Coral bleaching is something you’ve probably heard quite a bit about in the past few years, and, unfortunately, are likely to hear more about in the future. The Caribbean had a major bleaching event back in 2005, then later again in 2010, and then just recently experienced another bleaching event in September 2023. Australia’s great barrier reef has suffered six mass bleaching events since 2016, all of which make the headlines as it’s the largest coral reef system in the world, and arguably the most famous.
- Aside from corals losing their various colors and turning a bright white, what exactly is coral bleaching, and what causes it?
- Does a bleached coral mean a dead coral?
- All the corals on the reef were shades of brown and had white tips, does that mean they are dead or about to be bleached?
These are just a few of the questions we regularly get as snorkeling guides. So, to set the record straight here are some cold hard facts, as well as some anecdotal encounters.
Before we get into coral bleaching, we need to know a bit about the anatomy of the corals. Corals are often equated to terrestrial plants, trees, and flowers. However, one table coral, for example, is not just a single living thing, like a fern or a pine tree. A table coral, or any hard coral for that matter, is actually a colony of a bunch of identical coral polyps all living together in a symbiotic relationship. These coral polyps are all hosted together on the calcium carbonate structure that they have created through their own secretions. Each coral polyp has a set of tentacles that are used to capture food, ward off predators, and clean themselves. The tentacles also host zooxanthellae, a type of photosynthetic algae that also helps feed the coral. The zooxanthellae is actually what gives the corals their color.
When coral bleaches, the coral polyps are actually expelling their zooxanthellae. As coral polyps are typically clear, all the apparent color of the coral is lost, giving it that bone-white look. But what causes coral to bleach? Corals will expel their zooxanthellae anytime they are stressed out. Things that stress corals out are abrupt loud noises, people chewing with their mouths open, talking in a cinema, etc. Oh, sorry, those are just things that stress me out. Stressful events for coral include unnatural changes in the sea temperature, pollution, sedimentation, and unusual differences in the salinity of the water. Any one of these things may happen on a daily basis, for example, in the southern belt of islands in Indonesia the sea temperature can change by ten or more degrees Celsius as the tide rises and falls throughout the day. However, if the temperature were to remain at one of these ends—either high or low—for a couple of weeks, then the corals would likely start to show signs of bleaching.
When a coral releases its zooxanthellae the coral is not dead, it’s just stressed. The coral polyps live on, only they’ve lost a major part of their food source as they relied on the zooxanthellae to produce sugars which they consumed. If the stress-inducing event does not resolve itself in a given period of time, anywhere from a couple of weeks to a month or so, the corals will eventually starve to death or become covered in algae which will also kill the corals. Corals can return to their normal zooxanthellae-hosting selves again, but only if the events that caused them to stress out resolve themselves before the corals die.
In August 2023, Duxy and I spent the better part of a month snorkeling in Egypt’s Red Sea. The first thing we noticed was that anywhere from forty to seventy percent of the coral was bleached. It was very hard to see from an environmental point of view because the reefs in Egypt are truly stunning, and also from a business point of view as we run a lot of tours through Egypt now. However, when we spoke to the local guides about it, they didn’t seem to be too concerned about it at all. According to them, bleaching events like this are quite common in the Red Sea around August—the hottest month of the year—as the country’s oven-like air temperatures increase the sea’s temperature and cause the corals to stress and bleach. They said that in a couple of weeks the stressed corals will begin to recoup their lost zooxanthellae and the reefs will resume being their colorful selves again. Sure enough, by the end of our time in Egypt we could already see that a few of the individual corals that we had mentally marked out were no longer white, and had regained their beautiful natural color. In follow-ups a few weeks after our trip there with our friends at the Red Sea Diving Safari resorts, they were pleased to share with us that all the water temperatures had dropped and the corals that were bleached on our visit were back to their normal colorful and healthy selves.
In another large scale bleaching event back in 2016, the whole north coast of the southern chain of islands in Indonesia, which included Bali, Lombok, Sumbawa, and parts of the Komodo area, experienced a sustained increase in sea temperature of two to three degrees Celsius which resulted in many reefs bleaching almost entirely, and in a few cases the reefs actually died.
It was heartbreaking to see, but what was really interesting to notice was that there were certain reefs that had been bleached quite significantly, and then in a few weeks totally recovered. The reason for these particular reef’s speedy recovery was the fact that they were in the channels between the islands where there was more water movement and the colder water from the south was able to help cool the reefs off. The reefs that bleached and died were the ones in the north where the colder water never reached. The water movement between the islands is a large part of what makes the reefs of Komodo and Alor so healthy and robust.
A few other things I’d like to clear up about the general health of corals are the white tips you often see on hard corals and the general color of corals. If you’ve ever looked at hard corals closely, you’ll notice that nearly all of them will have white tips. This is nothing to worry about, this is actually a good thing as it indicates that the coral is growing. The white is the new calcium carbonate structure which will soon be inhabited by coral polyps and their accompanying zooxanthellae.
Now, the color of corals is also something that people get confused about as well. Quite often guests will see that the corals on a reef are varying shades of brown and assume that it’s all dead because they once saw some corals that were purples and blues. Shades of brown, beige, and yellow are actually the most common type of color for hard corals. The color of a coral, regardless of species, is entirely up to the zooxanthellae that coral polyps host. For example, on the same reef, you can have one table coral in a dark brown, another in a yellow, and another in a light reddish color. Yes, corals can come in a whole host of other colors and display beautiful blues, purples, and oranges, but that—at least in my experience—is far less common than your beiges, browns, and muted reds and oranges.
Alright, so there you have it, a very simple and hopefully helpful explanation of coral bleaching. As you can see, bleaching does not mean the instant and forever death of a reef. While fragile, corals are quite resilient and will recover in time. That being said, if we don’t slow down our destructive tendencies toward the planet, the future of coral reefs may not be able to recover from bleaching events caused by sedimentation, pollution, salinity levels, and sea temperature changes.