Coral bleaching could become an annual events in the Florida Keys between 2020 and 2034, earlier than predicted by existing computer models, according to a new study published in the journal Nature.
Bleaching occurs when reef-building corals, stressed by water temperatures rising by 1 to 2 degrees Celsius above maximum summer temperatures in a given location, expel the algae with which they live in symbiosis. The term ‘bleaching' is used because the algae give corals their coloration.
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The author of the Nature study -- Derek Manzello of NOAA's Atlantic Oceanographic and Meteorological Laboratories -- noted that, since the first recorded bleaching event in the Florida Keys in 1987, there have been six subsequent events.
Manzello examined in-situ water temperature for a number of reefs along the Keys, and examined the increase in the number of hours in August and September (when temperatures are at their highest for the year) in which sea surface temperatures exceeded 30.5, 31, 31.5 and 32 degrees Celsius -- temperatures that appear to correlate with bleaching events.
All of this matters, Manzello explained, because thermal stress as a result of elevated temperatures doesn't just cause bleaching, it can also bring about coral disease and declines in reproductive output and coral calcification.
The most important reef-building corals, said Manzello, have experienced the greatest declines over the last 30 years in the Keys and the wider Caribbean; and while some "weedy" corals have increased in relative abundance, they have little potential to construct or maintain architecturally complex framework structures that are vital to ecosystem function.
As Verena Schoepf, a marine biologist from the University of Western Australia and co-author of a separate study on bleaching, observed: "A lot of the diversity of other animals that live on coral reefs is based on the fact that you have a very three-dimensional reef structure which is made up of different sizes and shapes of coral. If there are only a few shapes or sizes of coral left then that will have cascading effects on all the other organisms ... because there will be less habitat."
This flattening of reefs, reported Manzello, has already occurred across much of the Caribbean. While the impact of warming on corals has long been acknowledged, Manzello concluded, the extent of temperature increases revealed by in-situ data suggests that "the impact of heat stress in the Florida Keys may be under-appreciated."