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 are prone to coral bleaching in Bocas del Toro 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.
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.”
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 the length of time 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 begin 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.
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; 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.
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.
Which 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. I even noticed some bleaching fire coral! In the past, I have also observed bleaching of leaf lettuce coral, Agaracia tenufolia, and I have heard reports from divemasters that the dive site at Hospital Point that the wall of leaf lettuce coral has been severely affected by bleaching within recent years.
Why was the bleaching event so strong last 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.
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 (https://blogs.ei.columbia.edu/2018/12/27/35-ways-reduce-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 are hoping that some genotypes of corals 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.
Want to learn more or get involved?
As part of Bocas Hope Spot I am planning to give trainings on identification of different coral species as well as signs of coral bleaching and disease with the hopes of organizing a community-based coral monitoring network in Bocas. If you are interested in participating in the trainings and conducting coral health surveys on a reef that you regularly visit, please send an email to CarolynJKovacs@gmail.com for more information.
Article written by Carolyn Kovacs. Follow her on Instagram @vidamarinabocas to learn more about the magic under the Bocas del Toro Caribbean Seas and check out more articles of hers on her website: https://www.carolynkovacs.com/
Editor’s note: Carolyn Kovacs is hosting a talk on Bocas del Toro coral species identification and coral bleaching awareness 7pm Monday, June 7, at Bocas Dive Center in the Divers Paradise hotel. Join her! For more information and to reserve a spot, contact her at CarolynJKovacs@gmail.com. Space will be limited due to COVID-19 distancing protocol, so reservations are recommended, though not required.