Dr. Laurie L. Richardson
Dr. James W. Porter
Dr. Richard Barber
Emerging coral diseases have increased dramatically in recent years, both in terms of increases in disease outbreaks and in the occurrence of new, previously undescribed diseases, worldwide.
Four coral diseases have been characterized to date. Black band disease, the first coral disease to be discovered (1973), consists of a dark line, or band, that migrates across coral tissue at rates up to 1 cm per day, completely degrading coral tissue and leaving behind bare coral skeleton. The disease consists of a specific community of bacteria that work together to produce and maintain a toxic chemical environment that kills corals. While the disease normally is present at low levels on reefs (an incidence of <1%) it is a serious threat in that it targets slow growing (<1 cm/year), reef building corals and routinely kills corals that are several hundreds to a thousand years old. In the last 5 years this disease has spread to coral reefs on a global basis. Coral plague, another bacterial disease discovered in 1977, reemerged in a new, more virulent form in 1995 on reefs of the Florida Keys. Within months it spread to infect 17 species of corals and affected over 200 km of reef tract. This is one of the most severe coral diseases in that it has been known to kill, by rapid tissue degradation, up to 38% of the most susceptible coral species in a matter of weeks. Since 1996 this disease has spread throughout the Caribbean. Aspergillosis is a newly discovered disease that affects sea fans, a soft form of coral. This fungal, lesion-producing disease is responsible for a massive sea fan die-off that occurred throughout the Caribbean and the Florida Keys, killing >90% of sea fans in regional epidemics since 1996. This disease is linked to an increased supply of dust from the African continent. The last disease, white band disease, is also responsible for massive coral die-offs via complete coral tissue degradation. It has killed over 95% of the important, shallow reef-building staghorn and elkhorn corals throughout the Caribbean and the Florida Keys, and to date, is the only disease shown to have completely restructured a long-standing coral reef (4,000 years old) in less than a decade. The white band pathogen is unknown. A number of other emerging coral diseases (red band disease, rapid wasting disease, white pox, yellow band, and others) are incompletely characterized at this time.
It is known that coral diseases represent a threat to coral reefs on a global basis. While the environmental causes of these diseases are just beginning to be understood, it is clear that multiple stressors are involved. Black band and coral plague disease activities are correlated with warmer water temperatures, thus supporting the notion that a global warming is contributing to the observed increase in coral disease. Additional, recently proven stressors (for black band disease) include increased nutrients and lowered salinities, with a positive correlation of disease incidence with lower coral diversity. While the environmental factors contributing to other disease outbreaks are not known at this time, factors such as prolonged elevated water temperature, increased turbidity, increased nutrient input and lower salinity are known to increase coral susceptibility to disease.
Corals are photosynthetic. Based on the presence of symbiotic algae within their tissues, reef-building corals produce more oxygen than they consume. Survival and reproduction of shallow-water coral is dependent on maintaining a Production/Respiration ratio in excess of one. Factors that significantly lower the P/R ratio kill corals. Like most tropical marine organisms, corals exist much closer to their upper tolerance level in terms of water temperature than to their lower tolerance level. Elevated oceanic temperatures of as little as 2.7 degrees F (1.5 degrees C) over the average summer temperature destroy the symbiotic algae resident in corals. Resultant loss of this pigmented algae causes the coral animal to become transparent, revealing the white limestone coral skeleton beneath, hence the term "coral bleaching." If bleaching persists for an extended period of time, the likely outcome is reef mortality. While several stresses can cause bleaching, virtually all known examples of mass bleaching to date have been caused by elevated water temperature. Although the onset of coral bleaching is a response to elevated water temperatures, it does not prove the existence of a global warming. However, a global climate warming best explains the recent occurrence of mass bleaching worldwide. 1998 was the warmest year on record, and recent reports from the Indian Ocean suggest that up to 70% of all corals there died as a result of these record-breaking temperatures. There is considerable concern that any additional, future global warming will cause an increase in both the frequency and severity of coral bleaching.
Elevated sea surface temperatures may also contribute to an increase in the reported incidence of marine diseases from both tropical and temperate oceans. For instance, some reefs in the Florida Keys have experienced a loss of coral cover and biodiversity due to disease (caused by a host of new pathogens). As a result of extensive surveys throughout the Florida Keys it has been discovered that there has been a quadrupling of the number of stations exhibiting disease, and a tripling of the number of coral species afflicted by disease. One locality, the deep (65 feet) reef at Carysfort Light, has experienced a 62% reduction of living coral cover during the three-year survey due, in part, to coral disease.
The 1995 Intergovernmental Panel on Climate Change's "business as usual" scenario (IS92a) projects that anthropogenic production of greenhouse gases will result in a doubling of the current atmospheric CO2 concentration from 355 ppm (parts per million) to nearly 700 ppm by the end of the next century. Recent research on corals suggests that by the middle of the next century, such elevated levels of CO2 will reduce by 30-40%, the ability of corals (and other tropical marine organisms) to deposit their limestone skeletons and to calcify normally. Adding to the influence of elevated CO2 on global temperatures, new research suggests that elevated CO2 concentrations will likely have serious consequences with regard to corals, resulting from the direct effects of elevated CO2.
Worldwide episodes of coral bleaching, coral disease outbreaks and macroalgal overgrowth of coral are increasing in frequency, intensity and range. These deleterious events occur in all regions supporting reefs including the Indo-Pacific, the Western Atlantic and the Caribbean. Surprisingly, both inhabited and uninhabited regions are affected. These recent coral reef changes have been attributed to (in order of assumed importance) global warming, oxygen starvation, sediment loading, overfishing of plant-eating animals and increased UV radiation. Only global warming and increased UV radiation (resulting from ozone depletion) have the global-scale influence that is characteristic of the scale of the coral responses observed. In addition to a global vs. local issue, there is a temporal enigma that may well provide a key to understanding causality -- many changes to coral ecosystems began very abruptly in the mid-1970s. Since the mid-1970s there have been increases in the frequency, intensity and range of outbreaks of a wide spectrum of "invader" micro-organisms, including numerous pathogens that affect coral, other invertebrates, amphibians and humans, and outbreaks of harmful algal blooms.
Global warming has a complex relationship to coral bleaching. Bleaching occurs during episodes of elevated temperature that appear to be the result of a combination of natural phenomena and human-induced changes to the climate system. Global warming appears to elevate seasonal temperatures while natural, short-lived climate phenomena such as El Nino, add to the new seasonal maxima resulting in temperatures that can be lethal to coral ecosystems, especially if sustained over a significant increment of time. Thus, the combined effects of a long-term climate warming, superimposed on the operation of short-term climate phemonena like El Nino, seem to best explain the current epidemic of coral bleaching which was especially widespread in 1998. These complex interactions are likely involved in the increased incidence of disease outbreaks because bleaching weakens the coral's ability to resist pathogens or competitors.
In addition, the observed global mean temperature increase may now be acting in concert with other recent climate-induced changes. For example, around 1976 there was a relatively abrupt climate shift in the Northern Hemisphere that was reflected most clearly in the change in the North Pacific and North Atlantic pressure systems. One consequence of this shift was a prolonged drought in the Sahel region of Africa that resulted in an increase by about a factor of five, the global supply of dust in the atmosphere. Because this dust is iron-rich, and because the productivity in tropical oceans is ordinarily limited by the lack of iron (which also serves as a nutrient in tropical waters), its transport to typically iron-poor regions of the tropical oceans leads to the reduction or removal of an otherwise natural limitation or check on microbial growth. Thus, this extra supply of iron may have spurred the growth of a variety of invader organisms harmful to coral ecosystems. The timing of this increased supply of atmospheric dust may help account for the peculiar timing of the change in the rate of disease outbreaks in coral ecosystems beginning in the mid-1970s.
As scientists struggle to understand the plight of coral, several ideas seem particularly noteworthy:
Dr. Laurie L. Richardson is an Associate Professor of Biology at Florida International University (FIU) in Miami, FL. Prior to her arrival at FIU in 1990, she spent three years in the Ecosystems Science and Technology Branch of NASA's Ames Research Center, California, first as a National Research Council Fellow and then as a senior research scientist.
Dr. Richardson's area of specialization is the relationship between microorganisms and the aquatic environment. She is particularly interested in how microbial metabolism affects aquatic chemistry, and the environmental cues that control the behavioral and mobility patterns of microorganisms. Her research has been conducted in hot spring outflows, lakes, hypersaline ponds, coastal and estuarine environments, and most recently coral reefs.
In recent years Dr.Richardson has focused her research efforts on identifying and understanding the biological mechanisms associated with coral diseases. This work involves characterizing pathogens of newly emerging coral diseases, determining the interactions between coral disease and reef degradation (reef stresses), and investigating the relationships between environmental perturbations and coral disease incidence. She is also actively involved in research in the area of remote sensing of aquatic ecosystems, including coral reefs. Dr. Richardson received her Ph.D. in microbial ecology and physiology in 1985, at the University of Oregon, Eugene, OR.
Dr. James W. Porter is Professor of Ecology and Marine Sciences in the Institute of Ecology at the University of Georgia. After teaching at the University of Michigan from 1973 to 1997, he joined the faculty at the University of Georgia, where he has won both the University's Outstanding Teaching Award and Creative Research Award.
Dr. Porter has also served as Editor of Ecology and Ecological Monographs from 1974 to 1978, as Graduate Coordinator for the Institute of Ecology from 1990 until 1997, and as Associate Director for the Institute from 1993 to 1997. He was later selected as a Fellow of the American Association for the Advancement of Science. In 1998, Dr. Hughes was selected as a Fellow of the American Geophysical Union, and in 1999, he was awarded a Bullard Fellowship by Harvard University.
Dr. Porter has also been called upon to testify before Congress on several occasions, most recently on coral reef conservation issues and the effects of global climate change on coral reefs. He is currently collaborating with the U.S. Environmental Protection Agency on long-term monitoring of coral reefs, studying the distribution of coral diseases from Key Largo, Florida to the Dry Tortugas. Dr. Porter received his B.S. degree from Yale in 1969, and his Ph.D. from Yale in 1973.
Dr. Richard T. Barber is the Harvey W. Smith Professor of Biological Oceanography in the Nicholas School of the Environment at Duke University. He also serves as director of the Duke/University of North Carolina Oceanographic Consortium, a program that operates R/V Cape Hatteras. Dr. Barber's research focuses on the interrelationship of large-scale thermal dynamics and oceanic productivity, emphasizing how biological/physical coupling contributes to partitioning of carbon between the ocean and the atmosphere. His interests also lie in the role of iron in the regulation of primary production in Antarctic waters as well as the equatorial Pacific. His interest in the global decline of coral reef ecosystems stems from observations on the apparent relationship between episodic influxes of iron to outbreaks of marine "invader" organisms.
Dr. Barber has chaired numerous advisory and editorial committees related to national and global research programs. He served as an advisor on the NASA SeaWiFS (Sea-Viewing Wide-Field Sensor) Science Team and Review Panel and on the U. S. JGOFS (Joint Global Ocean Flux Study) Synthesis & Modeling Project. On behalf of the National Academy of Sciences, he served on the Committee on Ocean's Role in Global Change, the TOGA (Tropical Oceans and Global Atmosphere) Advisory Panel, and the International Ocean Science Policy Committee.
Dr. Barber's honors and awards include the John Holland Martin Medal of Excellence from Stanford University, the National Science Foundation Creativity Award, the Rosenstiel Award in Oceanographic Science from the Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, and the Ecology Institute's Prize in Marine Ecology. He is a Fellow in the American Association for the Advancement of Science, the American Geophysical Union, and the California Academy of Science.
Dr. Barber received his B.S. in zoology and botany at Utah State University in 1962, and a Ph.D. in biological science at Stanford University in 1967. He later held a postdoctoral fellowship at the Woods Hole Oceanographic Institute (WHOI) from 1967-1968, was an Assistant Scientist at WHOI in 1969, and joined Duke University in 1970, as an Associate Professor. From 1987 to 1990, he was the founding Executive Director of Monterey Bay Aquarium Research Institute, and in 1990, he rejoined Duke University as the Harvey W. Smith Professor of Biological Oceanography.