Mississippi River flooding impacts reach beyond neighboring land, and this summer, a near record “dead zone” in the Gulf of Mexico will result. This hypoxic area of low top no oxygen can kill fish and other marine life. An annual occurrence, the dead zone is caused by excess nutrient pollution due to urbanization and agriculture. Once in the Gulf, these nutrients spur algae growth, which then die, sink, decompose and deplete oxygen in the water. The 2019 forecast of 7,829 square miles is close to the record size of 8,776 square miles set in 2017 and larger than the 5-year average measured size of 5,770 square miles.

Due to excessive rainfall in the Mississippi River watershed, discharge through the river was well-above the 40-year average. The USGS estimates a transport of 156,000 metric tons of nitrate and 25,300 metric tons of phosphorous were deposited into the Gulf of Mexico in May. Those nutrient loads were 18 percent and 49 percent above the long-term average, respectively.

The annual forecast issued by NOAA assumes average coastal weather conditions but the dead zone size could be changed by major events, such as tropical storms, that mix ocean waters. A survey to be done in August will confirm the size of this year’s dead zone and that information will be used to refine the model used for the annual forecast.

“The models help predict how hypoxia in the Gulf of Mexico is linked to nutrient inputs coming from throughout the Mississippi River Basin,” said Steve Thur, Ph.D., director of NOAA’s National Centers for Coastal Ocean Science. “This year's historic and sustained river flows will test the accuracy of these models in extreme conditions, which are likely to occur more frequently in the future according to the latest National Climate Assessment.