Human Impacts

There are three main factors threatening the life of the Great Barrier Reef: global warming, sediment runoff, and pollution. Because of these factors, a fifth of the reefs have been destroyed, a quarter of the reefs are endangered, and another quarter face long-term collapse.

Global warming is causing increased UV radiation, which as a result is preventing the plants and algae from photosynthesizing. This is a problem because coral reefs live in a symbiotic relationship with millions of microscopic algae. These algae foster the growth of corals by aiding in nutrient production. With this increased UV radiation, algae are leaving the coral reef and the coral inevitably dies.

Temperature and chlorophyll concentration in relation to the reefs.
http://upload.wikimedia.org/wikipedia/commons/thumb/c/ce/GBReef_TempChlorophyll_200602.jpg/415px-GBReef_TempChlorophyll_200602.jpg

Sediment runoff is a result of deforestation from farming. Because this sediment was previously used for growing plants, the sediment is filled with fertilizers such as nitrogen and phosphorous. These nutrients cause massive algae growth that leads to a depletion in oxygen. As a result of these algae blooms, crown-of-thorn starfish, which eat coral across the Great Barrier Reef, have been spiking in population. A doubling of chlorophyll in water leads to a tenfold increase in the crown-of-thorn starfish survival rate. Unfortunately, 80% of the land adjacent to the Great Barrier Reef is farmland that supports agricultural production, so an immediate solution to this problem is not possible.

Crown-of-Thorns Starfish are a problem in other reefs besides the Great Barrier Reef as well.
http://biodilloversity.files.wordpress.com/2011/09/starfish.jpg

Pollution in the air inevitably travels to the ocean though rain, which in return, slowly acidifies the water. Greater than 30% of carbon dioxide emitted by human activities goes into the ocean. The acidification of the sea causes two problems to occur: coral bleaching and low calcium carbonate accretion by organisms. 

Increasing acidification and temperature has caused a dramatic decrease in calcification.
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