This is the second installment in a three-part blog series about Coral written by Jess; read on to learn more about coral bleaching and how it doesn’t have to mean devastation.
Hello ocean lovers!
As I’m sure you all know by now, coral is one of my favourite animals. Although I love more than anything to be swimming on a reef, the more I see, the more I worry about the health of my beloved polyp friends and the future of these vital ecosystems. Coral bleaching is becoming more and more widespread every year. In 2016, on the Great Barrier Reef alone, bleaching hit 90% of corals, killing between 29-50% of the reef.
So, what is coral bleaching? Remember in the last blog we learnt that the tiny algae (zooxanthellae), that provide the wonderful colour of reefs, live in harmony with the coral. The most important thing that the zooxanthellae do, is provide carbohydrates for the coral, which in some cases can be up to 90% of the energy corals need to survive, grow, and reproduce. Like other plants, the zooxanthellae absorb energy from the sun and use it for photosynthesis. However, when the sun gets too hot, the zooxanthellae stop functioning as normal. The corals then get stressed and spit the zooxanthellae out, even though they rely on them for food. At night, the coral polyps use their tentacles to grab tiny zooplankton from the water, but they only get a fraction of the food they need, and they begin to starve. Because the zooxanthellae are the creators of the colour, without them, the coral becomes white, and their skeleton shows through the translucent tissue. Hence the name ‘bleaching’.
Temperature is the number one cause of bleaching, a small change of 1-2 degrees centigrade above or below the norm can cause their relationship with zooxanthellae to break down. But, corals are very fussy, and it’s not just temperature that causes them to become stressed and expel their algae. Changes in the salinity of seawater, as well as extra bright sunlight, pollution such as fertilisers from crops, and even sediment in the water from building work can all cause coral bleaching.
When we lose the corals, the structure of the reef begins to break down and it becomes more and more flat. Fish, crabs, shrimp, eels, and all the other creatures that live in the cracks and crevices of the reef lose their home and their protection, they lose a place to hunt and feed and breed. All these animals either flee to find a habitat elsewhere, or they die, and all the life which the coral reefs are known for disappears.
But, there is some good news! Bleaching does not have to be a death sentence. Not every reef that experiences a stress event is destroyed, and healthy reefs can bounce back. Some coral populations and their zooxanthellae may be able to adapt to the extreme conditions predicted during climate change. There are many different types of zooxanthellae and scientists have predicted that bleaching is a way for corals to find a better type of algae. For example, there are some strains of zooxanthellae which respond well to high temperatures, so when the water gets warmer, corals may spit out the algae they have to gain one which is more superior at working in these conditions. Next time we’ll talk about the corals which have mastered this technique and are the reefs of the future…
By Jess Kalisiak