From Vibrant to Vulnerable: The Truth Behind Coral Bleaching
Published on 9/1/24
Written by Jessica Colla
Coral reefs are some of the most vibrant and vital ecosystems on our planet, but they’re facing an increasing threat: coral bleaching. You may have heard the term before or even seen pictures of stark white coral, but do you know what causes it and why it’s such a big deal? This article will walk you through the basics of coral bleaching, explain how this process works, and why it poses a serious challenge not just for marine life, but for all of us. Whether you’re completely new to the concept or looking to deepen your understanding, we’ll cover everything you need to know by addressing the top 7 questions about coral bleaching.
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What Causes Coral Bleaching?
When scientists first observed this phenomenon in the 1980s, they were stumped. How could a coral suddenly lose its vibrant color? And what was causing it? After decades of research and countless experiments, we now know that coral bleaching is the result of stress. This is largely caused by rising ocean temperatures, which is tied to anthropogenic (human-caused) climate change. Since the early 1900s, the frequency and severity of bleaching “events” has increased. This coincides with an average global temperature rise of about 1 degree Celsius (or 1.8 degrees Fahrenheit). While that might not seem like much, it’s enough to have a huge impact on coral reefs.
You might be thinking, "1-2 degrees doesn’t sound like a lot. Daily temperatures fluctuate more than that. What’s the problem?” But think about it in terms of the human body. Our normal body temperature is around 37°C (98.6°F). But if your temperature goes up by just 2 degrees – 39°C (102°F) — you have a severe fever! As global ocean temperatures increase, coral reefs of the world are essentially experiencing a prolonged fever.
There are other causes of coral bleaching as well, including: physical damage from increasing storm activity, increased ocean acidity due to absorbed atmospheric carbon, excess erosion from coastal development, land-based pollution & marine debris, irresponsible tourism and toxic sunscreens, unsustainable fishing practices and overfishing, and more. But rising global ocean temperatures remain the most significant factor.
How does coral bleaching work?
When sea water changes dramatically (temperature, pH, salinity, etc.), the coral animal becomes very stressed. Imagine you are very warm, and then a thick fuzzy blanket was wrapped around you. Now you are overheating! This is how a coral feels during an underwater heat wave. As a stress response, the coral animals eject its symbiotic zooxanthellae (tiny algae that live inside the corals’ tissue), sending them out into the water. Without the algae, the corals lose their main food source and slowly begin to starve.
A side effect of expelling the corals’ colorful algae, is that the coral tissue becomes completely clear! The transparent tissues expose the bright white calcium carbonate skeleton beneath – giving it a “bleached” appearance. Imagine if this happened to you every time you got stressed out!
The diagram demonstrates how coral bleaches over time by losing its photosynthetic algae (zooxanthellae), leaving it "bleached"
Is a bleached coral dead?
The good news is that when a coral bleaches, it’s not always a death sentence – though it can be fatal if the stressors, like high water temperatures, aren’t reduced quickly enough.
Here’s how to tell the difference:
ALIVE - If you see a coral that’s bright white but still looks clean, that usually means it’s still alive. The coral’s tissue is still covering its skeleton, which prevents algae from growing on it. If the temperature drops back to normal quickly, the coral can recover by reabsorbing zooxanthellae (the algae it depends on) from the water, which helps it regain its color and start feeding again.
DEAD - However, if you spot a coral that is brown and covered in tufts of fuzzy algae, that means it’s already dead. The coral tissue has peeled away, exposing the hard skeleton underneath, which quickly becomes a surface for competitor algae to grow on (not symbiotic zooxanthellae). While some corals might only partially die, once a section is gone, it’s permanent. The fast-growing algae will take over and outcompete the coral for space, preventing any remaining coral from recovering.
Comparing coral health on the same reef over 1 year, demonstrating healthy coral, bleached coral & dead coral. Photo credit: XL Catlin Seaview Survey
Is Bleaching Permanent?
Corals can recover from a bleaching event and bounce back if conditions improve. Resilient individuals can reabsorb their symbiotic algae from the surrounding water when their tissues become “habitable” again. These colorful algae restore color to the individual and begin photosynthesizing again. With sufficient food, the coral gradually returns to their vibrant, healthy state. The key to recovery is creating the right conditions and time to allow this natural process to take place.
How does coral bleaching affect marine life?
Corals are more than just beautiful ocean animals—they play a crucial role in supporting marine life and protecting our planet. Often called the "rainforests of the sea," coral reefs are home to nearly 25% of all marine species, providing shelter, food, and breeding grounds for countless fish and other sea creatures. Eels and crabs hide in it, fish hunt around it, worms and urchins burrow into it, some organisms even eat it! During major bleaching events, the ripple effects can also be felt by larger predatory fish, sharks, sea turtles and even marine mammals like whales by disrupting food webs, breeding grounds and biodiversity. Since many species depend on coral, losing one species could trigger the loss of dozens or even hundreds of others that rely on it.
The loss of coral species can lead to the collapse of entire ocean ecosystems.
How does coral bleaching affect Us?
These unique ecosystems also fulfill roles that benefit humans! Over 40% of the world’s population lives within 100km (63 miles) from the sea! A healthy coral reef ecosystem is vital for sustaining local economies through fishing, tourism, coastal protection. More than 3 billion people rely on seafood as their primary source of protein, and 15% of all animal-based proteins consumed globally comes from the ocean. Coral reefs act as natural barriers, protecting coastlines from storms and reducing shore erosion by absorbing wave energy. We even derive many life-saving drugs, including those used to treat cancer, from reef organisms. By supporting coral reefs, we protect marine biodiversity, safeguard coastal communities, and ensure a healthy ocean for future generations.
what can we do to help?
Since the early 1900s, global coral bleaching events have become more frequent and severe. Each time corals bleach, they struggle to fully recover, making them more vulnerable to future bleaching and eventual death. Over time, smaller patches of coral survive, leaving them even more susceptible.
Some localized reefs have already lost over 90% of their coral cover, and globally, scientists estimate more than 50% of corals are gone. This paints a grim future, but we cannot afford to give up.
My dive team & I maintaining several "coral propagation trees" for Staghorn Coral (Acropora cervicornis) restoration near Andros Island, Bahamas.
Solutions and Hope
1. Coral Restoration Projects:
In the Florida Keys, the ‘1 Million Coral Project’ aims to propagate and transplant over a million corals onto degraded reefs. New techniques like micro-fragmenting have been proven to speed up coral growth for expedited recovery efforts.
2. Coral Resilience Research:
The Ruth Gates Lab at the University of Hawai’i studies coral bleaching to identify resilient coral and zooxanthellae species in hopes of fostering “super corals” in the future.
3. Coral Gene Banks:
Facilities like the Maui Ocean Center & the Marine Institute at MOC house rare, endangered and endemic coral species to preserve genetic diversity and restore lost species in the case of a mass bleaching event, like a living insurance policy!
4. Public Education:
Scientists & educators around the world are teaming up to educate and inspire the public through interpretive displays & signage, zoos & aquariums, citizen science programs and incorporating marine science into school curriculum.
5. Individual Action:
Making small daily changes, like reducing energy consumption, can lower our carbon footprint. With over 8 billion people on the planet, collective efforts can have a significant impact. By knowing the problem and taking action, we can help protect our reefs and ensure their future!
Want to learn more about coral biology and the physiology that makes coral reefs possible? Check out our blog on Coral 101! Want to learn more ways to help conserve coral reef ecosystems, these blogs provide tips and tricks for you at home, or at the beach!
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