http://www.teachoceanscience.net/teaching_resources/education_modules/dead_zones/learn_about/
http://www.marineinsight.com/marine/environment/what-are-the-creepy-dead-zones-of-the-oceans/
http://www.inspirationgreen.com/dead-zones.html
http://education.nationalgeographic.com/education/encyclopedia/dead-zone/?ar_a=1