Coral reefs are an excellent place to see the synergistic effects of man's alterations to the environment. They are one one of the richest ecosystems on the planet, they are generally accessible, occurring in warm seas and shallow water. Roughly 65% of salt water fish are reef dwellers.
Coral reefs are suffering from a variety of anthropogenic assaults. the one that has received the most media attention is coral bleaching. Coral is a small animal with tentacles to snare food, but it is also dependent on a species of algae that lives symbiotically in its body and produces additional food by photosynthesis. When the sea temperature rises above 28 degrees C., the coral expels the algae and if the condition persist it will starve. Out of 207 coral reefs monitored by the Global Coral Reef Alliance in 1998, 75% bleached, and in 50% most of the coral died.
The second major killer of coral is pollution, both in the form of sediment washed from the rivers, and swamps resulting from construction, agriculture, and most often from the destruction of mangrove swamps that trap the sediment. The second form of pollution is the dissolved chemical soup coming from sewage and agricultural runoff. The effect of the toxins is obvious, but the effect of the nutrients is just as severe, for the excess nutrient cause algae blooms which deprive the coral of light, invade the coral directly, and smother the coral with dead material.
Many coral reefs are being killed by diseases that have not been seen, or at least have not been a threat in the past. They may not bee new, but the combination of other environmental factors has opened the door for these pathogens to invade the coral ecosystem.
The increase in CO2 in the water corresponding to increases in atmospheric CO2 has two other effects working against the coral. The acidity of the water increases, making calcium, the substance from which reefs are made, less available. Secondly the sea level is rising as the global temperature increases, so just when the coral needs the ability to grow at a maximum rate to keep up with rising water levels, it is held back by lack of calcium, bleaching, disease, competition with algae blooms, and being smothered by sediment.
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