Coral Reefs: Fall of the Cities Beneath the Sea

“Coral reefs of the world have continued to decline since the previous GCRMN report in 1998. Assessments to late 2000 are that 27% of the world’s reefs have been effectively lost, with the largest single cause being the massive climate-related coral bleaching event of 1998. This destroyed about 16% of the coral reefs of the world in 9 months during the largest El Niño and La Niña climate changes ever recorded. While there is a good chance that many of the 16% of damaged reefs will recover slowly, probably half of these reefs will never adequately recover.”

— Status of Coral Reefs of the World: 2000

Five years ago I wrote a feature article on the plight of coral reefs entitled, “Coral Grief.” Some in the diving community objected to the title, saying that it had an unnecessarily negative tone. One reader even accused me of being an environmental extremist, and called the article “nothing but a Chicken Little warning about a nonexistent problem.”

If only that were true

Five years later I’m here to tell you that the original piece was not only an accurate assessment of the time, but that things have gotten even worse. As I stated in the first article, the dire warnings were heralded by a widely quoted plenary address to the seventh International Coral Reef Symposium by Dr. Clive Wilkinson of the Australian Institute of Marine Science, Global Coral Reef Monitoring Network (GCRMN). Wilkinson pronounced that as much as 10 percent of the world’s coral reefs were degraded beyond recovery, and that we were likely to lose another 60 percent by the middle of the 21st century.

As the new century dawned, Wilkinson and his colleagues at the GCRMN published an updated version of their hallmark report, “Status of Coral Reefs of the World.” But this time, I’ll let him speak for himself. His comments are in the “call out” to the left. Adding to Wilkinson’s argument, the World Resources Institute, in a comprehensive global assessment of coral reefs, found that 58 percent are under medium to high threat from human-related stress.

Just Why Are Reefs So Important?

While there’s no need to convince divers of the importance of healthy coral reefs, not everyone is a diver. In fact, most aren’t. So, why, for example, should an Iowa corn farmer care that the world’s coral reefs are in a tailspin? Well, while Farmer Brown from Iowa may never see a coral reef in his lifetime, he and everyone he knows and cares about is part of the biosphere — the envelope of Earth that contains all life on the planet.

Coral reefs are home to about a quarter of all identified marine species. And while that’s pretty impressive, when one considers that they account for an area of only 0.089 percent of the sea bottom — a mere 176,655 square miles (459,303 sq km) — the statistic is truly astounding. No terrestrial ecosystem comes even close to this figure. Furthermore, each year we humans take about 6 million tons of fish from coral reefs. At least 1 billion people on earth depend on coral reefs as their primary source of protein. But the list of benefits doesn’t stop there.

Coral reefs protect coastlines by absorbing wave energy. For many small islands the idea is simple: no reef to protect the island, no island. But this isn’t just a benefit provided to small islands. It’s been estimated that coral reefs protect around one-sixth of the world’s coasts, and that each square meter of reef protects about $47,000 in property value. In an attempt to further quantify their benefit, the World Resource Institute has calculated that each year the Earth’s coral reefs provide a total economic value (fisheries, tourism, coastal protection) of at least $375 billion.

Still, while dollars and cents tend to get one’s attention, there are far more incalculable — and, in the end, important — benefits of healthy coral reefs. One such benefit, as mentioned, is their diversity. Often termed the “tropical rain forests of the oceans,” reefs support an incredible array of organisms. While more species are found in tropical rain forests, more phyla — perhaps a truer measure of diversity — are seen on coral reefs. But why exactly is protecting biological diversity so important? To understand why, some have used the analogy of an airliner. One way you can think of our biosphere is by imagining every living thing on Earth aboard a grand flight of life — a “biospheric airplane.” In case you never noticed, what holds airplanes together are rivets. Now assume that the various species that populate the Earth are the rivets of the aircraft; and each species that goes extinct is like a rivet popping out of place. As you might expect, some rivets are more important than others to the structural integrity of the airplane. So, exactly how many rivets it will take before the plane comes apart is impossible to determine. But eventually, if too many rivets are lost — or only a few of especially critical ones are gone — the airplane comes crashing down. That’s why biodiversity is not only important, it’s vital. But the story isn’t over.

Where the analogy fails is that we know the function of every rivet in a real airplane; but in the biospheric airplane we haven’t even identified all the rivets, let alone understand their function. In fact, science has only identified about 1.6 million species on our planet, but conservative estimates are that 10 million to 30 million actually exist (some say as many as 100 million). It’s like having only a partial blueprint of an airplane to guide us if it breaks or malfunctions. It’s no mystery then why the eminent conservationist Aldo Leopold once said, regarding the need to protect diversity, “A wise tinkerer saves all the parts.”

From a purely ethical perspective, organisms should have the right to exist independent of their value to human beings. Yet the reality is that many of us only place value on other organisms if they somehow benefit humans. Here, too, coral reefs pass the test of human self-interest. Pharmacologists, for example, have found that coral reefs contain a plethora of biomedical compounds from antibiotics to anticancer agents (the drug AZT is one example); and they undoubtedly hold thousands more yet undiscovered. Just as the wanton destruction of the world’s rain forests could be tantamount to throwing away the cures for countless human diseases, the same thing could be happening by destroying the world’s coral reefs.

How Do Reefs Work?

It doesn’t take many dives on a coral reef before even the most scientifically inept begin to understand some fundamental principles. For example, common experience tells you that most corals do best in shallow, clear, warm water. In fact, they require a temperature range between 64 and 86 degrees Fahrenheit (18-30 degrees Celsius) (although there is some variation above and below this range). You may have also noted that, even if the water is warm, reefs that are too close to large landmasses — especially highly populated ones — are rarely luxuriant. This is often because of land runoff containing high levels of nutrients (such as human or agricultural waste), fresh water or high sediment loads.

Corals are colonial cnidarians — sort of like tiny upside-down jellyfish living together in mass — and all secreting a calcium carbonate (limestone) cup called a corallite. The individual coral animals, called polyps, are anchored within these limestone cups and collectively form massive colonies. They even share a connective tissue called a coenosarc.

Collectively, hard corals of the tropics are very impressive builders, forming the largest structure on earth manufactured by living organisms — the Great Barrier Reef of Australia. It’s even visible from outer space. Although not all corals build massive reefs, those that do are called hermatypic (mound building). This incredible capacity is due to a symbiotic relationship between the coral polyp and algae which reside deep within the polyp’s tissues. These forms of algae are called zooxanthellae (or just “zoox” for short) and, in essence, enable a coral colony to function as a combination plant and animal. The zoox produce food via photosynthesis, while the polyp catches plankton from the water column. Moreover, the byproducts of the algae (oxygen and sugars) are consumed by the polyp, whose byproducts (carbon dioxide and nitrogenous waste) are consumed by the zoox. Neat arrangement, huh? But because of the zoox’s dependence on light for photosynthesis, reef-building corals do not grow well below about 30 meters (100 ft). There’s just not enough light.

When a coral colony dies, either through nature or human-induced factors, it forms a substrate on which new corals grow. Sand and coral fragments are also cemented together by yet another major component of coral reefs — the coralline algae (a form of algae that itself secretes limestone). This cementing process consolidates sand and small pieces of coral, thereby filling in the spaces between the larger fragments of dead coral skeletons. Without this cementing process, coral reefs would be far less stable and highly susceptible to damage from waves and storms. Reef growth also occurs through consolidation of finer particles entrapped by turf algae (the fuzzy stuff you sometimes see on what appears to be a “dead” portion of a reef). Upward growth through continual deposition of limestone and consolidation of sediments allows a reef to keep pace with rising sea levels. That is, of course, provided the sea level rise isn’t too extreme. If it does rise too fast, the reef won’t be able to keep pace and will die.

Contrary to popular wisdom, all corals do not grow at the same rate. In fact, there are considerable differences among species. For example, branching corals like staghorn (Acropora cervicornis) can grow about 10 cm (4 inches) a year, while massive forms like boulder coral (Montastrea annularis) grow at one-tenth of this rate. There’s also a difference in regional growth. Some growth rates of branching (Acroporid) corals in Southeast Asia — the hot spot for coral diversity — are truly amazing.

Another aspect of these unique structures is that the term “coral” reef itself is a bit of a misnomer. At least, it doesn’t give one a full appreciation of the amazing menagerie of other organisms that call a reef home, nor of their importance to maintaining a functioning ecosystem. Bacteria and algae coat the sandy bottom and any portion of the reef not covered by living coral. This provides food for mollusks, crustaceans, sea cucumbers, sea urchins and herbivorous fish. These organisms, in turn, provide vital housekeeping functions that keep the ecosystem healthy, and become a source of food for other critters further up the food chain.

Other organisms play an important role in building the reef by eroding the massive limestone fortresses. It may sound strange to think of this erosion as a positive force, but it clearly is. The process produces additional habitat by creating living space within the reef. In fact, it’s estimated that as much as 40 percent of a “coral reef” is actually open space such as holes, caverns and grottos. Functionally, these voids provide both more space to live, and more diversity of habitat, than would a completely solid structure. Over time, boring organisms like sponges, worms and mollusks undermine the reef’s structure, making it susceptible to collapse during storms or intense waves. Yet the reef doesn’t die or wash away. Broken segments of coral provide new habitat and are eventually cemented back into the reef by coralline algae. The action of grazers such as parrotfish and sea urchins produce large quantities of sediment, which also become habitat for smaller fish and invertebrates.

Another surprising fact is the nature of the water surrounding coral reefs. While coral reefs are one of the most productive ecosystems on the planet, the waters that bathe them are among the most nutrient-poor, or as scientists term it, oligotrophic (low-food). This stark contrast to the teeming communities on coral reefs is why they are often described as being “oases in an oceanic desert.” Still, the water isn’t totally devoid of life. There is some plankton present, and it plays a vital role in the reef ecosystem. First, it provides food for sessile (stationary) organisms like corals and sponges, but plankton is also important in the life cycle of most reef inhabitants. The life history of the vast majority of reef fish and invertebrates has a larval planktonic stage, thus enabling them to disperse over long distances. This also explains why, although very little plankton exists seaward of the reef, the waters directly around it may contain quite a bit. As any night dive on a coral reef will prove, it can be alive with plankton. But this rich soup is mainly from local sources, and constitutes the larval form of reef creatures.

How Productive Are Reefs?

In biology, the term gross primary production (GPP) describes the total amount of living matter in a given area produced by plants and algae. (For a more in-depth look at the issue of primary production, see “Biological Oceanography: The Living Sea,” Dive Training, May 2001.) It’s a way of quantifying the very base of the food chain. In the open oceans of the tropics, GPP is very low, producing only about 18-50 grams of carbon per square meter per year (the traditional way that primary productivity is measured). In comparison, an apple contains about 50 grams of carbon, so you can see the ocean around coral reefs normally isn’t very productive.

On the reef itself, things are very different. Here, the GPP is between 30 and 250 times more than the surrounding ocean, often producing as much as 1,500-5,000 grams of carbon per square meter per year. This represents some of the highest rates of primary production in any natural ecosystem. This seeming violation of the laws of thermodynamics — the reef’s high productivity in a low-productivity ocean — is sometimes called “Darwin’s Paradox” and puzzled scientists until the 1950s. Although the full reason is quite complex and not completely understood, the basic answer to how this high gross productivity can occur in such nutrient-poor water is that corals and coral reef communities are extremely efficient at recycling nutrients (mainly nitrate and phosphate). So, any nutrients that do make their way on to the reef tend to be held there very tightly. Or, as one scientist has put it, “coral reefs just don’t leak much.”

Intuitively, from this high GPP it would seem reasonable to assume that coral reefs produce far more food than is needed by the local community, allowing for the export of a substantial surplus. This surplus or extra is termed “net productivity.” It’s the “profit” that’s left over after the “expenses” of living (respiration) have been deducted. But the reality is very different. The GPP produced by the reef is very nearly balanced by what it consumes. In fact, net productivity is often only 2-3 percent of the gross, and only slightly higher than the net productivity per unit area in the surrounding ocean water. It’s like a five-star restaurant where most of the food it produces is eaten by the staff, and therefore only has enough left over to serve one or two customers each day. This is yet another reason the “oasis” analogy is so apt in describing coral reef ecosystems. An oasis may contain water, but not enough to irrigate the desert.

This balance of gross and net productivity has important implications for coral reef fisheries. Unlike productive open ocean ecosystems in temperate and polar regions, the amount of organic matter (fish and invertebrates) that can be taken out of a reef without causing damage to the community is very limited. One source has calculated the amount of fish or other organisms that can be taken from a coral reef on a sustainable basis at a mere 50 tons per square kilometer. By contrast, a wheat field or a rice paddy can produce yields eight to 10 times that amount, and some temperate and polar fishing grounds even more. The take-home message is quite simple: Coral reefs are simply incapable of producing large amounts of food beyond what’s needed by the reef community itself. This is one reason a coral reef may adequately support a limited sustenance fishery for centuries, yet collapse within a matter of a few years once commercial fishing is introduced.

Still another important aspect of coral reefs is that they live within a very narrow range of tolerance with respect to light, temperature and nutrition. I call it the world of “little toos.” If the water gets just a little too warm or cool, corals can die from thermal stress. If the water gets a little too turbid or nutrient-laden, they can die from starvation (the zoox being unable to produce enough food) or by being outcompeted and overgrown by nutrient-loving macroalgae. Or if the sea level rises just a little too fast, the reef can’t grow quickly enough and slowly dies. This delicate nature has an important implication. Because of their sensitivity to even the slightest change in environmental conditions, coral reefs are likely to be one of the first ecosystems affected by pollution or alterations of the atmosphere. As a result, reefs could be the modern-day equivalent of the “canary in the coal mine.” And today, many scientists from a wide range of disciplines study coral reefs for early warning signs of environmental degradation and global climate change.

What’s Wrong With Reefs?

Coral reefs are threatened by both human and natural causes. Threats from nature include global weather anomalies such as El Niño, severe storms, freshwater inundation and exposure to air during extreme low tides. As coral reefs have been around for hundreds of millions of years, it’s quite apparent that they can readily deal with these conditions — provided there’s nothing else adding to the stress. In fact, the pressure placed on reefs by natural stress have actually been of benefit by helping them evolve a high level of diversity.

The most severe threat to coral reefs doesn’t come from Mother Nature but from humans, and how we’ve changed the environment. Basically, we can divide these human-induced threats into three categories: land-based stresses, ocean-based stresses, and stresses from global atmospheric change.

From land, stress is caused by increasing population pressure and coastal development. Population explosions in developing countries of the tropics, and massive migration to coastal areas have put enormous pressure on all coastal resources and especially coral reefs. Furthermore, unlike preindustrial societies which had minimal effect on the coastal environments, heavy machinery, mechanical dredges and other industrial building innovations easily transform the coastal zone into cities and resort communities without regard to the effect on nearby reefs.

Moreover, deforestation, overgrazing, and poor land-use practices — even when they occur far inland — lead to massive soil erosion and silt-laden rivers. The rivers, of course, lead to the sea where their sediment load is dumped onto coral reefs unable to cope with excessive light-robbing siltation. Then, in a one-two punch, direct discharge of domestic, agricultural and industrial wastes into coastal waters brings nutrient-rich waters to an ecosystem adapted to live in low-nutrient water. The result is that the corals die and once-healthy reefs, now festooned with algae, take on the drab appearance of algal reefs rather than colorful cathedrals of coral.

Stresses from sea-based sources include both overexploitation — such as overfishing and coral mining — and destructive fishing practices. Unfortunately, overfishing is often unnoticed in coral reef communities until it is too late because of the high species diversity. Indicators of overfishing such as decreases in average and maximum size, as well as changes in catch mix and fishing effort, often go unrecognized because of poor fisheries management. The dwindling catch, in turn, forces a downward spiral — a phenomenon called “fishing down the food chain” — as the response is targeting smaller, less desirable fish, and often using more destructive harvest practices such as dynamite fishing and cyanide poisoning (which kills or destroys everything nearby). Sadly, many of these practices are so commonplace that they have become part of the “traditional culture” of many fishing communities, particularly in Southeast Asia.

The overfishing further exacerbates the problem of reef degradation by removing fish and other algae grazers, thus allowing the algae to outcompete the corals for substrate. Other destructive industries, such as the harvest of coral for building materials and souvenirs, have devastated large tracts of reefs. The ornamental industry, in particular, is driven by demand by consumers in developed countries such as the United States. (Remember that the next time you see corals, shells and other marine products for sale in curio shops.)

Equally destructive are the methods practiced by indigenous fishermen, mainly though not exclusively in Southeast Asia, who supply the European and North American aquarium trade and “live fish” restaurants in Japan, Taiwan, Hong Kong and Singapore. Here the use of cyanide poison — which hopefully stuns rather than kills — is standard operating procedure. Still, poisoning often results in a mortality of more than 50 percent in the fish caught (to say nothing of the damage inflicted on the reef). On a positive note, the Marine Aquarium Council has begun addressing the cyanide issue within the aquarium fish industry (but not the “live fish” restaurant trade) with a new program to certify suppliers who use sustainable practices.

Another human effect on coral reefs is tourism. Although tourism can be an environmentally friendly way of generating income from coral reefs, this happens only when resort development and operations are carefully controlled. Certainly, some damage occurs to reefs from activities such as sport fishing, anchoring and accidental contact by snorkelers and divers. However, in most cases this is a relatively minor source of degradation. The bigger culprit is allowing sewage and other wastes from tourist facilities to pollute reef areas, and siting resorts in ways that promote destruction of coastal habitats such as beaches, mangrove forests and seagrass beds. These factors are far more degrading to the health of the reef than the direct damage caused by visitors.

Given its global nature and cause, the most poorly understood aspect of coral reef science is what threat human-induced changes to the atmosphere have wrought. As Wilkinson’s statement attests in the accompanying sidebar, a growing number of scientists believe that factors such as ozone depletion and increases in greenhouse gases are having a serious effect on the health of coral reefs. Ozone depletion permits the passage of greater quantities of potentially damaging UV radiation; and data indicate that this increase is highly destructive to corals and other organisms which host zooxanthellae. By the same token, global climate change appears to be wreaking havoc in several ways, including rising sea surface temperature, altering the pattern, distribution and frequency of tropical storms, changing rainfall patterns (which increase land-derived sediment) and by causing variations in ocean current patterns. In the final analysis, many scientists believe that the real problem is probably no single threat or “smoking gun” that will explain why coral reefs are dying, but a cumulative effect of interacting stresses which may induce some unknown synergistic effect. The final fate of coral reefs could be a cruel application of the Gestaltian tenet that “the whole is greater than the sum of the parts.”

No one knows if coral reefs will survive beyond this century. And if they do, whether what future generations consider a healthy reef will have any resemblance whatsoever to what I saw in my own youth is even more questionable. But one thing is abundantly clear: Coral reefs will survive only if we as a global society have the foresight and exercise the political will to make it happen. Perhaps Dr. Sylvia Earle has put it best. In her book, “Sea Change,” she asks a simple and obvious question. “How can someone who has never seen a coral reef be expected to care about them?” In part, it’s our job as divers to carry both the message and wonder of coral reefs to the unfortunates who have never witnessed one…and to make sure that they understand what the world would be like without them.

Recommended Reading

“Coral Reefs: Cities Under the Sea,” by Dr. Richard Murphy, The Darwin Press, Inc., Box 2202,

Princeton, NJ 08543 USA, Phone: (609) 737-1349, E-mail:

“World Atlas of Coral Reefs,” by Mark Spalding

Story by Alex Brylske

Photos by Ricky Ferguson