The gulf deadzone
Starting in the 1990's, "dead zones" in the ocean began to grow. Dead zones are areas of low oxygen concentration (hypoxic zones) - animals and plants suffocate and die. One of the biggest dead zones is located in the Gulf of Mexico. This dead zone also extends off the coasts of Louisiana, Texas, and Mississippi. These "dead zones" are caused by excessive nutrient pollution - things like waste water and agricultural runoff from human activities.
The hypoxic zones in 2005
Picture Source: http://www.noaanews.noaa.gov/stories2005/s2481.htm
The Gulf Dead zone as of 2014
Picture Source: http://www.noaanews.noaa.gov/stories2014/20140804_deadzone.html