Coral Reefs and Corals
Coral Reefs and Corals
Corals are colonial animals, that is, collections of separate creatures that combine to form a single organism. The individual coral organisms—polyps, tiny creatures a fraction of an inch across that resemble a tube crowned with tentacles—inhabit cells or pores in a rocklike skeleton made of calcium carbonate (limestone, CaCO3). To survive, corals harbor single-celled green plants called zooxanthellae in their body tissues. The relationship between coral polyps and zooxanthellae is symbiotic, or mutually beneficial: the coral polyps provide a home and nutrients for the algae, and the algae photosynthesize about 60% of the food the polyps need.
Coral polyps, by building new limestone structures or coral heads on top of older ones, can create reefs or submerged masses of coral many yards thick and wide and, in some cases, hundreds of miles long. These reefs support highly diverse populations of fish and other creatures and protect shorelines from storms and waves. When water temperatures are high, coral polyps lose their algae, which can cause the coral's death. Along with pollution or other pressures caused by human activity, increased acidity of the oceans caused by anthropogenic carbon dioxide in the atmosphere and global warming caused mostly by that same carbon dioxide are threatening coral reefs worldwide.
Historical Background and Scientific Foundations
Coral reefs are built by coral polyps, tiny animals that live in large colonies and which are themselves inhabited by thousands of microscopic golden-brown algae termed zooxanthellae. The tissues of the coral animals are mostly transparent and the limestone reef skeleton is white, so the coloration of a living coral reef comes primarily from the zooxanthellae. The zooxanthellae contain chlorophyll, the same chemical that colors green plants and enables them to exploit the energy of sunlight.
Using their chlorophyll, the zooxanthellae produce oxygen and food for the coral polyps. The zooxanthellae meet about 60% of the polyps' food needs, with the other 40% consisting of plankton caught by the polyps with their tentacles. The polyps in turn provide the algae with shelter and nutrients such as phosphorus and nitrogen. The polyps lodge in pores or cavities in the surface of a limestone skeleton and spread a living, skinlike covering over the skeleton's entire surface. A skinlike layer of their tissues is occupied by zooxanthellae, giving living coral its characteristic dark color.
Over centuries, successive generations of coral polyps manufacture the vast rocky heaps or shoals of limestone that are known as coral reefs. Most of a coral reef, its great hidden bulk, is dead: only the surface layer hosts living organisms. Since the zooxanthellae algae need sunlight, like any green plants, reefs grow only in shallow water; however, as sea bottoms subside slowly (or sea levels rise), dead coral can build up in thick layers below the living coral, which manages to stay up in the sunlit layer of the ocean. When a ring of thick coral builds up around an island as it sinks or as the ocean rises, it may become thousands of feet thick over millions of years and remain visible when the island itself is far below water. The origin of such structures, called coral atolls, was first explained by English naturalist Charles Darwin (1809-1882) in 1842.
Corals are often compared to underwater forests, manufacturing food and providing a complex environment in which other organisms can shelter. They are the most species-rich ecosystem in the oceans. About a million different species of plant and animal are harbored in a major reef, only about 10% of which have been described by science. Large fish often school along a reef while smaller ones swim in its crevices. Because of their beauty and accessibility, reefs are common destinations for tourists. For example, tourism at the Great Barrier
Reef generated about $4.5 billion in income for Queensland, Australia, from 2004 to 2005.
Corals can survive only in water that is 64°F (18°C) or warmer, and so are found only in the tropical and semitropical seas of the world. At the same time, as was only recently discovered in the 1990s, they cannot tolerate excessively warm water. A few days of water temperatures that are a few degrees above ordinary summertime maximum cause a coral reef to give up most of its zooxanthellae. Since the zooxanthellae give the coral its color, heat-stressed coral takes on a whitish appearance, a phenomenon called coral bleaching. Bleached coral is not necessarily dead, but its growth is greatly slowed and the coral may take years to recover. When bleaching continues too long, coral dies.
Large bleaching events have occurred several times since the 1980s, notably in association with the El Niños of 1982-83, 1987-88, and 1997-98. Severe bleaching also struck in 2002 and 2005. Coral bleaching events occur naturally due to the recurring weather cycles known as El Niño and La Niña, so cycles of damage and recovery are normal for corals. Although El Niños have increased in number, intensity, and duration since the 1970s, there is not yet any clear link between this trend and global warming.
Impacts and Issues
Coral reefs are stressed by a number of human activities other than climate change, but climate change is likely to increase the damage caused by these other stresses. For example, over-harvesting of fish encourages the marine organisms on which many fish feed, such as seaweeds, to compete with corals for space. Encouraging coral's competitors injures the coral. Nutrient loading occurs when nitrogen and phosphorus from agriculture or sewage are mixed with the ocean, encouraging phytoplankton and seaweed to grow in the shallow waters required by corals. Phytoplankton darken the water, reducing photo-synthesis in the zooxanthellae, and seaweeds compete with corals for space. Sediment loading occurs when small mineral particles washed to the sea by erosion interfere with polyp feeding and force the polyps to use up energy cleaning out the particles. Rapid burial under even a thin layer of sediment can kill a coral reef outright. By making bleaching events more common, global warming will probably make corals more vulnerable to these other forms of damage.
There is scientific dispute over whether corals will be able to evolve some resistance to increasing temperature. According to the adaptive bleaching hypothesis, corals shift the type of zooxanthellae on which they depend when repeatedly exposed to bleaching, thus adapting to higher temperatures. There is some evidence supporting this view. However, it is very uncertain whether, if such adaptation occurs, it could continue for more than a couple of degrees of warming in water temperature.
Although warming of the world's seas will make some presently cool regions suitable for coral, the area that will be made newly suitable for coral is forecast to be small compared to that which will be lost.
The greatest threat to corals from climate change, most experts agree, is not warming but the increasing concentration of carbon dioxide (CO2) in the atmosphere. As of 2007, the oceans were absorbing about a third of all anthropogenic (human-released) CO2. This was changing the chemistry of the oceans and thus affecting the ability of various organisms to produce calcium carbonate, which is the material not only of coral skeletons but of the shells of clams, whelks, mussels, and other shelly animals. When a CO2 molecule dissolves in water, it combines with the water in such a way as to release hydrogen ions. The hydrogen ions make the water more acidic. Fewer carbonate ions (CO32-) can exist stably in acidic water. It is carbonate ions, along with calcium ions, that shelly organisms, including coral polyps, use to produce their calcium carbonate shells. Fewer carbonate ions makes shell-building more difficult. There are far more calcium ions than carbonate ions, so carbonate ions are what limit the ability of shelly organisms to make shells in today's oceans.
Over the last 200 years, the average pH (a measure of acidity) of the ocean's waters has fallen by 0.1, a 30% increase in the number of hydrogen ions. This change has reduced the number of carbonate ions available to shelly organisms. At a certain acidity, the concentration of carbonate ions falls so low that corals have difficulty making their skeletons at all; this concentration may occur if today's atmospheric concentration of CO2, about 375 parts per million, rises to over 500 parts per million. This much CO2 will probably exist in the atmosphere by the end of the twenty-first century.
WORDS TO KNOW
ANTHROPOGENIC: Made by people or resulting from human activities. Usually used in the context of emissions that are produced as a result of human activities.
CORAL ATOLL: Low tropical island, often roughly ring-shaped, formed by coral reefs growing on top of a subsiding island. The rocky base of the atoll may be hundreds of feet below present-day sea level. Atolls, like other low-lying islands, are threatened with submergence by rapid sea-level rise caused by anthropogenic climate change.
CORAL BLEACHING: Decoloration or whitening of coral from the loss, temporary or permanent, of symbiotic algae (zooxanthellae) living in the coral. The algae give corals their living color and, through photosynthesis, supply most of their food needs. High sea surface temperatures can cause coral bleaching.
CORAL POLYP: Living organism that, as part of a colony, builds the rocky calcium carbonate (CaCO3) skeleton that forms the physical structure of a coral reef.
EL NIÑO: A warming of the surface waters of the eastern equatorial Pacific that occurs at irregular intervals of 2 to 7 years, usually lasting 1 to 2 years. Along the west coast of South America, southerly winds promote the upwelling of cold, nutrient-rich water that sustains large fish populations, that sustain abundant sea birds, whose droppings support the fertilizer industry. Near the end of each calendar year, a warm current of nutrient-poor tropical water replaces the cold, nutrient-rich surface water. Because this condition often occurs around Christmas, it was named El Niño (Spanish for boy child, referring to the Christ child). In most years the warming lasts only a few weeks or a month, after which the weather patterns return to normal and fishing improves. However, when El Niño conditions last for many months, more extensive ocean warming occurs and economic results can be disastrous. El Niño has been linked to wetter, colder winters in the United States; drier, hotter summers in South America and Europe; and drought in Africa.
GREAT BARRIER REEF: World's largest coral reef system, located along the northeastern coast of Australia. The reef system, which is approximately 1,600 mi (2,600 km) long, contains 3,000 individual reefs and 900 islands and is threatened by sea-level rise and ocean warming caused by anthropogenic climate change.
PH: Measures the acidity of a solution. It is the negative log of the concentration of the hydrogen ions in a substance.
SEDIMENT LOADING: Presence of moving mineral particles in rivers or streams. Faster-moving water can carry more sediment. Erosion increases sediment loading, limited by stream capacity for increased sediment loading.
SYMBIOTIC: A relationship or pattern of exchange that is mutually beneficial to two or more creatures. Algae are symbiotic with lichens and corals; intestinal bacteria are symbiotic with mammals.
ZOOXANTHELLAE: Algae that live in the tissues of coral polyps and, through photosynthesis, supply them with most of their food. The relationship is symbiotic, as the polyps supply the zooxanthellae with a hospitable environment. When water temperatures are too high, corals lose their zooxanthellae. If the loss is too pronounced for too long, the coral dies.
Primary Source Connection
Human impact on Earth's coral reefs is substantial. Global climate change is likely to exacerbate coral disease and reef death worldwide, with a negative impact on marine species and human economies dependent on healthy reefs. The following excerpt discusses the impacts of climate change on the health and development of coral reefs, highlighting problems such as bleaching and reef death.
The Pew Center on Global Climate Change is a United States-based think-tank that gathers scientific and economic data to inform policymakers on global climate change issues. The report's authors are researchers in atmospheric and marine sciences.
CORAL REEFS & GLOBAL CLIMATE CHANGE: POTENTIAL CONTRIBUTIONS OF CLIMATE CHANGE TO STRESSES ON CORAL REEF ECOSYSTEMS
The “Coral Reef Crisis”
Coral reefs have declined over the course of human history, culminating in the dramatic increase in coral mortality and reef degradation of the past 20–50 years. This “coral reef crisis” is well-documented and has stimulated numerous publications on the future of coral reefs and their vulnerability to environmental change. The causes of this crisis are a complex mixture of direct human-imposed and climate-related stresses, and include factors such as outbreaks of disease, which have suspected but unproven connections to both human activities and climate factors. By 1998, an estimated 11 percent of the world's reefs had been destroyed by human activity, and an additional 16 percent were extensively damaged in 1997–98 by coral beaching. Widespread coral bleaching, unknown before the 1980s, has brought recognition that reefs are threatened by global-scale climate factors as well as by more localized threats, and that different types of stress may interact in complex ways.
Although the crisis is widespread, individual reefs and even whole regions exhibit considerable variation in both health and responses to stress. The Caribbean region has been particularly hard-hit by problems, many of which are well-studied. Caribbean case studies and inter-ocean contrasts help to illustrate both the consistencies and the variations in coral reef responses to complex environmental changes.
Climate and Environmental Change
Over the past one to two centuries, human population growth and development have greatly altered not only local environments, but also the global environment as a whole. Major systematic changes include rising atmospheric concentrations of greenhouse gases (GHGs) that influence the earth's energy budget and climate. In addition, the global phosphorus and nitrogen cycles have accelerated because of artificial fertilizer use and massive changes in land use, the hydrologic cycle has been altered by river damming and water diversion as well as climate change, major natural ecosystems have been altered by fishing, forestry, and agriculture, and the ecological and biogeochemical implications of increased atmospheric CO2 levels go well beyond the effects on global temperature.
Because coral reefs occur near the junction of land, sea, and atmosphere, their natural habitats experience both the marine and terrestrial results of any climatic change and are vulnerable to human activities….
Climatic Change Stresses to Coral Reefs
Global climate change imposes interactive chronic and acute stresses, occurring at scales ranging from global to local, on coral reef ecosystems…. Gas bubbles preserved in polar ice caps show that atmospheric CO2 concentrations over the past 400,000 years have oscillated between about 180 and 310 parts per million volume, or ppmv; past temperature and sea-level variations mimic the CO2 fluctuations, with relatively constant minimum (glacial period) and maximum (interglacial) values. Accompanying this CO2 increase is an observed increase in temperature, and a decrease in pH of the surface ocean. IPCC projections show an even greater departure from geologically recent climates by the end of the present century.
The atmosphere and the ocean have warmed since the end of the 19th century and will continue to warm into the foreseeable future, largely as a result of increasing greenhouse gas concentrations. El Niño-Southern Oscillation (ENSO) events have increased in frequency and intensity over the last few decades. This combination (warming and intense El Niño events) has resulted in a dramatic increase in coral bleaching.
“Bleaching” describes the loss of symbiotic algae by the coral or other host. Most of the pigments in the usually colorful corals depend on the presence of these plant cells. The living tissue of coral animals without algae is translucent, so the white calcium carbonate skeleton shows through, producing a bleached appearance. Bleaching is a general stress response that can be induced in both the field and the laboratory by high or low temperatures, intense light, changes in salinity, or by other physical or chemical stresses. Bleaching is the extreme case of natural variation in algal population density that occurs in many corals.
Three types of bleaching mechanisms are associated with high temperature and/or light: “animal-stress bleaching,” “algal-stress bleaching,” and “physiological bleaching.” Although all are important to understanding climate-coral interactions, two are particularly relevant to present concerns: algal-stress bleaching, an acute response to impairment of photosynthesis by high temperature coupled with high light levels; and physiological bleaching, which reflects depleted reserves, reduced tissue biomass, and less capacity to house algae as a result of the added energy demands of sustained above-normal
temperatures. A rising baseline in warm-season sea-surface temperatures on coral reefs suggests that physiological bleaching is at least partly to blame in some bleaching events (e.g., in the Caribbean in 1987 and on the Great Barrier Reef in 2001). Such chronic temperature stress may also underlie some less obvious causes of reef decline, such as low rates of sexual reproduction.
The temperature threshold for bleaching is not an absolute value, but is relative to other environmental variables (especially light) and to the duration and severity of the departure from the normal temperature conditions of a reef. Bleaching due to thermal stress is not, therefore, limited to areas of normally high water temperature. However, regions where higher temperatures are the norm seem likely to be more vulnerable to increased physiological bleaching.
Coral bleaching events of greatest concern are acute episodes of high mortality and protracted debilitation of survivors in the form of diminished growth and reproductive rates. Corals with branching growth forms, rapid growth rates, and thin tissue layers appear to be most sensitive to bleaching, and usually die if seriously bleached. Slow-growing, thick-tissued, massive corals appear to be less sensitive and commonly recover from all but the most extreme episodes. Bleaching thus selectively removes certain species from reefs and can lead to major changes in the geographic distribution of coral species and reef community structures.
Global Warming and Reef Distribution
The global distribution of reef-building corals is limited by annual minimum temperatures of ~18°C (64°F). Although global warming might extend the range of corals into areas that are now too cold, the new area made available by warming will be small, and the countervailing effects of other changes suggest that any geographic expansion of coral reefs will be very minor.
Coral reefs require shallow, clear water with at least some hard seafloor, and their propagation depends primarily on ocean currents. The west coasts of North and South America, Europe, and Africa experience cool water flowing toward the equator and are thus “upstream” from potential sources, causing restricted distributions of coral reefs. In areas such as the southeastern United States and near the Amazon River, reef expansion along the coast is blocked by muddy coastal shelves, river deltas, and turbid water. Only southern China, Japan, Australia, and southern Africa present geographically realistic opportunities for reef expansion. Additionally, sea-surface temperature (SST) gradients are very steep in the vicinity of 18°C (the annual minimum temperature threshold for coral reef growth), and ocean model projections suggest that SST warming associated with doubled CO2 will only move the 18°C contour by a few hundred kilometers, especially in the critical western boundary areas. The overall positive effects of warming on habitat availability and ecosystem distribution will be very minor compared to the overall negative effects.
Reduced Calcification Potential
The oceans currently absorb about a third of the anthropogenic CO2 inputs to the atmosphere, resulting in significant changes in seawater chemistry that affect the ability of reef organisms to calcify. Photosynthesis and respiration by marine organisms also affect seawater CO2 concentration, but the overwhelming driver of CO2 concentrations in shallow seawater is the concentration of CO2 in the overlying atmosphere. Changes in the CO2 concentration of seawater through well-known processes of airsea gas exchange alter the pH (an index of acidity) and the concentrations of carbonate and bicarbonate ions. Surface seawater chemistry adjusts to changes in atmospheric CO2 concentrations on a time scale of about a year. Projected increases in atmospheric CO2 may drive a reduction in ocean pH to levels not seen for millions of years.
Many marine organisms use calcium (Ca2+) and carbonate (CO32-) ions from seawater to secrete CaCO3 skeletons. Reducing the concentration of either ion can affect the rate of skeletal deposition, but the carbonate ion is much less abundant than calcium, and appears to play a key role in coral calcification. The carbonate ion concentration in surface water will decrease substantially in response to future atmospheric CO2 increases, reducing the calcification rates of some of the most important CaCO3 producers. These include corals and calcareous algae on coral reefs and planktonic organisms such as coccolithophores and foraminifera in the open ocean….
The predicted rise of sea level due to the combined effects of thermal expansion of ocean water and the addition of water from melting icecaps and glaciers is between 0.1 and 0.9 meter (4–36 inches) by the end of this century. Sea level has remained fairly stable for the last few thousand years, and many reefs have grown to the point where they are sea-level-limited, with restricted water circulation and little or no potential for upward growth. A modest sea-level rise would therefore be beneficial to such reefs. Although sea-level rise might “drown” reefs that are near their lower depth limit by decreasing available light, the projected rate and magnitude of sea-level rise are well within the ability of most reefs to keep up. A more likely source of stress from sea-level rise would be sedimentation due to increased erosion of shorelines.
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