Schools of silver fish darted. Nearby, I spotted an impossibly elongate cornetfish. I slowly drifted over it, hoping to compare its body length to mine. The fish was about four feet long, and at its widest point only as thick as my big toe. Sunlight streamed through the clear water, illuminating the corals.
Fifty feet from shore, I encountered a large rock protruding through the sea surface. Currents churned rapidly, and glistening air bubbles swirled in the water like confetti. Nestled beneath the surface, feathery, purple gorgonians were anchored to the rock and swayed in the flow. After about an hour, I began thinking I was familiar with all the animals. Yet whenever this feeling arose, I’d encounter some new, spectacular, unfamiliar species. Such is the diversity of life in a coral reef community.
Tropical oceans are renowned for their azure clarity, while temperate waters typically are greener and more turbid. The transparency of tropical seas is generated by a virtual absence of phytoplanktontiny algaefrom the water, not a beneficial situation for the many sea creatures relying on phytoplankton as the base of the food chain. Comparatively, temperate oceans are a rich broth of phytoplankton and support abundant marine life.
Paradoxically, coral reefs, bastions of biological productivity and diversity, thrive near the equator, in those tropical regions of blue deadness. They exist in more than 100 countries, with hot spots of diversity in the Indo-Pacific and, to a lesser extent, the Caribbean. The largest barrier reef in the world is the well-known Australian Great Barrier Reef; runner up is the Belize Barrier Reef. “Within the U.S.,” said Eakin, “the largest reefs are in Florida, along the Florida Keys reef tract, with additional reefs on Florida’s east coast and small patches well offshore on the Gulf Coast. There are some reefs off Texas called the Flower Gardens Bank, and coral communities are in a couple other spots along the U.S. East Coast, but they’re very minor. Substantial reefs exist in Hawaii and in all of the U.S. territoriesPuerto Rico, the Virgin Islands, Guam, and American Samoa, as well as the Trust territories and such.
Successful adaptation
The paradoxical success of coral reefs in otherwise poorly developed locations can be traced to intriguing adaptations of the corals themselves. Reef-building, or hermatypic, corals are tiny animals that look like miniature sea anemones. Each coral polyp usually measures up to a few millimeters in diameter. Its base rests in or on a small calcareous structure, its external skeleton. The elaborate corals we see forming reefs are like apartment buildings. Each is constructed by a colony of polyps, with new individuals secreting their limestone skeletons atop those of dead polyps. As Simmons explains it, “Little creatures are on the outer layer, and as they grow, almost like rings in tree, you end up with this rocky substance” at the core of the coral. Reefs as a whole are “gigantic structures of limestone with a thin veneer of living organic material,” according to biologist Dr. Bruce Hatcher of Dalhousie University in Halifax, N.S. The volume of living coral on a reef, he said, “is approximately equivalent to a large jar of peanut butter spread over each square meter of reef.”
A polyp’s mouth is located at the top of its body and is fringed by tentacles used to capture zooplankton drifting by. Food obtained by a single polyp is shared with adjacent ones through a circulatory system linking all the polyps within a colony. Given the low abundance of plankton in tropical seas, however, corals must rely heavily on other food sources.
Their solution is symbiosis. Tissues of reef-building coral polyps contain numerous zooxanthellaetiny, single-celled algae. The algae are typically found in densities of one million or more per square centimeter of polyp, and their brownish pigments give corals their characteristic color. Zooxanthellae can live independently of corals, but the relationship between coral and algae benefits both.
Polyps provide zooxanthellae with nutrients and carbon dioxide, and the zooxanthellae employ sunlight to convert these to oxygen and carbohydrates. In return, polyps obtain energy-filled carbohydrates from the zooxanthellae, possibly even enough to fulfill their entire energy requirements. It is this symbiotic relationship that enables corals to thrive in otherwise unproductive waters. This symbiosis also explains why coral reefs appear only in shallow, clear waters: lacking these conditions, zooxanthellae cannot obtain sunlight.
But what explains the abundance of other reef species, like fish, that don’t participate in the symbiosis? Very little of the zooxanthellae-coral biomass is eaten directly, but waste products from the relationship are used efficiently by other creatures. In a reef community, “everything that’s excreted is eaten or reprocessed by somebody elseanother animal, bacteria, and so forth,” said Dr. Robert Ginsburg, a professor at the University of Miami. “The reefs are cavernous, and the basement is lined with these hungry mouths that eat it, excrete it, and then it’s reprocessed. There’s a real efficient recycling system.” According to one expert, even the nutrients in fish feces are consumed several times over before eventually being lost from the reef. Efficient recycling means that coral reefs arein terms of life-sustaining energyvirtually leak-proof, providing oases of productivity in the biological “deserts” of tropical seas.
Mutually beneficial relationships, such as the one between corals and zooxanthellae, typically arise out of dire need: symbiosis enables partner species to survive where others can’t. However, under conditions more favorable for life, symbionts may lose out to other organisms. Such is the case for corals. In places where phytoplankton grow abundant, they cloud the water, blocking the sunlight required by zooxanthellae. The corals suffer. And where large, bottom-dwelling algae thrive, they usually out-compete the corals for living space by growing faster; the algae grow on top of the corals, smothering them. Conditions for these doomsday scenarios exist at higher latitudes, where strong productivity leads to booming algal populations. Along with temperature (coral reefs persist primarily in water between about 70° and 90° F), competition with algae seems to figure prominently in setting the northern and southern limits to coral reef formation.
Damage to reefs
Therein, researchers say, lies the crux of recent declines in coral reefs. As the human population continues to expand worldwide, so too does its need for resources such as land and agricultural products. Increasingly widespread damage to reefs is caused by a 1-2-3 knockout punch from the sediments, nutrients, and chemicals contained in runoff from human development, sewage treatment, land clearing, and agriculture. For example, nutrients in fertilizers inevitably are washed to sea by runoff from the land. Because coral reefs tend to be located in shallow water near land, and the added nutrients in runoff stimulate tremendous explosions, or “blooms,” of algal populations, corals lose out as light becomes obscured by phytoplankton, and bottom-dwelling algae overgrow the sea floor.
Furthermore, “too much sediment in the water, especially if it’s fine sediment, reduces the penetration of light,” said Ginsburg. “And since corals are dependent on these algae that are in their tissues, it’s like turning down the sun. In some instances, depending on the size and character of the sediment, it can actually smother, or be an impediment to the growth of corals.”
“When we talk about this,” said Eakin, “most people think a bunch of mud comes out and buries the corals. It’s not that obvious. It’s more low-level and insidious in most cases. The level of sediment just increases above that which the corals are capable of tolerating. And so they’re spending more and more of their energy trying to get rid of these sediments [by removing them from their body surfaces] at the same time the sediments are cutting down on the amount of light that reaches them.”
Damage to coral reefs has been observed in more than 93 countries, and much of it is attributed to runoff. A report from the United Nations Coastal Zone Management Project in Belize identifies various forms of runoff as main threats to that country’s reefs, including the Belize Barrier Reef. Elsewhere in the Caribbean, “the coral reef of Cahuita National Park, on the Atlantic coast of Costa Rica, has been virtually destroyed by decades of siltation delivered by rivers originating in highlands destabilized by agriculture,” according to Dr. John Ogden, director of the Florida Institute of Oceanography. An increase of nutrient concentrations along the Barbados coast caused coral diversity there to decline, along with coral growth and reproduction. Simultaneously, algae increased in abundance. These changes signal a major ecological switch over from coral reef to ‘algal reef,’ triggered by runoff.
The problem is intense. Approximately 80% of all sediments entering the world’s oceans do so in the tropical western Pacific, where soil erosion rates in agricultural areas are as much as 10,000 times higher than those in undisturbed forests. Ironically, this is the global hot spot for coral reef biodiversity.
The Florida Keys, where Contship Houston ran aground, are strongly affected by runoff. But sources of runoff are not limited to the keys themselves, experts say. According to Eakin, “The problems are not just problems within the reef tract, or within the Florida Keys. They go all the way up to north of Lake Okeechobee. The entire southern half of the state is involved in the health of the coral reefs.”
“What’s happening is that agricultural runoff from Florida, South Florida, and around Lake Okeechobee is coming down through the Everglades and out into Florida Bay,” eventually reaching the reef tract, said Dr. Jim McVay of NOAA’s Sea Grant program. Sediments, nutrients, and pollutants in the runoff negatively affect the reefs.
Eakin notes that management efforts worldwide need to target this type of problem. “The trouble is that many of those efforts are directed at the reefs and not directed at the real causes,” he said, “So, you may have a country that goes in and protects the reefs, but won’t stop the severe logging, the improper farming practices, and the development that are contributing to an increase in the amounts of land-based pollutants and sediments that are running out and smothering the reefs.”
Instead, Eakin said, “You have to tackle the whole thing as an integrated action, and that’s much more difficult. It’s far easier to regulate what goes on at a coral reef than to, say, deal with all the agricultural and city water use issues in the entire southern half of the state. It becomes a much bigger, more difficult problem to deal with. The number of partners you get involved to handle this in a proper way becomes staggering.” According to McVay, “There’s probably 15 to 20 agencies, Federal and state, looking at nutrient flow in the Florida Everglades and Florida water systems to understand what exactly is happening and what can be done.”Leaching nutrients Coupled with these statewide problems, according to Simmons of the Florida Keys National Marine Sanctuary, are local ones. “Certainly the Florida Bay issue is influential, but we also have near shore water quality problems with numerous septic tanks and even some cesspools in a ground that’s very porous. A lot of that tends to leach out into the near shore water as nutrients that shouldn’t be there.” Boaters may contribute to the nutrient problem by discharging sewage into near shore waters, rather than using a holding tank.
Limited evidence suggests that, given effective management, degradation of reefs can be reversed. In Kaneohe Bay, Hawaii, sewage was discharged directly into the bay, thrusting corals into an inhospitable environment of high nutrients and sediments. A green bubble alga began to dominate, smothering the reef. Particle feeders such as sponges and barnacles became principal members of the community. After some years, the sewage outfall was redirected further offshore. Slowly, new corals reestablished a presence in the bay, hinting at potential recovery of the reef.
However, the pollution in the Florida Keys comes from diffuse sources, rather than originating from pinpoint places such as outfall pipes, complicating the task. Currently, the Florida Keys National Marine Sanctuary and other agencies are weighing various options for re-establishing semi-natural flow patterns throughout the South Florida ecosystem and for feasible ways to retrofit sewage treatment systems on the keys. Said Simmons, “It’s just very complex.”
Physical destruction, such as the Contship Houston grounding, doesn’t help, and its effects interact with poor water quality. The Florida Keys reef tract is among the northernmost of Atlantic reefs. Already a victim of natural stresses, the corals may be less resilient to additional impacts from humans. If water quality were excellent, said Simmons, the reef would stand a better chance of recovering from the Contship Houston grounding. “But,” she speculates, “water quality is not ideal, and there’s a chance that it may not come back at all. That added stress is the final blow.”
Around the world, various other threats to coral reefs have been identified: excessive fishing, use of harsh chemicals and dynamite bombs to harvest reef creatures, coral mining, and even global warming, which seems to stress the corals, causing them to expel their zooxanthellae in a phenomenon called “bleaching,” because the corals turn pure white.
“One of the things that many people don’t realize,” said Eakin, “is that physical contact with corals is damaging. A lot of people don’t realize the age of corals. A colony of a brain coral or a star coral, something the size of, say, a grapefruit is probably at least 20 years old. Anything larger is substantially older. Very large colonies are going to be in the hundreds of years old. Damaging that, it’s like knocking over a redwood tree. You’re talking about something that grows very slowly, and in many cases the anchoring practices of boaters have led to tremendous damage,” when anchors are used on reefs in lieu of moorings. Scuba divers and snorkelers contribute to the damage by grabbing or standing on corals. Despite their rock-like appearance, corals are fragile and can be abraded by human touches, debris, and fishing lines.
Although localized problems such as these are highly destructive, coral reef experts agree that poor water quality is the overriding threat. It took six days, several support vessels, a detailed hydrographic survey, and removal of a half million gallons of fuel, but Contship Houston was refloated successfully.
Now, the Florida Keys National Marine Sanctuary may implement restoration techniques developed to aid reefs in recovery from physical damage. And, with luck, the Sanctuary’s two-mile ship exclusion zone will continue to function more or less effectively, with only rare interlopers like Houston.
The larger and more daunting challenge is to counter the widespread degradation of coral reefs, both here and around the world, by water pollution. Recognizing this pressing need, the U.S. and other nations designated 1997 the International Year of the Reef and have established a worldwide network for monitoring coral reef health.
The primary long-term goal is to develop strategies for sustainable use of coral reefs, so that their biological and economic values can be maintained. Ultimately, however, if left unchecked, the problems affecting coral reefs may ripple to other natural systems. As McVay explains, “Coral reefs are like canaries in the coal mine. When they start going, you know that your environmental conditions are deteriorating.”
Pete Taylor, a writer and photographer specializing in ecology, is a biologist at the University of California, Santa Barbara.
