What depletes oxygen from water, creating "dead-zones"?

Question

What depletes oxygen from water, creating "dead-zones"?

Answer 1

Nate nailed it. Give'em the 10 spot.

Answer 2

algae blooms caused by chemical reactions from human pollution is the most common dead zone creator.  this can happen in lakes, lazy rivers, ponds, and anywhere else algae easily forms.  the algae reacts to certain chemicals and silt from runoff, multiplying and growing at an explosive rate.  after all of the wildlife dies from lack of air, the algae dies as well, leaving nothing alive.