Weekly Ocean News

13-17 November 2017


Items of Interest


Ocean in the News:


Concept of the Week: Living Coral and El Niño

El Niño episodes of 1982-83 and 1997-1998, the most intense of the 20th century, confirmed the connection between higher than average ocean temperatures and bleaching of hermatypic corals. (Hermatypic corals live in warm shallow water and build large reefs.) Water temperatures higher than 29°C (the normal maximum sea surface temperature in the equatorial eastern Pacific) can trigger expulsion of zooxanthellae, microscopic dinoflagellates whose symbiotic relationship with coral polyps is essential for the long-term survival of coral. Without zooxanthellae, coral polyps have little pigmentation and appear nearly transparent on the coral's white skeleton, a condition known as coral bleaching. If maximum temperatures are not too high for too long, corals can recover, but prolonged warming associated with an intense El Niño (that may persist for 12 to 18 months) can be lethal to coral. Most hermatypic corals thrive when the water temperature is 27°C, but do not grow when the water becomes too cold. Although the ideal temperature varies with species and from one location to another, the temperature range for optimal growth is quite narrow--only a few Celsius degrees. This sensitivity to relatively small changes in water temperature is an important source of information on past climates as fossil coral is a significant component of many limestones. Evidence of bleaching episodes in fossil corals may yield important clues to past changes in the world's tropical ocean.

Coral, sometimes referred to as "the rainforests of the ocean," provides a base for local ecosystems and have many benefits (e.g., fisheries, tourism) that are important in many parts of the globe. Hence, vulnerability to El Niño-associated warming is an object of considerable scientific interest. During the 1997-98 Niño, NOAA charted significant coral bleaching from portions of the Great Barrier Reef near Australia, French Polynesia in the south Pacific, in the Indian Ocean off the coast of Kenya, and around the Galapagos Islands off the coast of Ecuador. Closer to home, coral bleaching was reported in the Florida Keys, the Cayman Islands, and off the Pacific coast of Panama and Baja California. Fortunately, damage from the 1997-98 El Niño warming was less drastic than the 1983-84 El Niño when up to 95% of the corals in some locations died. Many of the corals damaged in the late 1990s have at least partially recovered including important reefs in the Florida Keys. For additional information on coral status, go to the NOAA website http://www.coralreef.noaa.gov/.


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Prepared by Ocean Central Staff and Edward J. Hopkins, Ph.D., email hopkins@aos.wisc.edu
© Copyright, 2017, The American Meteorological Society.