Every year for 34 years scientists have monitored the size of the Gulf of Mexico dead zone or hypoxic zone, which forms every spring. Last year’s dead zone was approximately 5,780 square miles, the size of Connecticut. This year NOAA scientists are forecasting this summer’s Gulf of Mexico dead zone will be approximately 7,829 square miles, the size of the land mass of Massachusetts. This is close to the record size of 8,776 square miles set in 2017.
According to NOAA the hypoxic zone is caused by excess nutrient pollution, primarily from human activities, such as agriculture, wastewater treatment and urbanization occurring throughout the Mississippi River Watershed. The excess nutrients stimulate an overgrowth of algae, which eventually die, then sinks and decomposes in the water. The resulting low oxygen levels near the bottom are insufficient to support most marine life, causing animal life to suffocate and die.
This year’s abnormally high amount of spring rainfall in many parts of the Mississippi River watershed is a major factor contributing to this year’s large dead zone. It has led to record high river flows and much larger nutrient loading to the Gulf of Mexico. This May, the discharge in the Mississippi and Atchafalaya rivers was about 67% above the long term average between 1980 and 2018.
The Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, a group working to reduce the Gulf dead zone through nutrient reductions within the Mississippi River watershed, has set a 5 year average measured size target of 1,900 square miles.
While nutrient inputs to the Gulf of Mexico vary from year to year because of natural swings in precipitation and discharge, U.S. Geographical Survey also tracks longer term gradual changes in nitrate and phosphorus loading into the Gulf of Mexico from the Mississippi River.
“Long-term monitoring of the country’s streams and rivers by the U.S. Geological Survey (USGS) has shown that while nitrogen loading into some other coastal estuaries has been decreasing, that is not the case in the Gulf of Mexico,” said Don Cline, associate director for the USGS Water Resources Mission Area. “USGS monitoring and real-time sensors, coupled with watershed modeling, will continue to improve our understanding of the causes of these changes and the role they play in the Gulf and other coastal areas.”
Photo courtesy of NOAA