I recently read an article in which Ove Hoegh-Guldberg, a researcher at the University of Queensland in Australia, suggested that if we don't throttle back our emissions of carbon dioxide into the atmosphere within the next two years, the Great Barrier Reef may eventually disappear. I'm sure some of my readers may think Dr. Bill is playing Chicken Little again. Yes, there are many who doubt global warming is a product of human activity despite the preponderance of scientific evidence. Even one of the scientists formerly touted by climate deniers recently announced that his data verify the same warming trend observed by others. But then there are still some who think the Water Planet is flat (wouldn't the water then run off the edge) or that the Sun revolves around the Earth. Heck, the Egyptians way back in Cleopatra's time (B.C.) knew that wasn't true
Having had the pleasure of diving the Great Barrier Reef a decade ago, I was well aware that it was being threatened by a variety of factors. One was agricultural fertilizer entrained in the run off from coastal and watershed farming. These fertilizers do just what they're intended to do... cause plants, and seaweed (algae) to explode. You may ask, "isn't this a good thing?" No, not when the algae smothers the coral reefs.
In the case of global warming the threat is different. Increasing water temperature can kill certain species of algae. For example, our giant kelp dies out when exposed to temperatures in excess of 68 F for two weeks or so. In large part this is due to the lack of nutrients in warm water. In the case of the Great Barrier Reef, the algae affected are tiny microscopic species that live within the individual coral polyp itself and the impact is largely due to thermal stress. You know how sluggish you feel during a hot spell (assuming we get some this summer).
Coral and some of their relatives including a few local sea anemones incorporate these tiny algae, known as zooxanthellae, into their cells. This gives them their array of colors, although they are usually somewhat greenish in nature. Now the coral don't do this just to show they have good fashion sense. The tiny algae also provide them with two benefits: food production and waste disposal.
Coral, like anemones, trap small critters including plankton using the stinging cells (nematocysts) on their tentacles. However, in the clear waters of the tropics, plankton are not as abundant as they are here because the numerous huge sponges filter the water very effectively. The corals need a secondary source of food, and they get it from the tiny algae embedded in their tissues. Through photosynthesis, the algae use sunlight as well as the nutrients contained in the waste products and the carbon dioxide from the coral to produce food (carbohydrates) that they share with the polyp.
Now most coral like it hot (why else would you choose to live in the tropics?), but their symbiotic algae may not like it too hot. Temperature changes of as little as 2 degrees F may cause decreased production of the pigments used for photosynthesis, the expulsion of the zooxanthellae from the polyp or the ingestion of the zoxanthellae by the coral. Such events are commonly referred to as bleaching events, and if they are severe the coral colonies and reef may die.
A few years ago it was discovered that some coral species may incorporate several different species of zooxanthellae rather than just a single type. In such cases, some of these zoxanthellae may be able to tolerate temperature stress and survive, reducing the impact on the coral itself. All the coral needs to do is invite the more temperature tolerant species to live inside it. Recently research has shown that only a quarter of the coral species that can host multiple types of zooxanthellae may overcome this.
Coral reefs are fixed habitats. The coral polyp is embedded in the calcium carbonate structure that forms the "backbone" of the reef. If conditions change, and the coral can't adapt... it dies. Motile critters like Dr. Bill, fish and many invertebrates can move to areas of more suitable "climate." The Great Barrier Reef, sometimes referred to as the largest living "organism" on Earth, can't.
Coral do produce larval forms that can drift with the currents to new regions. They can cross great distances under the right conditions, establishing new reefs in areas with slightly cooler temperatures. Hoegh-Guldberg calculated that new reefs would have to be created at distances of about nine (9) miles each year. At that rate I'm not expecting any of northern hemisphere coral reefs to appear in Catalina waters during my lifetime. I guess I'll just have to move myself to them!
© 2011 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 29, 10:00 AM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!
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Local anemone species with zooxanthellae and an individual that grew in a cave where the zooxanthellae couldn't
survive; and coral species from tropical reefs off Belize showing the color of zooxanthellae
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Dr. Bill Bushing.
Material and images © 2011 Star Thrower Educational Multimedia