Some people are aware of dead zones, also known as low-oxygen or hypoxic zones, in the oceans and seas of the world. We have a very large one in the Gulf of Mexico largely due to agricultural runoff collecting in the Mississippi River and then dumped when it enters the Gulf. Warmer water also contributes to hypoxic zones because warmer water has a lower ability to hold oxygen than cold water. But there are some different factors going on around the world that contribute to low-oxygen in deep seas. This article explains how hypoxia happens and the problems associated with it.
Ocean Dead Zones Are Growing March 10, 2010