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What Affects Coral?

Updated on October 2, 2014

Me diving on the coral reef in Borneo.

 Photo taken by Callum Pearson (2013).
Photo taken by Callum Pearson (2013).
  • Temperature
  • Depth
  • Salinity
  • Light
  • Sedimentation
  • Emergence into the air

Global Warming

Along with other gases like methane and nitrous oxide, CO2 acts like a blanket, absorbing IR radiation and preventing it from leaving the atmosphere. The net effect causes the gradual heating of Earth's atmosphere and surface. We currently do not fully understand the effect of climate change stressors (for example ocean acidification and warming) on coral reefs. To examine the simultaneous effects of these stressors, Williams et al. (2014) used coralline fungal disease (CFD) to mimic them and to study the progression of the disease. Ocean acidification causes lower ocean pH. This reduces the calcification of coral, meaning that it is more at risk of fungal erosion. However, it could also be argued that a lower ocean pH helps prevent the progression of CFD, since the lesion expansion rates are greatly reduced (Williams et al., 2014). Therefore, it is important to remember that global warming itself may not be the issue but the knock on effects due to it.

El Niño

During an El Niño season, easterly trade winds weaken, which depresses normal oceanic upwelling processes and affects the climate. Rainfall increases along the eastern Pacific, while Indonesia and Australia experience drought conditions. El Niño can lead to increased sea-surface temperatures, decreased sea level, and altered salinity due to excessive rainfall (Forrester, 1997). During the 1997-1998 El Niño season, extensive and severe coral reef bleaching occurred, especially in the Indo-Pacific region, and the Caribbean. Approximately 70 to 80 percent of all shallow-water corals were killed on many Indo-Pacific reefs (NMFS, 2001). During the same year, coral reefs in the Florida Keys experienced bleaching events ranging from mild to severe (NMS, 2001). The increased sea surface temperatures during recent El Niño events comes on top of gradually rising temperatures which are part of global warming. It is the combination that produces the temperatures high enough to cause mass coral bleaching. The cause for concern is primarily the global warming, not the El Niño by itself

Higher Water Temperature

Corals are susceptible to temperature changes, because of their symbiotic relationship with zooxanthellae. The coral protects the algae and provides it with its metabolic waste products, which are the compounds the zooxanthellae need for photosynthesis. The zooxanthellae produce oxygen, help the waste products from the coral be managed and removed and supply the coral with carbohydrates for the production of fats and the synthesis of its calcium carbonate skeleton (NOAA, 2011). Without the zooxanthellae, the tissue of the coral appears transparent, called bleaching and it begins to starve (GBMP Authority, 2011). An example of global mass bleaching of corals is in 2002, when 60% of reefs on the Great Barrier Reef suffered. It was due to two main periods of hot weather, which caused the sea surface temperatures to be a few degrees centigrade higher than long-term summer maxima. Although it seems like a small increase, if temperature increases by only one degree celsius for four weeks, a bleaching event can be caused (GBMP Authority, 2011). In El Nino-year 2010, preliminary reports show global coral bleaching reached its worst level since another El Nino year, 1998, when 16% of the world's reefs died as a result of increased water temperature (Cho, 2011).

The need for these dinoflagellates causes coral to have low adaptive capacity to thermal change (Abdullah, 2013). Dinoflaggellate death due to temperature appears to be due to heat stress on specific cells called B1 cells that are highly temperature-sensitive. When these cells are heated 8°C above ambient, increased nitric oxide synthesis happens, resulting in photoinhibition and, eventually, cell mortality. However, there is variation in the survival rates of zooxanthellae to high temperatures due to the different cell combinations, for example A1 cells are slightly more tolerant and C1 are significantly more so, meaning that the dinoflagellates who have larger numbers of these may be more likely to be tolerant (Hawkins and Davy, 2012). Therefore, corals can increase their thermal tolerance by associating with different types of Symbiodinium, for example, those which are more thermally tolerant (Cunning, 2013). This could suggest that global warming is not and will not cause the death of coral, but rather just a change in the relationships the coral forms. Furthermore, an important factor to consider is that although most corals struggle to survive without zooxanthellae, some corals are able to feed themselves and so are not reliant on the sensitive dinoflagellates, making them more robust to environmental changes (NOAA, 2011).

Even if global warming does cause coral bleaching due to increased ocean temperatures, it does not necessarily mean coral death. This is because if conditions return to normal, corals can regain their zooxanthallae, return to their normal colour and survive. However, this stress is likely to cause decreased coral growth and reproduction, and increased susceptibility to disease (GBMP Authority, 2011). That being said, those corals that associated with a greater abundance of symbionts were more susceptible to thermal bleaching. Thus if nutrient pollution causes higher densities of zooxanthellae, coral bleaching would be exacerbated (Cunning, 2013).

To conclude, it would appear that global warming causes the seas to warm which results in a reduction in zooxanthellae, resulting in coral death. However it is unclear if global warming is directly responsible for coral death or if other factors are at play also.

Increased CO2

Warming of the seas is not the only other effect of global warming. C​​arbon dioxide dissolves in sea water and is absorbed by phytoplankton and used during photosynthesis. When these organisms die, they sink to the ocean floor, removing huge amounts of CO2 from the ocean surface and act as an effective carbon sink. However, levels of CO2 in the oceans are increasing, and the oceans are slowly reaching saturation point. This is worsened by the way CO2 dissolves more readily into cold water than warm water, because sea surface temperatures are also rising (Griffin, 2011). Increased atmospheric carbon dioxide reacts with sea water and causes its acidification, which may influence the calcium carbonate precipitation of corals (Njegić Džakula, 2014).


As Njegić Džakula (2014) stated, “an accurate evaluation of global warming effects on corals requires the knowledge of coral biomineralization processes that is still poorly understood”. Thus, further research is required to see the effects of climate change on coral and how they can be reversed. It must be remembered though, that the populations of countries with higher levels of economic development have greater adaptively and resources to help stop coral reef degradation, thus countries will have to pool their resources to solve this (Abdullah, 2013).


Abdullah, S. (2013), ‘3.4 The Risks of Global Warming to Coral Reef Ecosystems’, World Social Science Report 2013, Available at (Accessed: 09/02/14).

Cho, R. (2011), ‘Losing Our Coral Reefs’, State of the Planet, Available at (Accessed: 19/02/14).

Cunning, R. ‘The Role of Algal Symbiont Community Dynamics in Reef Coral Responses to Global Climate Change’, Open Access Dissertations, Paper 1134, Available at (Accessed: 09/02/14).

Great Barrier Reef Marine Park (GBRMP) Authority (2011), Coral Bleaching, Available at (Accessed: 09/02/14).

Griffin, J. (2011), ‘Global Warming: A Storm in a Teacup?’, Climate Science Library, Available at (Accessed: 09/02/14).

Hawkins, T. and Davy, S. (2012), ‘Nitric Oxide Production and Tolerance Differ Among Symbiodinium Types Exposed to Heat Stress’, Plant and Cell Physiology, Volume 53 (issue 11), pages 1889-1898, Available at (Accessed: 09/02/14).

National Oceanic and Atmospheric Administration (NOAA) (2011), Symbiotic Algae, Available at (Accessed: 09/02/14).

Njegić Džakula, B., Falini, G. and King, D. (2014), ‘Influence of Natural Acidic Macromolecules Extracted from Sea Corals on Calcium Carbonate Precipitation’, The 22nd Croatian-Slovenian Crystallographic Meeting: York, Available at (Accessed: 09/02/14).

Williams, G., Price, N., Ushijima, B., Aeby, G., Callahan, S., Davy, S., Gove, J., Johnson, M., Knapp, I., Shore-Maggio, A., Smith, J., Videau, P. and Work, T. (2014), ‘Ocean Warming and Acidification have Complex Interactive Effects on the Dynamics of a Marine Fungal Disease’, Proceedings of the Royal Society of Biological Sciences, Volume 281, Available at (Accessed: 09/02/14).


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