Environmental▼News PERSPECTIVE Untangling the causes of coral reef decline age. However, the inability to sort out and quantify the effects of these multiple stressors is a conundrum for reef managers, who need to focus limited resources on the most serious problems in their area, says Billy Causey, superintendent of the Florida Keys National Marine Sanctuary.
PHOTO BY PETER D. GOREAU/COURTESY OF GLOBAL CORAL REEF ALLIANCE
Coral reefs are in big trouble. A new U.S. government report estimates that one-third of the world’s reefs are severely damaged and that 50– 60% of them will be lost in the next 30 years. Although the preliminary report from the U.S. Commission on Ocean Policy (COP), which was released on April 20, calls for immediate and long-term protection measures, some scientists say they still cannot provide definitive answers to such basic questions as what environmental threats are stressing any one particular reef. Known as “the rain forests of the sea”, coral reefs are home to onethird of all fish species and anywhere from 1 to 9 million species overall. The coral reefs also provide $375 billion per year in goods and services, says Nancy Knowlton, coral reef biologist at the Scripps Institution of Oceanography (La Jolla, Calif.). Global warming is one major cause of damage; sea temperature increases of as little as 1 °C over normal seasonal highs provoke bleaching events—loss of the symbiotic algae that give corals their bright color and food, she says. Massive coral bleaching events have increased dramatically over the past several decades, with worldwide episodes in 1982–1983, 1987–1988, and 1997–1998. In 2002, the worst mass-bleaching event on Australia’s Great Barrier Reef affected 60–95% of those reefs. Corals are also losing out to seaweeds, their chief competitors for space on the reef, Knowlton says. At Discovery Bay, Jamaica, coral cover has fallen from 52 to 3% of the reef area, while seaweed cover has risen from 4 to 92% of the reef since the early 1980s, she says. Diseases are transforming reef communities as emerging pathogens, such as white band and black band diseases, wipe out whole stands of coral. Facing impending collapse of the reef ecosystem, scientists support the COP report’s call for urgent action to curb global climate change, polluted runoff, and overfishing— the three largest causes of reef dam-
detrimental, Dodge explains. The debate has polarized scientists, especially those studying in the Caribbean, where reefs have suffered the worst decline and have lost 50% or more of their coral cover over the past three decades. Brian Lapointe and his colleagues at the Harbor Branch Oceanographic Institute (Ft. Pierce, Fla.) have new evidence to support the theory that nutrients are the main
Bleached (left) and healthy (right) corals. With one-third of the world’s coral reefs dead or dying, scientists are divided over the best way to stem further losses.
For instance, if a reef is impacted by a combination of nutrients, pesticides, and sediment, managers need to know what percentage of the damage is caused by each stressor, so they can target their management actions most effectively, he says. But the science is not yet there. One of the big unknowns is what is driving seaweed overgrowth on individual reefs. High nutrient levels and loss of algae-grazing fish and sea urchins can boost algal abundance, according to Dick Dodge, director of the National Coral Reef Institute at Florida’s Nova Southeastern University (Ft. Lauderdale, Fla.). However, determining which is more important, the loss of grazers due to overfishing or the elevated nutrient loads from runoff, is controversial, he says. One reason is that nutrients are difficult to measure in seawater. Also, if grazers are abundant, they can keep seaweed in check in the face of high nutrient inputs, which makes it hard to determine what levels of nutrients are
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cause of this decline. Their study finds that coral-smothering seaweed spread can be prevented at nitrogen concentrations below about 14 parts per billion. Reefs have evolved over tens of millions of years in nutrient-poor water, and the plants there have adapted to make efficient use of nutrients, which means that small increases can make a big difference, says Peter Bell, director of the Low Isles Research Station at the University of Queensland in Australia. Bell has found that algal overgrowth occurs above roughly 14 ppb nitrogen on the Great Barrier Reef, a threshold virtually identical to the one found on Caribbean reefs. “Nutrients can have detrimental impacts on coral reefs, but there is no question that the removal of grazers has a big impact,” Knowlton counters. In the 1980s, a disease knocked out 98% of the black-spined sea urchins in the Caribbean. The die-off, in concert with chronic overfishing of such algae-munching fish
as surgeonfish and parrotfish, helped convert the reefs fringing the north shore of Jamaica to a seaweed-dominated community, she says. Protecting reefs from over-fishing has led to dramatic recovery in some cases, says Jim Bohnsack, ecologist with the National Oceanic and Atmospheric Administration. About 10 years after Apo Island in the Philippines was closed to fishing, fish density more than quadrupled and the corals rebounded, he says. Bohnsack, a member of the U.S. Coral Reef Task Force, supports the group’s call for putting 20–30% of all U.S. reefs off-limits to fishing by 2010 in order to safeguard biodiversity. That goal is too modest, and the target should be closer to 50%, with zoning that could be adjusted with assessment and monitoring over time, following the experience of the Great Barrier Reef over the past 30 years, argues John Ogden, director of the Florida Institute of Oceanography at the University of South Florida. Others are less sure. “We don’t have a good rationale to set targets for protected areas because we don’t have a sufficient body of science to say overfishing is a problem on any specific reef,” according to Frank Muller-Karger, biological oceanographer at the University of South Florida and one of the authors of the COP report. Nonetheless, the COP report makes a clear call for strong legislation to immediately set up protected areas, based on science, that will help implement the most effective solutions for the problems that impact individual reefs, he adds. Despite a sanctuary system that has been growing since the early 1980s, protected areas in the Florida Keys continue to lose coral even as populations of grazers have increased, Causey says. Marine-protected areas are an important first step, but overfishing isn’t the only problem on Florida’s coral reefs, he notes. Nutrients and coral-killing sediment and pesticides are delivered from south Florida’s rivers, the Everglades, and local development on the Keys.
“Going after single causes of stress without proof of damage may miss the true source of the problem. We need to be able to sort out multiple stressors and measure how much each contributes to coral reef damage,” Causey says. Research on cellular biomarkers holds a lot of promise for identifying the contributions from various stressors on coral reefs, Dodge says. Scientists have recently developed a suite of biomarkers that assess the health of corals, detect stress from poisons, and even predict bleaching events three to six months in advance of sea-surface temperature monitoring, says Craig Downs, president of Envirtue Biotechnologies. For example, even though sea temperatures remained below the bleaching threshold of 30 °C in Biscayne National Park in 2000, a significant number of corals on Alina’s Reef that appeared healthy in March died by August, Downs says. He and his colleagues compared biomarkers for Alina’s Reef, which is located near agricultural and landfill runoff, to four sites in the Florida Keys National Marine Sanctuary, where corals did not experience high death rates. Biopsies of the corals on Alina’s Reef revealed that they were sick as early as March—afflicted with a severe oxidative damaging and proteindenaturing stress and exhibiting a pattern of cellular markers that indicated the corals were trying to detoxify an unidentified poison, Downs says. “We have provided evidence that coral decline in the Florida Keys may be caused by local factors, possible chemical toxins,” he says. Downs is now working on assays to identify the poison. Similar tools can help predict bleaching events caused by high sea surface temperatures on reefs off the Florida Keys, says John Fauth, ecologist at the University of Central Florida. Heat stress causes oxidative stress, which leads to bleaching, Fauth explains. Corals produce stress proteins to protect cells from oxidative damage and antioxidant proteins designed to sequester ox-
idative compounds. Fauth and his colleagues found that corals off Key Largo, Fla., with high levels of stress proteins and antioxidant proteins, typically those in shallow water, had less oxidative damage and did not bleach even though they probably experienced high temperatures. However, corals with low levels of the proteins, mostly those in deeper water, had high levels of oxidative stress and were bleached. As a result, coral health at the end of the bleaching season in September could be predicted by the levels of three proteins in spring and early summer, three to six months in advance of high sea surface temperatures, he says. By also monitoring sea surface temperature, the approach gives reef managers time to reduce other stresses on the reef, such as those caused by diving and boat traffic, and can help explain small-scale variations in bleaching, Fauth says. Remote sensing is another tool coming to the aid of reef managers by connecting reef damage to regional patterns in ocean temperature, current, and river plumes, Muller-Karger says. Remote sensing has already been used to plot the flow of an algal bloom that originated in Florida Bay and smothered and killed reefs in the Keys, he says. It is well accepted that reefs worldwide are being harmed by the combination of climate change, land-based pollution, and over-fishing, Causey says. “However, while there are many reefs for which a primary cause of stress is quite obvious, such as when sediments or nutrients can be clearly traced to adjacent land sources, for others the source of stress is not that obvious,” he says. It is imperative that specific threats be identified for each reef that shows signs of damage in order to address it by cleaning up water discharges or managing fishing practices, he adds. Action to curb these threats must be taken now, before it is too late, and new scientific findings can be used later to fine-tune and adjust management strategies, he says. —JANET PELLEY
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