New research led by scientists at Scripps Institution of Oceanography at UC San Diego suggests that by improving overall ocean health, corals are better able to recover from bleaching events. These events occur when rising sea temperatures force corals to expel their symbiotic algae, known as zooxanthellae. It is a phenomenon that is expected to rise in frequency as global climate change increases ocean temperatures.
The new findings, published in the journal PLoS ONE, show that following a major bleaching event, Mountainous star coral (Montastraea faveolata) was able to recover. The coral was from various reefs in Honduras and Belize and was able grow normally after two to three years when the surrounding waters and reef were relatively healthy. In comparison, corals living under poorer, polluted conditions were not able to fully recover even after eight years.
"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."
Core holes are plugged with concrete caps and tagged for future overgrowth monitoring.
(Photo Credit: Scripps Institution of Oceanography, UC San Diego)
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 pollution, overfishing, and sediment/nutrient run off from land.
By using x-rays, the researchers were able to examine the coral's annual growth rate records since 1950, including the period before and following a major bleaching event in 1998.
"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."
Corals are widely considered to be barometers for global warming and are important for biodiversity in the world's oceans. Coral reefs thrive in tropical oceans with moderate temperatures and low nutrient and sediment input from land-based sources.
Coral bleaching occurs when the tiny zooxanthellae that live within the tissues of coral polyp and are responsible for the coral's color are lost and the coral turns white.
Jessica Carilli is collecting a core sample using an air-powered drill system.
(Photo Credit: Scripps Institution of Oceanography, UC San Diego)
The fast-recovering corals were collected from Turneffe Atoll, which is farther offshore than the main Belize Barrier Reef and the marine biological reserve, Cayos Cochinos, 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 significant runoff impacts; meanwhile Utila is quite heavily populated and suffers from waste from sewage and other sources.
Coral reefs at Utila Atoll in Honduras shows signs of bleaching.
(Photo Credit: Woods Hole Oceanographic Institution)