What Does "Coral Bleaching" Mean?
Updated: Sep 23, 2023
There have been a ton of articles in the media recently about coral bleaching, and almost as much confusion about it. I’ve seen posts, articles, and videos talking about “coral bleaching” as if the term means “coral death,” but it doesn’t (thank god!).
First Things First - What is Coral, Really?
Corals are animals. Yep, this image to the right is an animal.
Well, actually, this is a colony of animals, known as lobed star coral. It's one of my favorite types of corals!
The image to the left shows individual coral polyps. This image is just a super zoomed-in version of the corals above.
So, from smallest to biggest, corals consist of:
Individual coral polyps (pictured here to the left). These are soft-bodied animals that secret calcium carbonate skeletons. The polyps group together into colonies which then behave as one organism.
Communities of coral colonies bind together with calcium carbonate to form the iconic reef structures you know and love.
So to put that in perspective, when we talk about coral reefs, especially like the Mesoamerican Barrier Reef where I live, we are actually looking at an uncountable number of coral polyps.
What is Coral Bleaching?
To answer this question, we first need to talk about what color healthy corals are, and where their coloration comes from.
Coral polyps are actually translucent/clear, and the calcium carbonate skeletons and structures that encase them are white.
The bright colors that we associate with healthy coral actually come from algae, not from the coral itself. A healthy coral polyp will absorb algae (called zooxanthellae), which will then provide the grand majority of the food and energy for the coral via photosynthesis. This is why corals do the best in shallower, clear waters where the algae can do their most productive photosynthesis.
When a coral becomes stressed, it will reject these algae. No algae means no color.
“Coral bleaching” can be a somewhat misleading term – it isn’t that something bleached out the coral’s color and turned it white, it’s simply that without the zooxanthellae, the white skeleton is the only thing that’s visible through the clear polyps.
Is Bleached Coral Dead?
No. This has been one of the biggest misstatements with recent headlines about coral bleaching events, especially like the one happening now in the south of Florida. While reporting from reputable media has spoken about this bleaching event correctly, social media has been full of errors.
Since late July, southern Florida and the Florida Keys have been under a massive bleaching event, and recent studies have found that some areas show 100% bleaching. Many people have then misinterpreted this to mean that the coral is 100% dead in this area, which is not correct (again, thank god).
Why Do We Care About Bleached Coral?
Corals only reject their algae as a result of severe stress. This is a way of scaling back and entering in pure “survival mode.” If the stressor is removed, bleached corals can and will reabsorb algae, regain their color, and continue onward.
But, this process takes a toll. Prolonged bleaching or too-frequent bleachings can weaken and kill coral colonies. Remember, the majority of a coral’s food comes from the same algae they’ve rejected. They can only live so long without it.
In short, we care about bleached corals because they aren’t dead…yet. However, they are distressed and at high risk of mortality if the stressor isn’t removed in time.
What Stresses Out Corals Enough to Cause Bleaching?
One of the biggest stressors for coral, and what is responsible for the current bleaching event in Florida, is heat. Water temperatures in southern Florida have been the highest ever recorded since recording began in 1985. The corals simply can’t take it, and if temperatures don’t begin to drop, the bleached corals will die.
Other factors that lead to coral bleaching include the introduction of harmful chemicals to the water. This can come from toxic sunscreens and bug sprays (for the love of all things holy STOP praying Off! before you go in the water. There are reef-safe options and ZERO reason to be so selfish).
Other toxins come from the commercial world. Dumping chemicals into the ocean. Oil spills. Runoff soil full of fertilizers from new construction. The removal of critical mangrove forests that capture and filter these types of runoff events. And so on and so forth.
Why Should I Care?
Coral is critical to the health and balance of the ocean and its coastlines. Barrier reefs
protect the coast from storms and erosion.
Even though coral reefs cover only 1% of the ocean floor, 25% of ALL marine life is dependent on them. According to NOAA, over a half billion people around the world depend on reefs for food, income, and protection.
Simply put, the ocean cannot survive without healthy reefs, and humans cannot survive without the ocean. And odds are good, you’re a human, so that means YOU!
I Don’t Live Near the Ocean. Does What I Do Matter?
YES! Rising ocean temperatures are part of a larger global pattern of warming. Driving your car, running your AC, buying a new shirt from a “fast fashion” brand every week, supporting companies that pollute, buying a condo built on the graveyard of a mangrove forest…these things are directly contributing to rising temperatures and ocean pollution.
It’s easy to feel like what you do when you’re landlocked has no effect on the ocean, but it does. Currents, winds, and commerce are connected globally and affect us all.
But don’t get discouraged. Positive changes can be small and add up over time. The reef isn’t dead yet, and you can help make sure that never happens!
I AM NOT A MARINE BIOLOGIST. THIS ARTICLE IS MEANT TO BE HELPFUL FOR THE LAY READER. IT SHOULD NOT BE CONFUSED FOR A SCIENTIFIC PUBLICATION. ANY ERRORS ARE MY OWN AND MADE IN THE SPIRIT OF ELEVATING GENERAL KNOWLEDGE.
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