August 22, 2023

Coral Bleaching 101

What is coral bleaching? What does it mean? Why is it bad? Will our grandchildren ever see a coral reef? Did you know that corals are animals? We constantly hear in the news that water temperatures are increasing and it is causing mass coral bleaching events. Coral bleaching, in short, is when the water is too hot for the corals to survive. Corals are very important for ecosystems across the world, so bleaching is detrimental to not only the coral reefs, but the overall planet.

What is the science behind coral bleaching? Put very simply, coral bleaching is when the coral gets too hot or undergoes other environmental stressors. There are three stages of coral bleaching: healthy coral, stressed coral, and bleached coral. In its healthy state, coral have a symbiotic relationship with their zooxanthellae, which are photosynthetic algae. These algae serve as the primary food source for corals, making up around 90% of their nutrients, and also give coral their vibrant colors! When coral is stressed, zooxanthellae start to leave the coral tissues because the coral becomes inhospitable. This means that the coral have decreased food supplies and decreased protection from the sun. When coral become fully bleached, they are white or very pale and are incredibly vulnerable. If left in this state for too long, the coral can succumb to disease and die. When corals are bleached, they can recover if the stressor that is causing the bleaching event stops, oftentimes requiring water to cool to normal levels. So a bleached coral can survive, but once coral are bleached, they are very vulnerable to dying.  

Some factors that can cause bleaching are: changes in ocean temperatures, polluted waters, overexposure to sunlight, and exposure to elements during extremely low tides. As global warming impacts ocean temperatures, corals everywhere are suffering under great stress. Runoff and pollution are causing waters to become overly rich in nutrients which negatively impact corals. Corals thrive  in waters that are nutrient poor, because nutrients in the water lead to a decrease in sunlight hitting their photosynthetic algae, which need the sun to make food for the corals. So a decrease in sunlight leads to less food for the corals to eat.

What does it mean for the world? Why are reefs important? To answer this question, we need to look at the data. Coral reefs are important because they cover less than 1% of the ocean, but 25% percent of the world’s marine life live on coral reefs. 1 billion people worldwide benefit from coral reefs through food production, coastal protection, and income from tourism and fisheries. Over 4,000 species of fish are dependent on coral reefs at some point in their lifespans. If you want to look at it economically, the total net benefit of coral reefs on the global economy is $29.8 billion dollars per year from tourism, fisheries, and storm protection. Coral bleaching is devastating for coral reefs because they grow extremely slow, at an average rate of .008 to .12 inches a year. Many corals are thousands of years old, and one bleaching event can kill them. These bleaching events are only going to become more common as water temperatures continue to increase, making it critical to protect the coral reefs we already have.

So is there any hope for our reefs? To answer this question, we interviewed CIRT Intern Sara Jane Shulman, who recently studied abroad in Fiji, where the reefs are very vibrant. The reason we use the word vibrant is because in Fiji, as well as other reefs around the world, there is a new phenomenon called Day Glo that is being studied by scientists from all over the world. This adaptation is now being found in corals that have experienced remarkably high temperatures repeatedly, it is helping these coral to survive bleaching events. Sara Jane says “this is huge for coral reefs and could help them to be more resilient in the face of increasing ocean temperatures.”

The way Day Glo works is that certain corals contain pigments capable of glowing and these pigments work as a sort of coral “sunscreen.” When corals expel their zooxanthellae, photosynthetic algae that provide food and protection for the corals, due to environmental stressors they are left with little nutrition and increased light exposure. The white branches of this bleached coral are inhospitable for their symbiotic zooxanthellae, thus some corals are expelling pigments to protect themselves from being sunburned and also try to get their zooxanthellae to repopulate. The corals go from being bleached, to being fluorescent, to slowly repopulating with algae and becoming normal colors once again. This is very important because instead of dying, they can survive mass bleaching events. This new adaptation is still being studied, but it is very good news for the coral reefs.

Maybe you have a new favorite animal! Corals are incredibly interesting animals that have a complex, but important, role in the marine ecosystem. We need to be doing every possible thing we can to save the reefs. Some things we can do are try and mitigate our personal climate impacts, by lessening our carbon footprint. We can use reusable items when possible to avoid more trash ending up in the oceans. We can also wear reef safe sunscreen and make a conscious effort not to touch or step on coral when we are entering these habitats. We can also research efforts and legislation impacting the reefs. Overall, coral bleaching is an issue that everyone should be concerned about and we need to do everything we can to save coral reefs around the world.

Photo taken on Lady Elliot Island, Australia by CIRT Intern Sara Jane Shulman.