The 2017 ‘dead zone’ in the Gulf of Mexico measured 8,776 square miles (22,730 sq kms) . This is the largest ever recorded and is significantly greater than the average Gulf ‘dead zone’ of 5,309 square miles (13,750 sq kms) since measurements began 32 years ago, according to the National Oceanic and Atmospheric Administration (NOAA).
‘Dead zone’ is a common term for hypoxia, which refers to a reduced
level of oxygen in the water. The hypoxic zone is an area where oxygen levels drop too low (dissolved oxygen concentrations of less than 2 milligrams per liter) to support most life in bottom and near-bottom waters. The low oxygen conditions cause fish to leave an area and can kill bottom-dwelling organisms that cannot leave. As a result, marine habitats in these areas that would normally be teeming with life essentially become biological deserts – hence the term dead zone.
The extraordinary extent of the 2017 Gulf dead zone was due primarily to heavy May stream flows in the Mississipi River water shed, which were about 34 percent above the long-term average, carrying higher-than-average nutrient pollution loads primarily from agriculture and developed land runoff. Each spring as farmers fertilize their lands preparing for crop season, rain washes fertilizer off the land and into streams and rivers. The US Geological Survey (USGS) estimates that in May 2017 these nutrient pollution runoffs from the Mississippi and Atchafalaya rivers into the Gulf of Mexico were in the order of 165,000 metric tons of nitrate (equivalent to some 2,800 train cars of fertilizer) and 22,600 metric tons of phosphorus. The USGS operates more than 3,000 real-time stream gauges, 60 real-time nitrate sensors, and tracks trends in nutrient loads and concentrations throughout the Mississippi-Atchafalaya watershed, which drains parts or all of 31 states.
Hypoxic zones can occur naturally. There are many physical, chemical, and biological factors that combine to create dead zones, but nutrient pollution is the primary cause of those zones created by humans. Dead zones occur in many areas of the USA, particularly along the East Coast, the Gulf of Mexico, and the Great Lakes, but there is no part of the country or the world that is immune. The Gulf of Mexico dead zone reported here is the second largest dead zone in the world. The World’s largest dead zone is located in the Baltic Sea.