English: Dead zones are bodies of water that do not have sufficient oxygen (3) levels in order to support most marine life. Dead zones are caused by oxygen-depleting factors which include, but are not limited to, human pollution (4). This is a process called eutrophication, where oxygen levels decrease as elements such nitrogen and phosphorous increase. A healthy river will have increased amounts of oxygen for consumption by organisms (1). As nitrogen increases, algae (5) produce large amounts of oxygen, but die from increased nitrogen. Decomposers then use all of the remaining oxygen decomposing the algae, resulting in no oxygen left and no oxygen being produced (2).