The coral, a kind of small anemone of the seas with a limestone skeleton, lives in symbiosis with a micro colored alga, the zooxanthelle. He offers him shelter, and to thank him, the alga will provide him with oxygen and nutrients. Without this exchange, the coral could not feed, plankton being rare because of the high temperatures, this symbiosis is therefore indispensable to the survival of the corals! But this balance is broken as soon as a stress situation intervenes, the coral then expels the zooxanthellae and becomes white, its limestone skeleton appearing. When the coral bleaches, it is not yet dead, it can even survive for another 2 or 3 months. It is only when it is covered with filamentous algae that it can be declared dead.
Increased temperatures
As soon as the water reaches 31 ° or more, a stress situation is created, and the coral whitens. If the temperature goes down fairly quickly, the coral can reborn and find its beautiful colors. There were 3 major waves of bleaching in 1998, 2002, 2008. 93% of the Great Barrier Reef is already affected, and in 30 years it has lost half its corals! Climate change and El Nino are the most important contributors to the rising temperatures of ocean water.
Water pollution
As a result of industrial wastewater discharges, shipping, or air pollution, more and more toxic substances pollute the oceans. Metals, organic pollutants, hydrocarbons, petroleum and pesticides rub shoulders. Sunscreens have joined them, used excessively by swimmers and divers, they also contribute to the death of corals.
The Acanthaster planci, the killer starfish
The Acanthaster, this giant starfish that feeds on coral tissue, is causing havoc in the reefs by weakening and even killing the coral! Coral reefs contain about 25% of marine biodiversity, that is, their importance. It would be high time to wake up!
