Coral is highly sensitive to its external environment. Changes in ocean temperature, pollution and run-off, and pH (ocean acidification) can cause them to 'bleach' (1). When bleached, they can survive if optimal conditions return. However, if the stress is sustained, they will eventually die. As such, coral reefs are deeply threatened by environmental change and therefore make a good marker for the health of ocean ecosystems.
What is coral bleaching?
Coral live in a symbiotic relationship with microscopic, photosynthetic, algae called zooxanthellae (2). Like plants, the zooxanthellae capture sunlight and convert it into energy, providing essential nutrients to the coral (3). In return, the coral provides a protected environment for the zooxanthellae to live. Aside from providing nutrients, the zooxanthellae are also responsible for the colouring of many stony corals.
Under conditions of stress caused by changes in ocean temperature, agricultural run-off, and ocean acidification, resulting from human activities releasing CO2 into the atmosphere, the coral polyps expel their zooxanthellae, causing them to take on a white appearance, what is known as "coral bleaching" (4). The coral will not die instantly when the zooxanthellae is expelled, but if the period of stress is sustained, the zooxanthellae will be unable to re-enter the coral tissue, the coral will not get the essential nutrients it needs to survive.
What is left is the corals calcium carbonate skeleton. Algae begins to grow over the coral skeletons, blocking any chance of regrowth. After time, the skeletons will erode, resulting in the collapse of the reef structure. This collapse is not just an eyesore, it has catastrophic impacts for the ocean ecosystem, fisheries and local communities that depend on the reef for an income.
Why is it Important That we Protect Coral Reefs?
Habitat - Coral skeletons are an important component of the reef architecture and provide a habitat for 25% of all marine species (5). They provide shelter, spawning grounds and protection from predators. In their absence, fish stocks will decline, this has a knock-on effect to local communities who are reliant on fish for their main source of protein.
Tourism and Recreation - For coastal communities that are reliant on tourism for their income, the health of coral reefs is paramount to their survival. Ecotourism provides thousands of jobs and millions of dollars in income. It has been estimated that coral reefs have a global economic value of $36 billion per year (6).
Coastline Protection - Coral reefs absorb 97% of a waves energy (7), acting as a buffer against storm events, such as hurricanes and tsunamis, to 150,000 km of global tropical coastlines in over 100 countries (8).
Pharmaceuticals - Due to the stationary nature of coral, it has developed a plethora of chemical defences in order to gain protection from predators and to fight disease. Coral is therefore becoming increasingly recognised as an important source of new drugs to treat diseases such as cancer, Alzheimer's and arthritis.
What Can we do to Help?
Although small changes at home will make a difference (e.g. buying organic food as to not contribute to the use of pesticides and herbicides), the real issue here is climate change which requires global cooperation and action to reduce emissions and slow global warming. The death of coral reefs is not just a great loss to the ocean but it will greatly impact millions of people, in over 100 countries, who rely on the reefs for their income and as a food source.