Coral bleaching is spreading as ocean temperatures rise

Photo Courtesy of Catlin Seaview Survey - National Geographic

Large-Scale Bleaching Events Endanger the World’s Largest Coral Reefs

By Rachel Gentile               December 10, 2017

In recent years, coral reefs worldwide have suffered severe bleaching events and destruction due to human activity. Though reefs only cover about 0.1% of the ocean floor they are home to 25% of all marine life. Coral bleaching occurs when a coral undergoes stress, typically when a change of water temperature forces coral to expel the algae (zooxanthellae) living in its tissues, turning the coral white. The algae are the coral’s primary food source and give it its distinctive vibrant colors. The algae and coral maintain a symbiotic relationship, cycling nutrients back and forth in order to survive in the nutrient poor tropical waters. Researchers believe that the expulsion of the algae is a regulatory mechanism used to maintain zooxanthellae density in the coral and under stressful conditions the rate of expulsion increases in order to avoid accumulating too much damaged algae. Bleached coral is not dead, but it is more susceptible to stress and disease because of this lack of nutrients. Coral can recover if the water temperature returns to normal quickly, but if it doesn’t, the coral will die. With global temperatures rising, more frequent and fatal bleaching events are becoming a major concern.

For the second year in a row, aerial surveys show massive bleaching across the Great Barrier Reef which has many scientists concerned for the safety of this important ecosystem. Many scientists attribute these extreme bleaching incidents to a particularly strong El Niño event as well as increasing global warming. Though the reef has been hit by multiple bleaching events in the last 20 years, two events only twelve months apart are significant. According to James Kerry, a senior research officer at the Australian Research Council Centre of Excellence for Coral Reef Studies in Queensland, Australia, “It takes at least a decade for a full recovery of even the fastest growing corals”. The 2016 event was centered around the northern portion of the reef whereas the 2017 event affected the middle of the reef, meaning that a small overlap area was affected by both bleachings. Any corals damaged by both bleaching events have a very low chance of recovering.

The Great Barrier Reef Foundation is a collaborative organization working with government agencies, research institutions, and volunteers to save the Great Barrier Reef from the dangers of global warming and human interaction. They have many different projects going on across the reef, focusing on different aspects of coral restoration as well as developing new methods of increasing coral resilience to stress. Some of these projects include sequencing coral DNA to determine why some species are more resilient than others, protecting coral islands as well as their adjacent reefs and marine life, and creating the first detailed map of the entire reef using satellite and drone technology.

The damage is not just limited to Australia, the Mesoamerican reef is undergoing bleaching as well as other man made dangers. The Mesoamerican reef has undergone multiple bleaching events, but they have not been nearly as severe as the bleaching in Australia. The Mesoamerican reef is the second largest reef in the world and  the largest in the western hemisphere, which makes it a popular tourist destination. Between tourism and fishing it is estimated that the reef generates about 15% of Belize’s GDP. Increased waste production due to tourism influxes, overfishing, and unequally enforced environmental regulations all pose a danger to this important ecosystem. Fish like the parrotfish feed on macroalgae, a type of seaweed that competes with the coral for space. In areas with overfishing the macroalgae take over and coral population declines. There are environmental regulations in place protecting certain high diversity areas from fishing and tourism, but in many places such as the coast of central Belize, they are seldom enforced.

Although the dangers are extensive, the Mesoamerican reef has become a success story in recent years because of coral restoration projects. Coral restoration is conducted by growing small corals in a relatively controlled environment and then introducing them to coral reefs with the hope that they will survive and replenish the naturally occurring coral. Fragments of Hope is an organization that has been growing coral in nurseries for ten years and has made great progress in restoring the coral of Laughing Bird Caye National Park in Belize. They grow coral by stretching ropes across the sea floor and then inserting small branches of coral in between the strands of the rope. Once the coral is grown, the rope can be cut into fragments and easily transported and distributed among the reef.

The Mesoamerican Reef is also home to the Smithsonian’s Healthy Reefs for Healthy People Initiative (HRI), an international collaboration of coral research, management and conservation organizations. The HRI has tasked itself with monitoring the health and recovery of the Mesoamerican Reef and reporting on global bleaching events. Global bleaching events occur when multiple reefs around the world experience mass bleaching at the same time. These mass bleaching events have occurred in 1998, 2010, 2014, 2016, and now in 2017. Despite recent global bleaching events, the HRI reported that coral cover of the Mesoamerican reef has increased from 10% to 17.5% since 2006.

Although restoration and monitoring will improve the health of coral reefs globally, they are not a permanent solution. If ocean temperatures continue to rise, global bleaching events will continue to occur across the globe, endangering the important ecosystem of coral reefs. Many marine scientists predict that if nothing is done to stop mass coral bleaching, coral reefs could cease to exist within the century.



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