It becomes a dead zone
And I found this on the web to explain it
“Green stuff”, as you point out, liberates oxygen when it is producing carbohydrate. IOW it only liberates oxygen when it is physically growing larger.
But the other half of the equation is why “green stuff’ forms those carbohydrates. And quite simply they primarily do it for the same reasons that animals eat carbohydrate: because it provides something that they can burn to produce usable energy. IOW “green stuff” respires exactly like you do. They absorb oxygen, combine it with carbohydrates, and expel carbon dioxide.
Two sides of the same coin. The problem is that while respiration and the consumption of oxygen goes on continually, 24/7, the liberation of oxygen only occurs while the plant is physically getting larger.
As soon as the light levels drop, such as at night or when the water becomes silty, the plants stop liberating oxygen. But they continue to absorb oxygen at the same rate.
As soon as the nutrient levels in the water decline, the plants stop liberating oxygen. But they continue to absorb oxygen at the same rate.
As soon as the water temperature becomes too high or too low, the plants stop liberating oxygen. But they continue to absorb oxygen at the same rate.
As you can imagine, when you have umpteen thousand cells per milliliter of water, and they are all actively respiring and consuming oxygen and nothing at all is producing oxygen the water becomes anoxic incredibly fast.
http://boards.straightdope.com/sdmb/showthread.php?t=645268