Scientists attribute dead zones to a process that begins when nitrogen from agricultural runoff and sewage stimulates the growth of photosynthetic plankton on the surface of coastal waters. As the organisms decay and sink to the bottom, they are decomposed by microbes that consume large amounts of oxygen. As oxygen levels drop, most animals that live at the bottom cannot survive. "The overwhelming response of the organisms in our coastal areas is to migrate or to die," [lead author Robert J.] Diaz said. "To adapt to low oxygen water, it has to be a part of your evolutionary history. It's not something you can develop in a 40- or 50-year time period."