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December 08, 2005
Coral Reefs' Fight for Survival
Coral reefs are a part of one of the most beauitful and spectacular ecosystems on the earth. They are home to at least 1/4 of all marine plant and animal species, while also protecting many of the world's coastlines from erosion. In order to understand human impact on coral reefs, I'll give you a little summary of what they are. A reef is built of tiny coral animals who contrust calcium carbonate shells around their bodies. In turn, the reef grows when the larva from a young coral polyp attaches itself to an ideal spot, typically on top of an existing coral, and then begins to build itself a new shell. From this building, the reef structure rises about one to two inches per year. Of course coral dies, and when it does, new larvae build their shells on top of them, allowing reefs to grow to keep up with a moderate rise in sea level. A single reef can grow for millions of years and become hundreds of meters thick--the world's largest, the Great Barrier Reef of Australia, is 1,250 miles long.
Since coral reefs consist of many diverse organisms, its animals have a "symbiotic" relationship with minute algae known as zooxanthellae (quite a word). In short, this algae provides the coral with oxygen and food made from photosynthesis. The zooxanthellae then get a home and nutrients from the coral. This algae is what gives the coral its fascinating and bright colors. The environmental conditions the coral live in determine different kinds of algae that live there, wuch as light availability, depth and water temperature. Coral need a certain set of conditions to thrive, as do the zooxanthellae in order to provide photosynthesis to the coral. The water has to be warm, clear and moderately saline with constant salinity. Without this, coral reefs are in danger. Now, human activity is estimated to have killed ten percent of known coral reefs, while thirty percent of them are currently in critical condition. Sadly, only thirty percent of the total are thought to be stable.
Humans are not the only cause to coral depletion. Many environmental factors take their toll, including the tsunami in the Phillipines, which damaged thousands of miles of coral reefs. Scientists are currently trying to establish ways to replenish this damaged coral. Also, sediment runoff can and has killed reffs because zooxanthellae cannot photosynthesize in cloudy or murky water; logging and plowing has increased this runoff, especially in the Philippines and the Costa Rican coast. In these areas, 75% of the reefs have died as a result. Just a list of a few more environmental problems include ultraviolet radiation from ozone depletion, pesticide and industrial runoff, oil spills, coral mining and damage from tourists and divers that have resulted in killed reefs in othre various locations.
When these environmental changes occur, it puts stress on the coral animals. Under stress, they expel their zooxanthellae and cause the coral to turn white, rather than die. Since this algae turns the coral color, the reef results in white, known as "coral bleaching". Corals can recover from one bleaching event, but multiple times can kill them. A theoretical cause of bleaching is the contamination of the reefs by extra nutrients from sewage, or the increase of seawater temperatures due to global warming.
Natural selection may also be playing a part in this. A recent research study has concluded that when a coral expels its zooxanthellae, it may actually be aiding in its own survival. When the environment rapidly changes, the algae that the coral gets rid of is no longer optimal, which in turn allows the coral to host a different type of algae that is better suited to the coral's survival in the new environment. This was discovered by Wildlife Conservation Society's (who conducted it) and Dr. Andrew C. Baker, a scientist at the New York Aquarium. He switched corals found off the coast of Panama, putting deep water species of coral in shallow water and shallow water species in deep water. The coral that went from deep water to shallow bleached but then took on new algae and ended up surviving. The shallow to deep water coral didn't bleach, which says it did not change its algae, and most of the coral died. It was concluded that while bleaching is a risky strategy for survival, it does indeed help the coral respond to rapid environmental change, which we all know is fast upon us.
Posted by Keely Gerhold at December 8, 2005 04:21 PM | 1. Critical Thinking in Environmental Science | 7. Humans, Knowledge, and Technology