Bleached Out

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Rising sea temperatures, pollution and runoff from coastal development, and impacts from overfishing have made tropical oceans a difficult place for environmentally sensitive corals to survive.

A new study led by scientists at Scripps Institution of Oceanography at UC San Diego suggests that by improving overall ocean health, bleached corals are better able to bounce back to normal growth rates more quickly.

Coral bleaching occurs when rising sea temperatures force the invertebrates to expel symbiotic algae living with the tissues of coral polyps known as zooxanthellae. Coral bleaching, in which coral turns white, is a phenomenon that is expected to increase in frequency as global climate change increases ocean temperatures worldwide.

“You can imagine that when you are recovering from a sickness, it will take a lot longer if you don't eat well or get enough rest,” said Jessica Carilli, Scripps graduate student and lead author on the study.  “Similarly, a coral organism that must be constantly trying to clean itself from excess sediment particles will have a more difficult time recovering after a stressful condition like bleaching.”

Corals are widely considered to be barometers for global warming and are important for biodiversity in the world’s oceans.  Coral reefs thrive in warm tropical oceans under just the right conditions that include moderate temperatures and low nutrient and sediment input from land-based sources. Protecting reef health from local sources of stress, such as runoff, can improve resilience to global warming stress.

Carilli and colleagues analyzed 92 coral cores collected from four reef sites off the coast of Honduras and Belize. The cores were collected from reefs with different degrees of local stress from pollution, overfishing, and sediment and nutrient runoff from land. By using x-rays, the researchers were able to examine the coral’s annual growth rate records since 1950, including the time before and after a major bleaching event in 1998.

The findings, published in the July 22 issue of the journal PLoS ONE, show that following a major bleaching event, mountainous star corals (Montastraea faveolata) on various reefs in Honduras and Belize were able to recover and grow normally within two to three years when the surrounding waters and reef were relatively healthy.  In comparison, those corals living with excessive local impacts, such as pollution, were not able to fully recover after eight years.

The fast-recovering corals were collected from Turneffe Atoll, which is farther offshore than the main Belize Barrier Reef and Cayos Cochinos, a marine biological reserve off the northern coast of Honduras.  Those that took longer to recover to pre-1998 conditions were from the Sapodilla Cayes in southern Belize and Utila in Honduras. The Sapodilla Cayes are a marine protected area, but experience the effects of significant runoff. Local impacts from heavily populated Utila probably result from development, sewage, and other sources, the researchers said.

In a related paper by the authors recently published in the journal Global Change Biology, the researchers looked at a century-long record of thermal stress experienced at the Mesoamerican reefs. The study compared thermal stress against a century-long record of bleaching events as recorded in coral cores.
  
 “We were surprised to find that although temperatures were much warmer in the 1950s, corals did not bleach,” said Carilli. The authors’ results suggest that massive bleaching events in more recent decades appear to stem from the coupling of mild warming and local stress such as runoff, human-induced coastal impacts, and increased fishing pressure.

“It is clear that Mesoamerican corals really fell off a cliff in 1998 -- nearly everybody suffered mass bleaching,” said Dick Norris, Scripps professor of paleooceanography and co-author of the study.  “There are no pristine reefs in the region, but the ones in the best shape clearly are more resilient than those that are long-suffering. It shows that a little improvement in growing conditions goes a long way in recovering coral health.”

 

-- Annie Reisewitz

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