image credit NASA
"The so-called dead zones are caused by agricultural runoff, especially nitrogen-rich fertilizers, as well as the burning of fossil fuels. Pollutants from these sources cause marine eutrophication, whereby the ecosystem receives too many nutrients, triggering massive algae blooms, which eventually die and are broken down bacteria. In breaking down the algae blooms, the bacteria consume excessive amounts of oxygen, essentially starving the marine system."
It was just so nicely put that I figured I wouldn't try to re-word it. Check out the link for a larger version of the image!
Full article and image here (mongabay.com)