Coral Reefs Currently Under Stress from a Changing Planet

Filed Under: General Care, Fish

Did you know that corals are actually marine animals? There are more than 2,000 known species of coral. These delicate creatures are under stress around the world due to warmer temperatures, changing pH levels, and even agricultural and fresh water runoff. Coral reefs provide homes to more than a quarter of the world’s saltwater fish. Coral reefs are typically found in shallow water areas, often close to the coast or island locations.

Most of the coral reefs are found where ocean temperatures run from 80º to 89ºF. The ideal temperature for most corals is from 72º to 78ºF. The most sensitive corals to warmer temperatures are those that have developed a symbiotic relationship between themselves and zooxanthellae algae, which are typically found in the large corals that are some of the major reef building species. The algae are responsible for providing energy for the host coral through photosynthesis, and therefore require access to sunlight limiting the water depth in which coral may locate. Even the algae’s waste products help the host animal with coral calcification and are responsible for the bright colors of the coral. The zooxanthellae is very sensitive to water temperature changes. If the algae is killed, the coral will starve, as it is dependent upon the algae for its food production.

When the change of water temperature is temporary or not dramatic it is possible for the algae to repopulate the coral. If the temperature change occurs for a long enough period the coral will undergo bleaching (lose its color) and die, leaving behind its exoskeleton as a reminder to what once was a thriving ecosystem. In 1998 a temperature change in the ocean waters was so severe that nearly 16% of the coral reefs around the world underwent bleaching.

Another factor generating additional stress on the world’s coral reefs are the changing pH levels of the oceans. Most coral prefer a pH level of 8.1 to 8.4. Approximately half of the carbon dioxide, the most notorious of the “green-house gases” that is released, becomes dissolved into the ocean. The dissolved carbon dioxide actually becomes a weak carbonic acid that is capable of damaging coral and any marine animals with shells. Since the Industrial Revolution of the 1800’s the pH of the ocean has decreased from 8.3 to 8.2. This may not seem like a great change, but the pH scale is logarithmic. A decrease of .1 in pH actually represents a 10-fold increase in acidity! The full impact of this change has yet to be measured.

Agricultural runoff contains fertilizers and pesticides which increase the chemical nutrients in the oceans causing an algae overgrowth, a process termed eutrophication. The algae whose growth is stimulated are not the same species as the zooxanthellae algae and will outcompete the resident algae or produce toxic products that further compromise the coral. A toxic algae by the name of Pseudonitzchia australis is the cause of amnesic shellfish poisoning. P. australis blooms in the presence of nitrogen, usually due to urea in sewage runoff. Shellfish are poisoned by the production of domoic acid. In certain instances, even toxic red tides are suspected be a result of nutrient runoff. A red tide can kill all marine life it comes into contact with through oxygen depletion and the production of potent toxins.

Runoff does not even have to contain fertilizers and pesticides to affect delicate reef systems. More fresh water is running off the land without being absorbed into the soil due to extensive paving.

Fresh water near shorelines pours into delicate reef ecosystems causing the marine organisms, especially the young organisms, to absorb the fresh water and swell up. The marine life literally explodes with dramatic changes in the salinity of the water.

The majority of the human population lives within 50 miles of the coastline. With such a density of people so near coastlines, the coastal oceans are among the most vulnerable habitats to human activity. We must all work to limit urban, industrial, and agricultural effluents to ensure the survival of the world’s reef systems.


Dawes, John. “Warmer Seas Threaten Large Corals.” Pet Products News International. May 2007. Pp. 90-91.

Jessup, David, and Melissa Miller, et al. “Sea Otters in a Dirty Ocean.” JAVMA, VOL 231, No. 11. December 1, 2007. Pp. 1648-1652.

Lewis, Darcy. “Saving our Coral Reefs.” Pet Age. April 2009. Pp. 34-38.

Murphy, Meghan. “Captivating Corals.” Pet Product News International. May 2007. Pp. 86-87.

Topics: algae, climate change, coral reefs, ocean temperatures

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