Dead zones can occur naturally however it is no coincidence that dead zones are increasing around the world. They occur often at river mouths and receiving waters that are collection points for runoff. Increased human activities have lead to greater amounts of nutrients and other pollutants coming off the land from wind and water.
The link below is for an educational presentation describing the history of research related to Chesapeake Bay, with an emphasis on new insights into what controls the size of the dead zone, how its size has varied in past, and what we should expect in future decades. There are other case studies of interest also.
find out more at: http://www.teachoceanscience.net/teaching_resources/education_modules/dead_zones/learn_about/
For Australian specific information see