Dead Zones

On NPR they were talking about dead zones, which are patches of ocean that can no longer sustain life because of depleted oxygen levels. Apparently these are growing, and while there are a few fringe people who are really concerned about it, it's hard for me to tell just how serious this really is. I wonder how it's been affecting the world economically and/or socially. It is largely caused by agriculture runoff into the ocean, mainly fertilizer. There is a budding movement for green agriculture, which would help.