Coral growth and climate change

Published: March 30, 2010, 5:53 pm

Updated: December 8, 2021, 12:49 pm

Author: J. A. Kleypass

Topic Editor: C. Michael Hogan

Topics:

Coral Reefs (main) Oceans and seas (main) Meteorology (main) Climate Change (main)

The growth and subsistence of coral depends on a number of requirements: temperature, irradiance, calcium carbonate saturation, turbidity, sedimentation, salinity, pH and nutrients. The level of these variables influences the physiological processes of photosynthesis and calcification, and also survival. In turn these requirements are affected by meteorological processes, which results in coral reefs occurring in only select areas of the world’s oceans. The growth rate of corals varies with species, location on the reef and age of the colony. There are many different morphologies of corals such as brain (or massive) coral, branching coral and plate coral. These all grow at different rates. For example, the growth rates of the massive coral, Montastrea annularis, measured in the Caribbean, was 0.06 - 1.23 cm yr^-1 , while the growth of the branching coral, Pocillopora eydouxi, measured in the Eastern Pacific, was 2.1 - 3.9 cm yr^-1 . This article describes some of the factors that influence coral growth and survival, from climate change to coral mining and mentions some of the approaches that may mitigate against coral reef destruction. Figure 1 summarizes the connections between different meteorological processes and coral requirements for growth and survival. These processes affect the distribution of corals on both global and synoptic scales.

Figure 1. Schematic diagram summarizing key meteorological processes and coral requirements controlling calcification, photosynthesis and survival.

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Carbonate Chemistry

Corals grow by the deposition of a calcium carbonate skeleton (calcification) in the form of aragonite by combining calcium ions with carbonate ions. The concentration of calcium ions in sea water is much higher than the concentration of the carbonate ion, therefore the rate of calcification is controlled by the saturation state of carbonate ions in the sea water. The saturation state of calcium carbonate is determined by the concentration of carbon dioxide (CO₂), which dissolves in water to form an acidic solution consisting of three species of inorganic carbon; carbonic acid: H₂CO₃, bicarbonate ion: HCO^-₃, and carbonate ion: CO^2-₃. These are related by the following equilibrium equation:

CO₂ + H₂O ↔ H₂CO₃ ↔ H+ + HCO^-₃↔ 2H+ + CO^2-₃

The concentration of CO₂ in water is largely controlled by the atmospheric concentration of CO₂ and temperature. Therefore the concentration of calcium carbonate in the ocean is highly correlated with temperature.

Calcium carbonate also reacts with CO₂ and water as described by the following equation:

CO₂+ H₂O + CaCO₃ ↔ Ca²⁺ + 2HCO^-₃

This means that the more CO₂ dissolved in the water, the more readily the calcium carbonate will dissolve. CO₂ is more soluble in cold pressurized water and less soluble in warm non-pressurized water. Therefore the concentration of CO₂ is smaller in shallow tropical waters, which reduces the solubility of the calcium carbonate, allowing corals to precipitate calcium carbonate skeletons in these conditions.

Dissolved CO₂ also affects the pH of water. An increase in CO₂ concentration causes a decrease in pH, which results in a decrease in the levels of the carbonate ion. As corals use the carbonate ion to form their skeletons, a decrease in the levels of carbonate ion will lead to a reduction in the calcification rate, less carbonate accumulation on average, and probably lower extension rates or weaker skeletons in some corals. The result of this would be a reduction in the ability of the coral to compete for space and to withstand erosion.

Many studies have been carried out to determine the influence of temperature and carbonate concentration on the growth rate of corals. Some studies have measured the growth rate directly through extension rate, or increase in weight, others measure the calcification rate. These rates can be measured directly from living corals or from cores drilled from the ground. Using the process of X-ray microanalysis, discrete seasonal banding can be detected and measured in the coral cores, and from this, coral growth rates over many decades can be measured. The calcification rate is a product of the growth rate and the linear extension rate. This means that a coral that has a lower calcification rate could have the same extension rate as a coral with a higher calcification rate, if the skeleton that was being formed had a lower density. It is the precipitation rate of calcium carbonate that determines the growth or accretion rate of the reef and therefore the net loss of calcium carbonate as well as net calcification rate needs to be considered.

From predictions of future atmospheric CO₂ levels that the surface waters of the extra-tropics may well reach a level of undersaturation in the future. The tropical and warmest subtropical waters are unlikely to become undersaturated. Even though the waters are likely to remain supersaturated, the degree of supersaturation affects the rate of coral calcification. And when the saturation state of calcium carbonate is greater than one (supersaturated), the calcification rates of all calcifying organisms, including corals, decrease in response to the decreasing saturation state.

Direct Destruction by Humans

Corals grow within very narrow limits of temperature, irradiance, salinity, pH and turbidity; all variables which are influenced by climate and weather. Corals are also influenced by direct human intervention – bomb fishing, coral mining, coastal development, marine pollution (Water pollution), overfishing and overexploitation of resources, and inland pollution and sedimentation. Such a manifold of stressors make for a bleak outlook for corals.

Water Pollution

Coral reefs can be damaged by a variety of water pollutants that are produced by a number of anthropogenic sources. Agricultural runoff can contain herbicides, pesticides, and nutrient fertilizers. Nitrogen and phosphorus addition can fertilize algae and result in algal blooms. Because algae can potentially grow much faster than coral, they can out-compete corals. For example, in China, river discharges carrying nutrient runoff to Yulin Bay and Sanya Bay caused significant damage to the receiving coral reefs compared to Yalong Bay which gets little runoff and coral reefs are healthy.(Kimura, 2008)

Human sewage, often untreated, can add nutrients, microorganisms, and other pollutants to coral reefs; moreover, nutrients in sewage can cause eutrophication. Bacteria added by sewage pollution are suspected causes of increased incidences of coral diseases such as white band disease. For example, urban sewage discharged largely untreated into Daya Bay has resulted in the loss of coral communities.(Kimura et al, 2004)

Chemical pollution can also harm coral reefs. For example, oil spills, the result of spills from drilling or discharge of oil from vessels can harm reefs. They can be especially harmful if they occur during coral spawning because the oil can kill eggs and sperm.

Solid pollution such as plastics and discarded fishing nets (ghost nets) can also damage reefs.

Sedimentation

Human activities on land such as the clearing (Deforestation) of forests, slash and burn agriculture, road building, and other development can lead to increased rates of sedimentation. High sediment load can reduce light penetration and reduce the photosynthetic activity of zooxanthellae. The problem of sedimentation of coral reefs has been increased by the removal of mangrove and seagrass communities that naturally filter out sediments.

Destructive Fishing Practices

Dynamite fishing. (Source: ATN)

Many fishing practices harm the reef by physically damaging the reef or by killing non-targeted reef fish or other reef organisms. Blast fishing, a method of fishing in pars of the Caribbean, East Africa, and Southeast Asia, uses underwater explosions to damage the swim bladders of fish so that they float to the surface where they are easily captured. The blast of the explosions destroys coral and flattens the reef structure. In parts of China fishermen use cyanide to stun fish so that they can be captured alive; another Chinese practise, a style of fishing called muro-ami involves scaring into a net fish by pounding on the reef with heavy objects that severely damage the coral reef.(Kimura et al, 2004)

overfishing has badly damaged coral communities around Hong Kong and China's Xisha islands, causing most high-value fish species to become locally extinct.

Traps and monofilament nets also snag corals and cause damage to them when retrieved. Abandoned nets can smother corals. Other threats include bomb fishing, which has occurred in the South China Sea for more than 100 years, and recently there has been the use of cyanide to catch fish. Illegal fishing and the sale of living corals for the aquarium trade are problems around Hainan Island. Bomb fishing has been widely practiced in Hainan Island and the remote Nansha Islands, which has resulted in 50-80% coral mortality. Cyanide fishing by large-scale commercial operators and for collecting aquarium species occurs in the remote Xisha and Nansha islands.

Unsustainable Fishing

In 1950 no fisheries were reported as being overexploited whereas by 1996 35% of fisheries were reporting overexploitation and an additional 25% were reported as being near overexploited. Overfishing can have a variety of negative effects on the environment. Overfishing can reduce genetic variation in a population making it harder for species to adapt to environmental change and mate. In addition, overfishing can alter trophic interactions and cause unexpected indirect effects on the environment. For example, in areas where predators have been removed increases in population size of their prey have resulted in unintended effect on the environment.

Coral Reef Human Exploitation and Contact

In some locations direct human contact is the major cause for reef decline. This activity may include impact from scuba divers, specimen collection or damage by boating vessels. In the case of the Great Barrier Reef in Australia, Daley has conducted extensive documentation illustrating that besides overfishing, a major degradation impact to the coral reefs has occurred due to collection of reef specimens, which activity was especially intense in the 20th century,. but continues until present time (e.g. early 21st century. (Daley, 2014) Such removal of corals has taken place both legally and illegally; in fact since "No Take" zones have been expanded by the Queensland authorities, there has been measurable improvement in coral health. Considerable amounts of coral removal have been done overtly as commercial gathering for specimen sale.

In Fiji, major commercial exploitation is ongoing, with over 800,000 kilograms of coral being harvested and exported for aquarium use. The economic pressure for this trade is immense with the average harvest employee making more than the average wage per Fiji resident. (Lovell et al., 2004)

In the Hainan Island area of China, 95 percent of coral reefs have been destroyed by commercial mining of corals for construction materials and souvenirs; moreover, in the South China Sea, reefs and atolls have been paved for military installations and armaments. Most of these islands belong to nations other than China.(KImura et al, 2008) In Belize, major reef loss has occurred by marine sand mining used for coastal fill. (Arrivillaga, A. & M. A. Garcia, 2004)  

References

• Arrivillaga, A. and M. A. Garcia , 2004 , Status Of Coral Reefs Of The Mesoamerican Barrier Reef Systems Project Region, And Reefs Of El Salvador, Nicaragua And The Pacific Coasts Of Mesoamerica. . p: 473-492. in C. Wilkinson (ed.). Status of coral reefs of the world: 2004. Volume 2. Australian Institute of Marine Science, Townsville, Queensland, Australia. 557 p
• Buddemeier R W et al. (2004) Coral Reefs & Global Climate Change: Potential Contributions of Climate Change to the Stresses on Coral Reef Ecosystems. Pew Center on Global Climate Change, Arlington, VA, 42 pp.
• Christianson G E (2000) Greenhouse: the 200-year story of global warming. Penguin. ISBN: 0140292586.
• Crabbe M J C (2006) Challenges for sustainability in cultures where regard for the future may not be present. Sustainability: Science, Practice & Policy. 2.
• Crabbe M J C et al. (2008) The impact of weather and climate extremes on coral growth. In: H. Diaz and R. Murnane (Eds.) Climate Extremes and Society. Cambridge University Press. In the press.
• Done T J (1999) Coral community adaptability to environmental change at the scales of regions, reefs and reef zones. American Zoologist 39: 66-79.
• Hoegh-Guldberg O (1999) Climate change, coral bleaching and the future of the world’s coral reefs. Marine Freshwater Research 50: 839-866.
• Kimura, T., C.F. Dai, H-S. Park, H. Hui and P.O. Ang. 2008. Status of Coral Reef Resources in East and North Asia (China, Hong Kong, Taiwan, South Korea and Japan). In: Wilkinson, C. (ed.). Status of Coral Reefs of the World: 2008. Global Coral Reef Monitoring Network and Reef and Rainforest Research Center, Townsville, Australia. p145-158. 
• Lovell, E., H. Sykes, M. Deiye, L. Wantiez, C. Garrigue, S. Virly, J. Samuelu, A. Solofa, T. Poulasi, K. Pakoa, A. Sabetian, D. Afzal, A. Hughes and R. Sulu , 2004 , Status of Coral Reefs in the South West Pacific: Fiji, Nauru, New Caledonia, Samoa, Solomon Islands, Tuvalu and Vanuatu. . p: 337-362 . in C. Wilkinson (ed.). Status of coral reefs of the world: 2004. Volume 2. Australian Institute of Marine Science, Townsville, Queensland, Australia. 557
• Kleypass J A et al. (1999). Geochemical consequences of increased atmospheric carbon dioxide on coral reefs. Science, 284, 118-120.
• Knowlton, N. 2001. The future of coral reefs. Proceedings of the National Academy of Sciences of the U.S.A. 98: 5419-5425.

Websites

• Coral Reef Targeted Research and Capacity Building for Management (CRTR) Program.
• International Society for Reef Studies.N
• ReefBase: An online information system on coral reefs that provides information services on management, research, monitoring, conservation and education.
• REEFVID: A resource of free coral reef video clips for educational use from Professor Peter Mumby.
• The Royal Society: Information on causes and effects of climate change.

Citation

J. A. Kleypass (2012). Coral growth and climate change. ed. C. Michael Hogan. Encyclopedia of Earth. National Council for Science and Environment. Washington DC. Retrieved from http://editors.eol.org/eoearth/wiki/Coral_growth_and_climate_change