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What is coral bleaching?

Updated: Mar 10, 2021

This past September and October Bocas del Toro was full of sunny days, blue skies… and white corals? Many residents of Bocas del Toro noticed white corals in our reefs, visible even from the surface on calm days when we had aquarium-like visibility. The reefs in Bocas are prone to bleaching during the warmest times of the year, with events often taking place in June and September-October. However, this past year seemed worse than normal.

Bleached brain corals near Isla Carenero.

What is coral bleaching, and why do corals turn white?

When you look at a coral, you are actually looking at a colony of small animals working together to act as a larger animal. Corals get most of their nutrition from microscope algae living inside of them, a type of dinoflagellate known as zooxanthellae. These zooxanthellae use the sun's energy to photosynthesize and produce food energy that the coral then uses. In return, the zooxanthellae get a home and nutrients.

However, if corals get stressed - most often due to high temperatures, but also caused by low oxygen or other stressful environmental conditions - they will expel their zooxanthellae. Zooxanthellae give corals most of their color, so without these algae all you see is the white calcium carbonate skeleton underneath the coral tissue. The resulting bright white color of the corals led to the term “coral bleaching.”

A mostly-bleached boulder brain coral (Colpophilia natans)

What happens to corals once they bleach? Are they dead?

Corals can live without their zooxanthellae for a little while, so if the temperatures return to normal within a couple of weeks, zooxanthellae can recolonize the corals and everything will be fine. However, if the corals are without zooxanthellae for too long, they will no longer have enough nutrition and will die. Unfortunately, we don’t know much about how long corals can survive without their zooxanthellae, as it seems to vary based on species and the conditions.

How do I tell if a coral is bleached or dead?

Bleached corals will be bright white, and if you get a close view you would still see the clear tissue and polyps of the coral. Once the coral is dead, there will be no tissue present on top of the skeleton. As anyone who has had a boat or dock knows, algae quickly begins to grow on anything that stays still in the ocean, and dead corals are no exception. Within a few weeks, dead corals typically have a layer of green algae growing on top.

The portion of coral in the lefthand side of the photo is starting to bleach, the coral in the center is fully bleached by alive, and the coral on the righthand side has died due to bleaching and is starting to be covered in algae.

Bleaching doesn’t necessarily happen all at once, and sometimes corals partially beach in areas that see more direct sunlight, or become fainter in color without turning completely white.

Some coral species, including the great star coral, Montastea cavernosa, turn neon green before they bleach (below, left); the fluorescence is a byproduct of an attempt by the coral to protect itself from UV radiation. The massive starlet coral, Siderastrea siderea, turns bright purple when it begins to bleach or is recovering from a bleaching event (below, right).

If you see corals that are white just around the edges or the tips, don’t worry, these corals are still healthy - they are just growing! New coral tissue at the growth edges hasn’t had a chance to be colonized by zooxanthellae yet, so they appear white.

What corals in Bocas del Toro are prone to bleaching?

Based on personal observations, the majority of corals bleaching this past September and October were brain corals. This included many boulder brain corals, Colpophilia natans, as well as symmetrical and knobby brain corals, Pseudodiploria strigosa and P. clivosa. I also observed staghorn corals, Acropora cervicornis, which were pale in color and partially beached. Massive starlet corals, Siderastrea siderea, were frequently seen with their bright pink-purple warning colors, as well as pale in color. Many other species of corals can bleach as well - I even noticed some bleached fire coral!

Partially beached symmetrical brain coral

In the past, I have also observed bleaching of leaf lettuce coral, Agaracia tenufolia (below), and I have heard reports from divemasters that the wall of lettuce corals at Hospital Point has been severely affected by bleaching within recent years. Have you noticed coral bleaching either now or in the past? Let me know about it down in the comments.

Why was the bleaching event so strong this year?

According to the monitoring conducted by the STRI here in Bocas, the water temperatures were warmer in 2020 than the average in recent years. In addition, we had an unusually sunny July, which is typically a time that corals have a respite from the intense tropical solar radiation between our warm months in June and September. Without this dip in solar radiation, the corals may have been more susceptible to bleaching in September.

Monthly sea surface temperatures at the STRI platform on Isla Colon in 2020 (blue) and averages from 2002-2019 (red). Data and graph from the STRI monthly physical monitoring.

What can we do to prevent coral bleaching?

The primary cause of coral bleaching is increased water temperatures due to global climate change. Water temperatures have been increasing over the past decades and will continue to do so. The root of this issue is the burning of fossil fuels, releasing carbon dioxide into the atmosphere and leading to more heat being trapped in the atmosphere. Unfortunately, the solution to this problem is a tough one and will require businesses, governments, and individuals to all come together to reduce our reliance on fossil fuels. We can each do our part to reduce our carbon footprint, but the big changes need to come in the form of regulations by governments and changes in policy by industries. The only way for this to happen is to demand regulation of industries and vote for governments that prioritize science and our environment.

There are also local causes of bleaching, and in one of the main issues in Bocas de Toro is hypoxia – low oxygen levels. Hypoxia is caused by increased nutrient runoff, stemming from agriculture and improper waste treatments. Reforestation near rivers, regulation of banana plantations and farms, and the creation of proper waste treatment facilities will all benefit our coral reefs, not only by reducing hypoxic events, but also by reducing sedimentation, algal growth, and vectors for disease. The only way we can expect corals to survive is if their environment becomes habitable once again.

We also have several local organizations in Bocas del Toro working to grow coral fragments in nurseries and outplant them on reefs. These organizations hope to determine which genotypes of corals that are more resistant to increased temperatures or other debilitating environmental factors and plan to speed up the natural process by which these corals can grow and repopulate the reefs. If you are interested in this work, you can check out Mother of Corals and Caribbean Coral Restoration.

Have you seen coral bleaching in Bocas?

Have you noticed coral bleaching in Bocas del Toro recently? Have you been able to go check on the corals since the waters have cooled off? I am curious to know if the corals have recovered or if they are now dead and starting to become covered in algae. Let me know the location, dates, species (or a photo or description if you are unsure), and a report and how the corals are doing now down in the comments if you have a chance!

Want to learn more or get involved?

Check out some of the educational resources posted on my website, and feel free to ask a question in the comments or send me an email. In addition, I am planning to start a Bocas Coral Monitoring network in 2021 – if you are interested in participating in the training and conducting coral health surveys on a reef that you regularly visit, please let me know!

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