Coral bleaching: what is it and why should we care?

Like the rest of the world, much of the Western Indian Ocean is currently experiencing coral bleaching. Coral bleaching reports have already been received in Seychelles, including Cousin Island Special Reserve. On April 15, the United States’ National Oceanic and Atmospheric Administration (NOAA) confirmed that we are in the midst of the fourth global coral bleaching event. Mass events are declared when extensive bleaching occurs at a regional or global level. In this two-part series, Charlotte Dale, a member of Nature Seychelles’ Reef Rescuers explains what coral bleaching is, its impacts, and what scientists are doing in response to the current event.

Coral bleaching reports have already been received in Seychelles-(Photo: Hugo Bret)

What is coral bleaching and why is it a concern?

Coral bleaching occurs when these aquatic animals expel their photosynthetic and symbiotic algae known as zooxanthellae that provide them with nutrients and pigments. Without these algae, they become white or pale. Corals expel the algae when they become stressed as result of increased sea temperatures, pollution, or other environmental factors. Coral reefs are especially vulnerable to predicted climate change because they bleach rapidly in response to increased sea surface temperatures. These mass coral bleaching events we have experienced recently are primarily triggered by abnormal increases in water temperature, often associated with intense El Niño activity.

Coral reefs are critical because they support diverse marine life and provide essential shoreline protection, tourism, and fisheries support. A bleaching event compromises all these services that depend on healthy coral reefs and therefore coastal communities' livelihoods.

What are the signs of bleaching?

The fourth global coral bleaching event has been declared (Photo: Hugo Bret)

A change in colour, where they lose their vibrant hues, is the most visible sign of bleaching. They become white or display neon colours, such as pink, yellow, or blue. Some severely stressed corals exhibit fluorescence due to fluorescent pigments.

Do corals bleach differently? Are some species more vulnerable than others?

Corals do not bleach at the same level; some species are more vulnerable than others. Bleaching severity can range from mild to severe, depending on factors such as environmental stress, species type, and depth. In massive bleaching events, more sensitive corals may bleach earlier than others.

Growth forms, which refers to the shape the coral takes as it grows such as branching, columnar, plating, encrusting or massive, tissue thickness, and growth rate can all influence how vulnerable a species is to bleaching. Generally, massive growth forms, thicker tissues, and slower growth rates are more resistant to bleaching.

Bleached coral-(Photo: Mehdi-Boudault)

Can corals recover from bleaching?

Despite bleaching's detrimental effects, corals can recover and protect themselves. Recovery involves zooxanthellae algae restoration. When warming events are short-lived, corals can sometimes regrow their algae quickly. But when events are long-lived, extensive and repeated, corals can die and recovery can be difficult and take decades.

Corals that bleach in vibrant colours are more likely to recover than those that bleach white. To mitigate coral bleaching impacts and promote reef resilience, it is crucial to understand these differences in bleaching susceptibility among coral species.

How are marine scientists monitoring coral bleaching events? What methods and technologies are used?

Coral bleaching events are monitored using various methods and technologies. Satellite data is used to monitor coral bleaching on a large scale. On-site surveys and observations are conducted to assess local events. Reef assessment technologies, such as underwater imaging systems, provide detailed information about coral health, including bleaching extent and severity. Citizen science initiatives where the public logs their observations are also being utilized to enhance data collection and expand monitoring capabilities.