What Is Coral Bleaching?

What is coral bleaching?

Bleaching is a coral’s response to stress, in this case warming ocean waters. Corals bleach or turn white when they expel the symbiotic algae that live inside and nourish them. These algae are known as zooxanthellae, and they are the main food source for corals. They also give corals their vibrant colors, so without them, corals appear pale or “bleached,” and begin to starve. Over time, coral can die without the nourishment that zooxanthellae provide.

What causes coral bleaching?

Many stressors can trigger bleaching in coral, but elevated ocean temperatures are the most common cause. When ocean temperatures rise higher than normal and stay that way for weeks or months, the corals expel their zooxanthellae, leading to bleaching. A rise in ocean temperature as small as 3° F can cause coral bleaching. Hawai‘i’s waters were 3-6 degrees warmer this year.

Can corals recover from bleaching?

If a reef is healthy to start with, it is much more likely to resist or recover from bleaching events. But like humans, corals that are already stressed – from sediment, algae, or pollutants in the water – often cannot take the added stress of a severe rise in temperature, and they will bleach and die in response. For those that don’t die, recovery can take months to years depending on the severity of bleaching. By reducing the stressors that we have control over -- like overfishing, sediment, and invasive algae -- we can improve a reef’s ability to resist and recover from coral bleaching.

Why is coral bleaching a problem?

Bleaching can cause a cascade of decline on a reef if the coral dies. For example, when coral reefs bleach in an unhealthy ecosystem, invasive algae can quickly move in, covering the coral skeleton and smothering any remaining living coral, thereby preventing recovery. Over time, the skeleton breaks down to rubble, leaving no habitat for the marine life it previously supported. Reef fish that eat the smaller animals that live in or on the reef leave in search of food. Lost reefs mean fewer fish for food, less tourism, fewer recreational activities, and the loss of coastal protection that reefs provide by buffering the effects of powerful waves.