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How is coral bleaching affecting national marine sanctuaries?

By Casey Brayton

June 2016

Gliding through the crystal blue seas of Florida Keys National Marine Sanctuary, you glance down into a scene straight out of a kaleidoscope: dazzling yellow and orange parrotfish dart in and out of view, while anemones dance softly in the current. Below, you spot rainbow sponges clinging to corals of emerald, lilac, pink, and…white?

panoramic view of a coral reef in florida keys
Coral reefs like those in Florida Keys National Marine Sanctuary support an abundance of marine life. Photo: XL Catlin Seaview Survey/NOAA

Since 2014, our ocean has been in the middle of its largest and most damaging coral bleaching event in recorded history. A recent NOAA forecast anticipates that coral reefs will likely be exposed to above average sea temperatures for an unprecedented third year in a row, leading to increased bleaching – with no end in sight. However, corals within national marine sanctuaries may prove to be bright spot in an otherwise bleak future for coral reefs.

bleached coral in Kaneohe Bay, Hawaii
In August 2015, coral bleaching was recorded in Kaneohe Bay, Hawaii, in Hawaiian Islands Humpback Whale National Marine Sanctuary. Photo: XL Catlin Seaview Survey

What is coral bleaching and why is it important?

Corals depend on colorful photosynthetic algae called zooxanthellae for food and oxygen. When exposed to stress, like the extended periods of elevated sea surface temperatures predicted in the bleaching forecast, corals lose these symbiotic algae. This loss exposes coral's white calcium carbonate skeleton, giving the coral a "bleached" appearance.

Coral bleaching is one of the most obvious visual signs of climate change in the marine environment. When we burn fossil fuels like oil and gas to heat our houses and drive our cars, we emit carbon dioxide into the atmosphere; this carbon dioxide builds up and acts as a heat-trapping blanket, warming Earth and the ocean's surface. This warming can be enough to trigger bleaching. Because corals are sensitive to environmental changes, bleached coral serves as a "canary in the coal mine" and warns scientists when the marine environment in an area is unhealthy.

photos of the a coral reef taken at three different time and coral condtions. Healthy - Dec. 2014, Dying - Feb. 2015 and dead - Aug. 2015
Bleaching was recorded in coral reefs of National Marine Sanctuary of American Samoa between 2014 and 2015. Photo: XL Catlin Seaview Survey

How will the bleaching forecast affect our sanctuaries?

Bleaching, fortunately, is not necessarily a death sentence. If stressors like elevated temperatures or poor water quality are resolved in time, symbiotic algae can return to the coral's tissue. However, if corals cannot recover their algae within a few months, they are likely to die.

National marine sanctuary reefs are no strangers to bleaching events, but not all of our reefs have responded similarly to bleaching. According to G.P. Schmahl, Superintendent of Flower Garden Banks National Marine Sanctuary, it all comes down to three things: location, location, location. Some reefs, by virtue of location, are more naturally protected than other reefs. For instance, the deeper coral reefs in Flower Garden Banks have historically had higher rates of recovery from bleaching than shallower reefs in Florida Keys. Similarly, geographically isolated reefs in Papahānaumokuākea National Monument have shown higher resilience, mostly because they are more removed from other stressors like pollution and overfishing.

close up of bleached coral
Without their colorful photosynthetic algae, bleached coral colonies appear bright white. Photo: Wendy Cover/NOAA

What can we do to manage future bleaching?

Working with your community to reduce your carbon footprint will help prevent climate change from getting worse and may reduce future warming of the ocean, helping to reduce the severity and frequency of future bleaching events. Volunteering your time to organizations that help protect sanctuary ecosystems and following all sanctuary regulations while visiting are excellent ways to help coral reefs recover.          

You can help, too, when you visit our sanctuaries: Always follow good ocean etiquette and keep your distance from corals while diving and swimming. If you notice bleaching or other problems in corals, notify sanctuary representatives and continue to check up on the reef's progress.

panoramic view of Hanauma Bay, Hawaii with snorkelers looking at the coral and fish swimming
Divers swim in Hanauma Bay, Hawaii, in 2014. Photo: XL Catlin Seaview Survey