With 2016 being a record year for coral reef die off, it sadly looks like 2017 will not be too different. Not surprisingly, rising ocean temperatures are not slowing down just because the New Year has arrived, and scientists predict that the Great Barrier Reef must brace itself for another massive bleaching event as early as this month. To say the least, 2016 was not a good year for coral reefs. In fact even the greatest of them all, the Great Barrier Reef at 1,400 miles long, saw massive bleaching events last March and April.

Coral Bleaching Fundamentals

In today’s coral news the term “coral bleaching” and “bleaching events” gets thrown around a lot and sadly not many people actually understand what is happening to coral. First of all, coral is living; it is an organism that lives and dies. It is not just a rock like many people think. Second, coral bleaching is so much more than just coral losing its color or becoming “bleached”. This appearance is of course how the event earned its name, but is a much more complex process than what appears and this process begins with coral’s relationship with zooxanthellae.

Coral has a very important relationship with zooxanthellae. Zooxanthellae are dinoflagellates that are present in the cytoplasm of many marine invertebrates like coral. They are photosynthetic algae that live in the tissues of coral organisms and many times give coral their vibrant hues. Zooxanthellae and coral have a mutualistic symbiotic relationship, thus they benefit each other by a cooperative existence. Coral gives the zooxanthellae a safe environment and compounds they need for photosynthetic events while zooxanthellae produces oxygen and helps the coral remove waste. Zooxanthellae also provide coral with essential products of photosynthesis like amino acids, glucose, and glycerol which is critical for the coral in order to make essential molecules such as fats, proteins, and carbohydrates. They also use the products zooxanthellae provide to produce calcium carbonate used to build their hard exterior. The relationship between zooxanthellae and coral reefs is so important that it drives the growth and productivity of coral reefs or the vast groupings of hard coral outgrowth.  This relationship is responsible for a tightly knit web of recycling nutrients in the often nutrient poor tropical ocean waters coral is most commonly found in. It is so prominent and important is the ocean, that 90% of organic matter that is photosynthetically produced in seawater is produced by zooxanthellae and is transferred to a host coral tissue.

Besides the critical productivity and recycling of nutrients, zooxanthellae are also the component of coral that give stony or hard coral species their beautiful, vibrant colors.  Zooxanthellae is not a true part of coral; that is in stressful or less than optimal conditions it leaves or expels from the coral tissue. These conditions could include any range of water quality or a taxi the zooxanthellae is exposed too.  This could include but is not limited temperature, pH, dissolved oxygen, salinity, light, and water movement.  When an exodus of zooxanthellae occurs from a coral, the coral becomes a white or bleached color. This of course gave rise to the nickname “coral bleaching” which sadly is very prevalent in ocean news today. Coral decline is often associated with coral bleaching or coral appearing bleached or less bright. Coral bleaching is a term used so frequently today that it is easy to read about it and not understand what is actually happening to coral reefs that suffer such a loss. Since zooxanthellae is what gives coral their characteristic vibrant hue, when the zooxanthellae is expelled it causes the coral to become white or appear bleached. While coral bleaching brings the coral closer to mortality, it does not kill the coral instantaneously.  In other words, the coral survives the bleaching events but it leaves the coral very vulnerable and at high risk. While the symbiotic relationship can be re-established, it often takes too long to save the coral.

Coral Bleaching in 2017

The coral bleaching event just last year led to 63% of corals in the northern part of the Great Barrier Reef dying as well as 6% of the central part of the Great Barrier Reef dying. Scientists are quite concerned that they are seeing the same events occur this year, just in the next few months.

It is notable that the sea level temperatures in the Australian region have risen and stayed elevated since the events last year. In combination, the Australian coast has been struck with a heat wave that has caused cloud cover on the mainland to be low in general. With little rain fall and lots of sunshine, the next few weeks will be critical to the outcome for the Great Barrier reefs in 2017. Many researchers worry the events will strike the southern part of the reef this year. And they have every reason to worry since; a recent report shows that the water around the reef is warmer than it was this same time last year. It is no surprise then that the reef is showing more heat stress than before the 2016 bleaching began.


References and photos courtesy of: