrounding reefs are clear blue and have extremely small populations of phytoplankton. The waters over the reef itself have somewhat higher phytoplankton populations and primary productivity, but these populations are extremely small in comparison with those in upwelling-zone ecosystems and cannot account for the high productivity of coral reefs.

Factors Affecting Coral Reef Productivity

The reasons for the anomalously high productivity of coral reefs are somewhat complex and not fully understood, but they appear to be related to a combination of physical conditions and the unique relationships among the reef organisms.

First, hermatypic corals and their associated zooxanthellae act together in a mutualistic (mutually beneficial) relationship to create a very effective mechanism for collecting, concentrating, and rapidly recycling nutrients. Zooxanthellae live embedded in the coral’s tissues and use solar energy that penetrates the coral’s transparent tissues to produce food by chemically recombining the coral’s waste products. Carbon dioxide and nutrients, which are released by the coral through its digestive processes, are transferred directly to zooxanthellae and converted by photosynthesis into organic matter. In turn, as much as 60% of the organic matter created by the zooxanthellae through photosynthesis is released through the zooxanthellae cell wall directly into the coral tissue, providing food for the coral. In this way, nutrients are continually recycled, and food is continually produced by the algae and consumed by the coral. The coral–zooxanthellae association ensures very little loss of either nutrients or food to the surrounding water. Corals feed on zooplankton to supplement the food supplied by their internal zooxanthellae. Thus, the small amounts of nutrients that are lost from the coral–zooxanthellae partnership are continuously replaced. This efficient nutrient retention and recycling mechanism is believed to be the main reason for the high productivity of coral reefs.

The second reason for their high productivity is that most reefs are built on the sides of submarine mountains or on the fringes of landmasses with very narrow continental shelves. Indeed, the growth of corals off coasts where sea level has risen during the past several thousand years has created very steep drop-offs at the outer edges of many reefs (Figs. 4-28, 15-4). Ocean currents that flow past such steep continental shelves and islands form eddies. The eddies create vertical water movements that can bring nutrient-rich deep water, at least episodically, into the photic zone of the reef.

Figure 15-4. The structure of a typical coral reef.

The third reason for the high productivity of coral reefs is that many reef ecosystems are partially closed systems, within which most nutrients not retained in the coral–zooxanthellae association are continually recycled. Most of the reef floor is shallow and within the photic zone. Hence, organic detritus created on the reef settles largely on the reef floor, where it is consumed by decomposers that release nutrients to be recycled. In addition, most fishes are permanent residents of the reef. Nutrients in their urine and feces are released almost entirely back to waters of the reef, where they are rapidly recycled and taken up again by zooxanthellae or phytoplankton.

Primary Producers in the Coral Reef Community

Although zooxanthellae are important primary producers in coral reef ecosystems, they account for only a small proportion (generally less than 5%) of the biomass of photosynthesizers in these ecosystems. Reef ecosystems are dominated by benthic or encrusting microalgae and a variety of attached macroalgae (Fig. 15-4). The biomass of these algae generally exceeds the animal biomass in the reef ecosystem by as much as three times. In fact, the hard parts of calcareous algae are responsible for building much of the reef structure.

Algae attached to the solid substrate of the reef are favored over phytoplankton in coral ecosystems because nutrients are recycled, made available, and rapidly reassimilated at these surfaces. Attached algae remove most of the recycled nutrients before they can diffuse into the water column and become available to phytoplankton. In addition, attached algae remain in the reef ecosystem, where nutrients are available, whereas phytoplankton may be transported away from the reef into nutrient-depleted adjacent deep water.

Coral Reef Niches and Topography 

Coral reef ecosystems contain a bewildering variety of species. These species are distributed in niches that are defined by current and wave action, and by variations in salinity, water depth, turbidity, temperature, and other factors.

Figure 15-4 shows the general topographic features of a typical coral reef. In shallow waters of the lagoon, currents are generally weak and there is little wave action. Hence, sediments tend to accumulate in the lagoon and are removed primarily during major storms. Lagoon sediments sustain a wide variety of invertebrates that feed on suspended or deposited detritus. These detritus feeders include suspension feeders such as sea pens (Fig. 14-6a), and a variety of bivalve mollusks that live in the sediment and extend their feeding apparatus into the water column. Also present are many species of deposit feeders, including mollusks and worms that live in and sift through the sediment, sea cucumbers (Fig. 15-5a), urchins (Fig. 15-5c), sand dollars (Fig. 15-5d), and other species that feed on benthic algae growing on the sediment surface, and predators such as goatfishes (Fig. 14-13d) and sea stars (Fig. 15-5b) that hunt mollusks and other animals living in or on the sediment.

Figure 15-5. Inhabitants of the coral reef lagoon and outer reef flat. Many invertebrates—including (a) sea cucumbers (family Synaptidae, Indonesia), (b) sea stars such as this rhinoceros, or horned, sea star (Protoreaster nodosus, Papua New Guinea), (c) urchins (Astropyga radiata, Indonesia), and (d) the urchins’ close relatives the sand dollars (Clypeaster sp., Indonesia)—feed on algae, detritus, and microorganisms on sandy seafloors. (e) Spiked forms of hard corals, such as Acropora sp. (Papua New Guinea), grow in patches on sandy lagoon floors. (f) Elkhorn coral (Acropora palmata, Puerto Rico) grows in many areas on the outer reef flat of reefs that experience low wave energy.

In the lagoon, the growth of most corals is inhibited by the blanket of sediment that covers the seafloor. However, certain corals, such as Acropora (Fig. 15-5e,f), that grow up into the water column can prosper if sedimentation rates are sufficiently low and sediments are rarely resuspended by waves. Once established, these corals provide a sediment-free substrate for other species. Consequently, in quiet lagoons with little suspended sediment input and where salinity is not altered by freshwater input, irregularly shaped mounds of coral and associated species develop. These are called “patch reefs.”

At the lagoon’s outer edge is a reef flat, or reef terrace, that is relatively free of sediments because they are swept off the terrace by waves. It is an ideal location for coral growth. Coral grows upward until the reef terrace is only a few centimeters below the low-tide line. Further growth is inhibited because corals cannot survive for long periods out of water, although the surface of the reef terrace may be completely exposed to the atmosphere for short periods during low spring tides without killing the corals. Reef terrace corals are generally encrusting corals because the water is too shallow and the wave energy too high for corals that grow in other forms (e.g., Figs. 14-8a,b, 15-6e,f). The surface of the reef terrace is not smooth. It has many grooves and holes created primarily by invertebrates that eat or drill into the coral to obtain food or to create safe areas to shelter from larger predators.

On some reefs, a low island is formed by sediment accumulated during storms at the landward side of the reef terrace (Fig. 15-6). This island may be well enough established to support palm trees. The seaward edge of the reef usually has an irregular ridge, parts of which are shallow enough to emerge from the water, especially at low tide. The ridge is formed by intense wave action that periodically smashes against the reef’s outer edge and dislodges large chunks of the limestone substrate of the reef. The chunks are cemented back onto the reef by the calcareous algae that live in abundance in this region. Calcareous algae are abundant because wave energy is too intense for corals to grow effectively. The algae cement themselves to the reef surface with their calcium carbonate hard parts. The cemented algae can withstand intense wave action and can quickly colonize any new surface created by storm wave damage. These algae benefit from a continuous supply of very low concentrations of nutrients brought to the reef edge by currents and waves. Because calcareous algae are abundant and help to maintain this ridge, it is called the algal ridge.

Figure 15-6. Islands often form behind the outer reef flat as wave-driven debris accumulates in this area. The island can become high and stable enough to sustain vegetation, such as the palm trees and other plants on this island on a fringing reef in Fiji.

On the few reefs where wave energy is very low, no algal ridge or reef terrace is present. The outer reef may consist primarily of relatively robust branching corals, such as elkhorn coral (Fig. 15-5f), or the reef terrace may simply end at the reef edge. Stands of elkhorn coral characterize the seaward edge of several sheltered Caribbean reefs. Some reefs within tectonically active island chains, such as Palau in the Pacific Ocean, are protected from wave action by a barrier reef that surrounds groups of several or many islands. Inside some of these barrier reefs, the sheltered island shore has a fringing reef with a reef terrace that simply ends abruptly at the reef wall, which plunges vertically or even undercuts to depths that can exceed several hundred meters (Fig. 15-4).

Seaward of the algal ridge (or reef terrace if no algal ridge is present), the downward slope of a reef can range from very gradual to nearly vertical. The upper part of the outer slope (Fig. 15-4), called the “buttress zone,” is subject to strong scouring action of waves down to about 20 m. It is usually cut across at intervals by deeper channels separated by relatively high ridges, sometimes called a “tongue and groove” formation. The channels or grooves are cut by waves and may also be called “surge channels.” Sand and debris created by wave action on the reef are transported seaward through these grooves. Coral growth is limited in the grooves, but many invertebrates and fishes feed on detritus transported along them. Coral growth is more extensive on the relatively sediment-free ridges. However, because wave energy is high, these ridges sustain primarily robust massive varieties of coral, such as brain coral (Fig. 14-8a), and encrusting corals and algae.

Farther seaward, on the outer slope between about 20 and 50 m, is a transition from robust massive corals and encrusting species to still strong but less robust forms, such as Acropora (Fig. 15-5e,f), and then to more delicate varieties of corals, including black coral (Fig. 15-7), sea fans (Fig. 14-8e), and soft corals (Fig. 14-8d,f). The depth at which these transitions occur depends on the intensity of wave action. Delicate corals are present at shallower depths on leeward sides of coral-fringed islands than on windward sides.

Figure 15-7. Black coral colony (probably Cirrhipathes sp.) with a longnose hawkfish (Oxycirrhites typus, Papua New Guinea) hiding among its branches. The hard parts of black corals are a deep black color and are often used to make jewelry, but the polyps may be a variety of colors in the living coral.

Between about 50 and 150 m, the reef is dominated by delicate coral species that grow outward in slender fingers or arms. Such growth enables hermatypic corals to extend beyond the shadows of their neighbors in the never-ending competition to obtain light for their zooxanthellae. It also enables nonhermatypic corals, including the delicate and beautiful soft corals (Figs. 14-8d,f), to extend into the water that flows along the reef face to capture suspended food. Soft corals are most abundant in areas where currents are strongest and thus expose them to the greatest possible food supply.

Just as the types of coral that grow on the reef’s seaward side are determined primarily by depth and the intensity of wave action, species of invertebrates and fishes that inhabit or feed on the corals change with these factors and in response to changes in coral species. The variations are too complicated to review here but can provide fascinating study for scuba divers when no big animals such as sharks, manta rays, or turtles are present to capture their interest.