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Why are there suddenly so many bleached corals along the Caribbean coast of Panama?

March 6, 2024

By Vanessa Crooks

A massive coral bleaching event in Panama’s Guna Yala islands along the eastern Caribbean coast signals a major problem with rising ocean temperatures and their long-term effects.

Bleach is a popular method to whiten clothes and other materials. But when marine biologists warn of the dangers of coral bleaching, no bleach is involved. Rising water temperatures cause corals to expel the tiny, colorful algae that live within coral polyps, exposing their white skeletons.

Most recently, a massive coral bleaching event in the Guna Yala Archipelago along the eastern Caribbean coast of Panama, indicates that something is the matter in the ocean.

“I had never seen such massive bleaching in the Caribbean of Panama,” said Juan Maté, Manager for Scientific Affairs and Operations and Marine and Coastal Advisor at the Smithsonian Tropical Research Institute (STRI) in Panama. Maté was visiting the archipelago last November with a group of students when he came across white corals all the way from El Porvenir Island to a small chain of islands around Punta San Blas and to Cayos Limones. The extent of bleaching probably reached the eastern sector of Guna Yala, according to locals.

“It looked like it had snowed underwater,” Maté explained.

Massive coral bleaching events have occurred before during El Niño years. This year’s El Niño, however, has been especially harsh. “This is one of the few times that an event this massive occurred simultaneously in the Caribbean and the Pacific,” Maté notes.

Maté is part of a research group that has the oldest database on corals in the Panama Pacific, started in the 1970s with former STRI staff researcher Peter Glynn. Glynn documented the first mass coral bleaching in the Panamanian Tropical Eastern Pacific, during the El Niño event of 1982 and 1983. Marine ecologist and Cornell University professor Drew Harvell, who accompanied Glynn during the diving field trip where they saw bleaching for the first time, wrote about this discovery in her book A Sea of Glass: “Here I was, a brand-new coral reef researcher, with the most respected, experienced coral reef scientist of our time, and he didn’t know why his reef was devoid of all pigments? At the time, none of us were aware of the importance of our discovery.”

Why do corals react like this?

Biologically classified as an animal, corals catch small organisms for food with the tentacles in their polyps. But shallow-water corals also get a lot of nutrients from yellow and brown zooxanthellae microalgae, which inhabit the coral and give it color. All reef-building corals have a symbiotic relationship with these algae: microalgae use sunlight to perform photosynthesis and produce nutrients like oxygen and energy in the form of glucose. The coral feeds on these nutrients to grow its skeleton, fertilizes the algae with its waste, and provides it with a safe place to live. Thus, they both benefit from this relationship.

But when the coral becomes stressed, it reacts by expelling the microalgae from the structure. Some corals become completely white, others are partially colorless, if the bleaching occurs more gradually.

The diversity of coral species in the Caribbean is also broader, compared to the Pacific, where there is one predominant species.

Credit: Jorge Alemán, STRI

This doesn’t mean that a coral has died; a bleached coral can still recover, if the algae return. But this needs to happen fast. If stressful conditions persist, their chances of recovering diminish. “They can take anywhere from one to three months to die, depending on the species and their level of tolerance,” Maté explains.

However, corals have adapted so much to their symbiosis with the zooxanthellae, that they rely on them for up to 95% of their nutritional intake; without the zooxanthellae, the coral must capture food to compensate.

Corals can reproduce both asexually, by cloning themselves from broken off pieces, creating a new genetically identical colony, and sexually, by releasing eggs into the water to be fertilized by neighboring corals, or by releasing coral larvae. These require ideal conditions and a time frame that allows them to survive.

Credit: Sean Mattson, STRI

In the Pacific, where corals have gone through several bleaching events, the dominant species Pocillopora have adapted to other types of algae that are more tolerant. “Those corals can look fine, like nothing bad is happening,” Maté establishes.

In 2004 the last massive coral bleaching event, was reported in areas of the Caribbean of Panama, including Guna Yala, resulting in coral death. But reefs can survive as long as there is enough material left to regenerate from. In the meantime, conditions need to improve so that the corals have a chance to recover and grow again.

What can be done?

Other factors threaten coral survival, besides the increasing temperature of the ocean waters.

Rising sea levels affect how much sunlight the microalgae receive and photosynthesize, forcing corals to grow faster and change their shapes to capture more sunlight. And as the waters reach further inland, they drag sediments which causes the water to be murkier, affecting how the sunlight penetrates as well. Bigger, more violent storms, and more towns and agriculture near the coast also lead to more sediment in the water.

If there’s coral death, the skeleton can become the home of new organisms, like macroalgae, which grow faster and can take up a lot of space, forming a canopy that shades the coral and makes it more difficult for the microalgae to photosynthesize and for the coral to regenerate. Some animals that usually control the growth of the macroalgae, like the parrotfish and the surgeonfish, are being affected by overfishing, excessive tourism, and more. Another important herbivore that controls algae growth in coral reefs is the long-spined black sea urchin, which experienced a massive mortality in 1983 and has yet to recover to its pre-mortality numbers.

New research by STRI scientists working in Bocas del Toro explores if lack of oxygen in warm water may be another major factor.

But right now, the main cause of coral bleaching and subsequent death is usually thought to be the ocean warming caused by El Niño. And as global climate change worsens, these phenomena become more aggressive, and less reversible.

In the Panamanian Tropical Eastern Pacific, where the El Niño effects are more widespread and visible, STRI marine scientists study corals as part of the Rohr Reef Resilience Program, trying to understand how corals adapt to increasingly stressful conditions.

By collecting samples from the Las Perlas Archipelago, where corals are exposed to upwelling (a phenomenon where trade winds push away warmer surface waters and cause colder, more nutrient-rich waters from the bottom to rise), and in the Coiba National Park, where there is no upwelling, they can analyze what factors influence coral resilience and adaptability to extreme conditions.

The data they collect on how corals can resist the effects of climate change may make it possible to develop solutions to make other corals and coral reefs more resilient, protecting one of the world’s most biodiverse ecosystems before it’s too late.

The Smithsonian Tropical Research Institute, headquartered in Panama City, Panama, is a unit of the Smithsonian Institution. The institute furthers the understanding of tropical biodiversity and its importance to human welfare, trains students to conduct research in the tropics and promotes conservation by increasing public awareness of the beauty and importance of tropical ecosystems.

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