Coral Reefs and Climate Change

This topic submitted by Paul Suprenand ( at 2:40 AM on 6/10/06.

A class picture at Poas Volcano in Costa Rica, 1997.

Tropical Field Courses -Western Program-Miami University

The often debated question is no longer, ‘if climate change is occurring,’ but what factors are causing the warming of our planet. As governments and scientists around the world agree that a global warming trend is occurring the variables that contribute to the concern are still being examined. Many people attribute some climate change to increases of carbon dioxide gas (CO2), which as a greenhouse gas, traps solar heat in our atmosphere. This temperature increase causes a redistribution of heat budget around our planet. Temperature influences oceanic movements, the hydrologic cycle and chemical biological and geological processes. It is not that CO2 does not have it’s benefits to ecosystems around the world, but the amount of CO2 produced today theoretically exceeds the planet’s current ability to ‘fix’ or chemically change the molecule into a more useful molecule(s); such as the production of oxygen from plants using carbon dioxide during photosynthesis. One of the major photosynthetic ecosystems are rainforests. Since there are a limited number of plants in the rainforests, there will be a limited conversion of CO2 to O2. The majority of the remaining atmospheric CO2 is taken up by our oceans, becoming a ‘carbon sink.’

The ‘rainforest’ of the ocean, coral reefs, also requires a constant input of atmospheric CO2. The coral reefs utilize carbonate chemistry in saline water which creates the molecule CaCO3, calcium carbonate (the same basic molecule as human bone). This molecule creates the structure to the reef and the colors associated with reefs are the organismal tissue over that structure. The color of coral reefs, or more precisely coral polyp tissues, is from a symbiotic relationship with a single-celled microalgae (plant) named zooxanthellae. This plant lives within the corals’ tissue and gives the colony food from photosynthesis as well as tissue color. In turn the nutrient ‘wastes’ from coral polyps benefit the microalgae. It is this relationship that makes the building of coral reefs possible. The relationship is responsible for the distribution of coral reef around the world (Figure 1.).

FIGURE 1: ( )

(Red dots indicate a coral reef)

Coral reefs offer an incredible value to society as they provide medicine, fish nurseries, coastal protection, food and many other benefits. The coral reef ecosystem is exceptionally complex and not fully understood. Just as rainforests produce a limited conversion of CO2, so do coral reefs. The remaining CO2 begins to change environmental conditions. To fully appreciate the effects of changing chemical, biological and geological processes on coral reefs in the environments in which they live, we need to examine each of these processes to understand how each may contribute to the success or failure of coral reefs.

We will first consider the broader environmental changes when looking at coral reef ecosystems. As our human population increases the manipulation of the global environment is a constant concern. Climatic and environmental “change” can also be termed as a “stress.” Logging trees allows greater sediment/soil run-off into coastal waters, artificial fertilizers accelerating aquatic phosphorus and nitrogen cycles, and sewage piped into the ocean causing eutrophication are just a few examples of the environmental changes.

Environmental changes are also responsible for the shift in reef carbon budgets (Kleypas, 2000). If the coral reefs need the balanced atmospheric/aqueous CO2 interface, by definition an unbalanced interface may affect coral reefs, Figure 2. For instance, as atmospheric CO2 enters the ocean it quickly dissociates and becomes one of three chemical species; H2CO3, HCO3- and CO32-, Figure 3. The dominant or most abundant species depends on the amount of CO2 present at the interface and existing aqueous conditions such as pH. At present, the marine environmental conditions make HCO3- the dominant species, making the much valued CO32- a growth limiting variable for coral reefs.

FIGURE 2 (Kleypas, National Center for Atmospheric Research)

With CO32-, the dissolved cations in the water, namely Ca2+ combine and the coral reef has its resource to grow. Unfortunately, the increase of CO2 today, is pushing the chemical equilibrium further towards the HCO3- chemical species, making CO32- less available, and coral are either slowing in growth and/or losing density/dissolving (like osteoporosis). The result in the chemical species change is that the ocean environment is becoming more acidic (termed ocean acidification) creating an additional environmental stress, Figure 3 calculation table.

FIGURE 3 (Kleypas, National Center for Atmospheric Research)

Stress is further subdivided into acute and chronic stresses (Table 1). An acute stress is short-term usually causing rapid damage, such as a hurricane. A chronic stress takes place over longer periods of time and is usually associated with a more gradual influence on the environment. The most devastating combination is an already chronically stressed reef (i.e. suffering from ocean acidification) suddenly suffering an acute stress (i.e. hurricane), where reef survival or recovery after the acute stress becomes much more bleak as larger percentage of the reef is damaged. Combinations of factors are most likely what is happening to species such as, Acropora in the Caribbean and around Florida. This “species have suffered a 97% decline in areas off the Florida Keys and the in the Caribbean since 1985.” (Sainz, 2006) The National Oceanic and Atmospheric Administration (NOAA) has recently made great contributions to positive legislation in coral reef protection as deeming the Acropora a threatened species, under the Endangered Species Act. (Sainz, 2006)

The NOAA stated, “If these losses are not arrested and reversed, Florida’s corals could go extinct within the foreseeable future, resulting not only in the loss of these irreplaceable forms of life, but also billions of dollars in tourist, recreational, medicinal, and subsistence income.” Acropora is suffering more than one chronic stress and when large hurricanes travel into coastal waters, the coral are easily damaged and recovery is a much greater challenge.

TABLE 1 (Kleypas, National Center for Atmospheric Research)
Chronic Stresses Global Regional Local Comments
Carbonate ion decrease and reduced calcification X Cooler areas will be stressed first, opposing possible warming benefit
Temperature increase X Gradual increase may be chronic stress in warm areas, benefit in cool
Over harvesting X X X Fishing, commercial, recreational, souvenir and trade
Nutrient Loading X X Land use, agriculture, sewage treatment, burning and increased run-off
Introduced/invasive Species/Disease X Increased competition and debilitation by parasites, predators or disease
Ocean/Atmospheric circulation change X Specific predictions are difficult
Coastal and watershed alteration X X Alteration of circulation patterns, run-off and land-ocean coupling
Sedimentation X X Land use for agriculture, land clearing, construction, increased erosion and run-off

Acute Stresses
Temperature increases X X Transient high-temperature episodes are major stresses
Intensified climate change X X Linkage to climate change uncertain, major factor in acute temperature stress
Diseases; introduced or invasive species X X
Storm frequency and intensity increases X X Increased virulence and frequency of disease outbreaks may be linked to climate change
Sedimentation X X Land use for agriculture, land clearing, construction, increased erosion and run-off
Urbanization, watershed modification X X Increase in waste, other discharges into the environment, alteration of land-ocean coupling
Commercial and incidental destruction X X Transportation, tourism and recreational use, mining, dredging, destructive fishing

Other stresses can be seen in Figure 4.

Coastal zone modification and mining of coral reefs are stresses directly attributed to humankind. A classic example would be the modification of the Florida Keys, the third largest coral reef tract in the world (EPA online). The Florida Keys, with bridges now attaching once separate islands, have resulted in waterways being redirected and human pollutants being introduced…greatly impacting these coral reefs over the last several decades. These modifications to the marine environment in conjunction with, “relentlessly growing human populations…poor water quality from land-based sources including sewage, fertilizers, and sedimentation; over-fishing; and global climate change,” may have changed the coral reefs surrounding Florida indefinitely (Ogden and Miller, 2001). Another attack to coral reefs by human influences occur when building supplies for industry and resident homes are in low supply and reefs are used for construction materials.

Invasive marine species are often introduced into coastal environments by the massive trades of goods from port-to-port around the world. “Accidentally introduced into Tampa Bay (U.S.A.) in the Gulf of Mexico in 1999, the green mussel (Perna viridis), a native to the Indo-Pacific region, has proliferated and dispersed southwards along peninsular Florida.


During 2002 another introduction of P. viridis occurred on the northeast coast of Florida and larval dispersal has also carried the species northward in Atlantic coastal waters.” (Powers et al., 2004) These invasive species are disruptors to a balanced and seemingly fragile ecosystem, when they out compete endemic organisms for resources. The major concerns for coral reefs are the introduced diseases or parasites from other parts of the world that produce additional stresses. And, as Dr. Harvell wrote in 2002, “changes in El NiĖo-Southern Oscillation events have had a detectable influence on marine and terrestrial pathogens, including coral diseases, oyster pathogens…” The bacterial diseases of concern for Caribbean coral are called, “black-band disease and white-band disease.” (Patterson, et al., 2002) With in introduction of invasive species, climatic changes can create more inhospitable environments to reproduce, resulting in greater short-term and long-term damages.

Coral reefs also have predators. The crown-of-thorns starfish is a powerful biological stress and like a sickness has population explosions or outbreaks when large numbers of them attack a coral reef simultaneously. The crown-of-thorns starfish moves over live coral colonies and inverts its stomach onto the coral tissue, releasing digestive enzymes, and sucks up the dissolving coral tissues. It is thought that a decrease in crown-of-thorn starfish predators from over fishing and increased anthropogenic nutrients in the marine environment have created an epidemic-like population of the species.

El NiĖo is often connected to global warming and remains complex in its far reaching effects. El NiĖo essentially transfers the world-wide heat or energy budget into shifting areas of climatic extremes. For instance, winds created by El NiĖo can cause water to be about 2 feet higher in Indonesia than Ecuador (NOAA online). The changes in water depth, along with heat and wind can also change the water column stratification either creating colder water or warmer water at abnormal depths depending where on earth the phenomenon is being observed. The warmer waters tend to collect in latitudinal areas directly effecting coral reefs communities, increasing sea surface temperature. Storms are also created by El NiĖo, but are usually more intense and severe. The reference to the, El NiĖo-Southern Oscillation, takes into consideration the variable of climate change in addition to a list of other stresses. In work done by Dr. Peter W. Glynn, he showed wide-spread coral bleaching in direct relationship with the El NiĖo events in 1982-1983, 1987-1988 and in 1997-1998.

Coral bleaching viewed as a combination of stresses (chronic, acute and/or climatic) results in discoloration/whitening of the coral reefs. Bleaching is the expulsion of zooxanthellae from the coral tissue, which is naturally translucent, and the calcium carbonate structure can be seen underneath. Coral reefs have “bleached” for centuries for natural adaptation. The cycle is to remove zooxanthellae of the symbiotic relationship when conditions are physiologically taxing, and re-absorb the zooxanthellae when conditions return to normal, or more tolerant zooxanthellae are found. If the zooxanthellae do not return to the coral tissue, coral reefs most often die. Specific factors also causing bleaching are: high and low temperatures, intense light and salinity change. Coral bleaching is further defined into three types of bleaching 1) animal-stress bleaching, 2) algal-stress bleaching and 3) physiological bleaching (Fitt et al, 2001). Extremely problematic coral reef bleaching events occur when a large percentage of the overall coral reef population bleaches; mortality is high, and the recovery growth of the remaining species remaining is naturally slow.

It is thought that in approximately thirty years we could see the complete extinction of coral reefs around the world (IMAX film quote, “Coral Reef”). The projected model by Dr. Kleypas shows that as sea surface temperature (SST) increases with increased atmospheric CO2 the available building blocks of coral reef, CO32-, continually decrease, Figure 5.

To further explain the effect of ocean acidification on carbonate species, as they relate to coral reefs, Figure 7 illustrates that as pH changes the availability of the three carbonate species changes. The mean pH of the ocean is approximately 8.3. As pH becomes more acidic Figure 7 shows that the CO32- decreases in availability and of HCO3- increases. If we model a monoprotic acid in water, water compromises the following species: H2O, H3O+, H+ and OH-. The increasing acidity makes it chemically inefficient to subtract an H+ from HCO3- than it does to add H+ to HCO3- (the characteristic of decreasing pH is the greater concentration of H+), Figure 8 (Honeyman, Colorado School of Mines). The addition of H+ to HCO3- is carbonic acid, H2CO3, Figure 6.

The future impacts of increased sea surface temperatures and a lower availability of CO32- are almost certainly going to have a negative affect on coral reefs over the next 30 years. A widely accepted prediction is, “a doubling of the pre-industrial pCO2 by the year 2065.” (Kleypas et al, 2000) The ocean temperatures will also increase, but rhe exact increments can not be accurately determined. There is a theory that global warming will create the potential for coral reefs to live at higher latitudes assuming, “a 2°C warming of SST and (assuming) that reef growth is limited to depths where irradiance was 200-300 Ķmol m-2 / s. The pole-ward shift in the 18°C isotherm resulted in a 2.5-3.5% increase in potential reef habitat area. This does not account for loss of habitat due to too-warm temperatures, and assumes that reduced light at higher latitudes is not a factor in reef development” (Kleypas et al, 2000).
Gattuso et al. 1999, continues to state that the same doubling affects will result in a decrease at an estimate of 30% of available CO32- in tropical regions. Again the result of decreased chemical availability for structure building molecules will most likely result in a decrease of coral reefs in tropical areas. A number Kleypas et al, 2000 predicts at about a 14-30% decrease in reef calcification and “those reefs which already have a low surplus of carbonate production will become non-reef coral communities.”
To look one of the United States’ most valuable coral reef ecosystems, the Florida Keys, which extends from Miami to the Dry Tortugas are included in the Florida Keys National Marine Sanctuary (FKNMS), and the Biscayne and Dry Tortugas National Parks. The FKNMS covers an area approximately, “9850 km2 with 1400 km2 of coral reef and hard bottom habitat” (Australian Institute of Marine Science, Status of Coral Reefs of the World: 2004). According to this report, the Caribbean possesses some of the best documented reef ecosystems in the world and consist of a species diversity of 64 hard coral species, 2 fire corals, and 55 octocoral species. It is to be noted that in this sanctuary the diseased stony coral has, “increased alarmingly from 20 stations in 1996 to 95 stations in 2003…a disease outbreak in 2003 affected (Acropora) corals and prompted Sanctuary management to close (affected areas)…” (Australian Institute of Marine Science. Status of Coral Reefs of the World: 2004). Not to harp on conditions too much, but poor water quality and pollution from fertilizers, sediments and other nutrients from Florida still remain considerable threats to the coral reef ecosystem. In response, the FKNMS increased water sampling everywhere including the mangrove estuaries.
Over-fishing remains a constant problem in the Florida Keys, showing 65% of the 35 fish species examined were over-fished (Australian Institute of Marine Science. Status of Coral Reefs of the World: 2004). In addition, “400-600 vessels have run aground each year in the FKNMS…damage also occurs from anchors and chains…fiber optic cables, gas pipelines…drilling and trenching” (Australian Institute of Marine Science. Status of Coral Reefs of the World: 2004). The same 2004 report states that there are considerable problems with nutrient loading (sewage) causing algal blooms, cyanobacteria, increased bioerosion rates (possibly linked to coral diseases), and increased problems with nitrogen and phosphorus from agriculture.
The brighter side of identifying all of the problematic factors in deteriorating coral reef health is the possibility of enacting social change and potential legislation to preserve coral reefs as a valuable resource. The Florida Keys now prohibits, “oil exploration, mining, large shipping traffic, anchoring on or touching corals, and collecting coral of ‘live rock’ in the Sanctuary” (Australian Institute of Marine Science. Status of Coral Reefs of the World: 2004). Referring in part back to the invasive species portion of this paper, and considering other pollutants, ships are no longer permitted to ‘discharge’ vessel water (i.e. tankers ballast water and cruise line wastewater, both of which could carry invasive organisms) inside of a specified boundary from the coast. Depending upon the area of Florida’s coral reefs, no fishing zones have been designated and the Tortugas Ecological Reserve was designated inside of the FKNMS; totaling a protective area for coral reef to about 10% of the entire sanctuary area.

A partnership between the FKNMS, the National Oceanic and Atmospheric Administration (NOAA), and the State of Florida Department of Environmental Protection (FDEP) to focus each organization’s respective talents in four areas:
(Australian Institute of Marine Science. Status of Coral Reefs of the World: 2004)
 Land-Based Sources of Pollution
 Fishing, Diving and Other Uses
 Awareness and Appreciation, and
 Maritime Industry and Coastal Construction Impacts
Under current U.S. national environmental law the National Pollution Discharge Elimination System (NPDES) controls ‘point source’ release of pollutants into State waters. Fishing is regulated by the, Florida Fish and Wildlife Conservation Commission, locally, and by the South Atlantic and Gulf of Mexico Fishery Management Councils, internationally. There are protective systems in place and without public and governmental support, coral reefs could be much worse off. But, they could also be better.

In considering all of the wonders of coral reefs as an organism and as an ecosystem, our lives would be dramatically changed if they were lost forever. Environmental transformations, whether chemical, biological, geological or any combination therein are often difficult to study, but the reality is that changes, good or bad, are taking place on a global scale. As global temperatures increase, concerns of the potential consequences fuel scientific exploration and action. Our time to find solutions in now as this responsibility falls to the talents and hard work of many in the next few decades. It is hopeful that governments are recognizing that action must be taken to protect coral reefs in this time of change. Whether it is in Indonesia trying to curtail dynamite fishing or in the United States controlling sewage dumping, social awareness is slowly turning into social responsibility. We all have a stake in preserving coral reefs, and it is conceivable that our actions, or lack there of, can be the determining factor in the health of the oceans, the health of our coastal communities and the existence of coral reefs as we have know them for centuries.


Australian Institute of Marine Science. Status of Coral Reefs of the World: 2004; Status of Coral Reefs in the U.S. Caribbean and Gulf of Mexico: Florida, Flower Garden Banks, Puerto Rico, U.S. Virgin Islands, Navassa. Edited by Clive Wilkinson. Volume 2, Section 16, pgs 1-20

EPA (online) Climate Change, Wildlife and Wildlands: Case Study – Everglades and South Florida.

Fitt, William K., B. E. Brown, M. E. Warner, and R. P. Dunne. 2001. Coral bleaching: interpretation of thermal tolerance limits and thermal thresholds in tropical corals. Coral Reefs 20: 51-65

Gattuso JP, Allemand D, Frankignoulle M (1999) Interactions between the carbon and carbonate cycles at organism and community levels in coral reefs: a review on processes and control by the carbonate chemistry. Am Zool 39:160-183

Harvell CD, Mitchell CE, Ward JR, Altizer S, Dobson AP, Ostfeld RS, Samuel MD. Climate warming and disease risks for terrestrial and marine biota. Science. 2002; 296(5576):2158-62 (ISSN: 1095-9203)
Honeyman B (2006) Environmental Aqeuous Chemistry course, Colorado School of Mines, Spring 2006.

Hughes TP, Baird AH, Bellwood DR, Card M, Connolly SR, Folke C, Grosberg R, Hoegh-Guldberg O, Jackson JBC, Kleypas J, Lough JM, Marshall P, Nyström M, Paulumbi SR, Pandolfi JM, Rosen B, Roughgarden J (2003) Climate Change, Human Impacts, and the Resilience of Coral Reefs. Science. 301:929-933

Kleypas JA, Buddemeier RW (2000) The Future of Coral Reefs in an Age of Global Change. Internation Journal of Earth Sciences. 90:426-437

Ogden JC and Miller SL (2006) Decline of Florida’s reefs is not a mystery. Submitted to the Miami Herald.

Precht WF and Aronson RB (2004) Climate flickers and range shifts of reef corals. The Ecological Society of America. 2(6): 307-314

Sainz, A (2006) Coral species put on ‘threatened’ list. Assoicated writer, Miami Herald, May 5, 2006

Sarmiento JL, Slater R, Barber R, Bopp L, Doney SC, Hirst AC, Kleypas J, Matear R, Mikolajewicz U, Monfay P, Soldatov V, Spall SA, Stouffer R (2004) Response of Ocean Ecosystems to Climate Warming. Global Biogeochemical Cycles. 18:1-23
Shwartz M (2006) Marine Scientists Look to the Bahamas as a Model for Coral Reef Conservation. The Stanford Report.

Williams FP and Aronson RB (2004) Climate Flickers and Range Shifts of Reef Corals. The Ecological Society of America. 2(6):307-314

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