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What is Coral bleaching?


To understand coral bleaching, first of all, we have to understand what coral actually is.
Corals are animals. However, corals make a calcium carbonate skeleton that looks similar to a rock and have a symbiotic relationship with plant-like cells called zooxanthellae. Coral itself is actually a small individual soft-bodied organisms, called a polyp. A coral polyp closely resembles a very small anemone. They grow in warm shallow waters that receive plenty of light. Most coral species live in waters close to the warmest temperature they can tolerate around 85° F or 29° C, meaning a slight change in ocean temperature can harm corals.

Coral tissue itself is actually mostly transparent and their calcium carbonate skeletons are white. So when the zooxanthellae leave the coral or is expelled, it leaves the coral appearing white. When this happens on a large scale, the phenomenon is termed coral bleaching. Coral bleaching is theorized to be a generalized stress response of corals that may be caused by a number of factors. If the conditions necessary to sustain the coral’s zooxanthellae cannot be maintained, this will lead to the expulsion of the zooxanthellae.

Factors that can cause bleaching consist of:
• Increased (most commonly), or reduced water temperatures
• Changes in water chemistry mostly acidification due to chemicals and herbicides which find their way through runoff also increased sedimentation (due to silt runoff)
• Bacterial infections
• Changes in salinity increased rate of the melting of the ice caps also increasing sea level
• Low tide and exposure
• Cyanide fishing used to paralyse fish to catch live
• Mineral dust from African dust storms caused by drought
• Four common sunscreen ingredients, that are non-biodegradable, and can wash off of skin (The effects of these chemicals in sunscreen are toxic at the concentration equivalent to a drop of water in an Olympic pool).

Although the coral polyps feed on zooplankton and other food particles, the majority of reef-forming corals rely for a large proportion of their nutritional requirements on their zooxanthellae. This means that without them they are liable to starve. Coral growth and reproduction are reduced and the coral becomes increasingly susceptible to disease. If stress factors reduce and the zooxanthellae return, the coral can recover, but prolonged bleaching causes the death of the coral.


Why should we care?
• Coral reefs support more marine life species per unit area than any other marine environment, even though they only cover less than one percent of the ocean floor. Reefs also provide spawning, nursery, refuge and feeding areas for a large variety of organisms, not just fish.
• Tourism such as Diving which is one of the key components of reef tourism and recreation.
• Coral reefs are vital to the world’s fisheries. They form the nurseries for about a quarter of the ocean’s fish an estimated one billion people have some dependence on coral reefs for food and income from fishing.
• Coastal protection, coral reefs are complex structures capable of dissipating a large amount of the force of waves and storms. Reefs can save us billions every year in terms of reduced reconstruction, insurance costs and displacement of people. (It’s not called the Great ‘Barrier’ Reed for nothing!)

What can we do? To prevent coral bleaching, we need to avoid actions that induce stress on the corals, or keep these at a minimal level, such as the increase or decrease in water temperature due to global warming. Here on Koh Tao this month, the water temperature has increased to about 31°C and bleaching can now start to be seen in some coral. Other ways we can help avoid touching the coral. Minimise the use of chemically enhanced fertilizers, insecticides, pesticides, and herbicides. These kind of chemicals are non-degradable and they will eventually end up in our oceans. Apply sun cream in plenty of time before entering the water or choose eco-friendly brands. Join in on any beach or reef clean ups. Here at Master Divers, we organise a weekly clean up of our local beach in Mae Haad, every Friday so feel free to come down and join us! We also run conservation dives such as the Coral Watch health chart which involves checking the colour of the coral and sending this data to Coral Watch who use this to help analyse how healthy coral is worldwide and bleaching events. If we all do our little we can make a difference.


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