Terminology Tuesday

Published on October 21st, 2014 | by GrowWNY Intern


Terminology Tuesday: Bleaching Event


Up until recently, I always thought a bleaching event was what my mom did for fun on the weekends when she was doing laundry. Apparently, I was wrong. Not only is it not related to laundry whatsoever, it is actually a very serious issue for coral reefs residing in oceans around the world.

Staying true to its name, a bleaching event is when the coral organisms that make up coral reefs become covered in a white substance, making them appear as if they’ve been bleached. What is this white substance, and where does it come from? A majority of coral species have a mutually symbiotic relationship with an extremely small species of algae known as zooxanthallae. The coral provides this zooxanthallae with shelter and protection and in return it produces nutrition for the coral. In fact, this tiny algae is able to produce in upwards of 90 percent of the total nutritional energy that coral requires in order to live, grow and reproduce. Clearly this is quite the beneficial relationship for the coral.

Unfortunately, coral is a very sensitive organism and can only survive in a specific temperature range. When the water becomes too warm, the coral expels the zooxanthallae that is so important to its survival. However, it is not the zooxanthallae that causes the coral to turn a bleached white color. Surprisingly, the bleached white color is just the structure of the coral itself without the algae inside it. Little known to the general public, the zooxanthallae is not only a nutrition source but the source of coral’s stunning array of colors as well. Without the zooxanthallae contained within, the coral reveals its true white form and is left on its own to gather nutrition. Ultimately, this leads to the death of the coral unless they reabsorb the algae before it is too late.

At this point, the true risk of bleaching events becomes evident. Nowadays, climate change is causing oceans to become warmer and bleaching events to become more frequent. Warming oceans can’t support the survival of many of the incredible coral reefs that currently inhabit our oceans. Furthermore, bleaching events can also be directly caused by human interactions with their environment. One example is our massive pollution of the seas. As we continuously dump contaminated sewage water and other waste into our oceans, we are contributing to this natural phenomenon. Pollution of this sort and on such a large scale can easily cause bleaching events. In addition, dredging ocean floors for sand can throw tons of sediment into circulation in the water which can also be a factor.

We have already seen large scale bleaching events threaten some of the world’s most pronounced coral reefs. In 1998, the Great Barrier Reef off the coast of Australia was 50% bleached and in 2002 another event saw that figure rise to 60%. If bleaching events continue to happen, they will be detrimental to the coral reefs that house some of the greatest examples of biodiversity that our planet has to offer.

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