Source: Phys.org 
September 19, 2012
"Researchers at Oregon State University for the first time have confirmed some of the mechanisms by which overfishing and nitrate pollution can help destroy coral reefs – it appears they allow an overgrowth of algae that can bring with it unwanted pathogens, choke off oxygen and disrupt helpful bacteria.
These "macroalgae," or large algal species, are big enough to essentially smother corals. They can get out of control when sewage increases nitrate levels, feeds the algae, and some of the large fish that are most effective at reducing the algal buildup are removed by fishing.
Scientists found that macroalgal competition decreased coral growth rates by about 37 percent and had other detrimental effects. Other research has documented some persistent states of hypoxia."
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