The oxygen-depleted “dead zone” that forms each year in the Gulf of Mexico off Louisiana and Texas is forecast to cover about 4,155 square miles (10,761 square kilometers) this year.
That's about 1,200 square miles (3,107 square kilometers) smaller than the average during the 36-year history of dead zone measurements in the Gulf, the National Oceanic and Atmospheric Administration announced Monday, but still more than twice as large as the long-term hypoxia-reduction goal set by a federal-state task force.
This year’s forecast would be about 1,000 square miles (2,589 square kilometers) larger than what was actually measured last year.
Scientists from Louisiana State University, the Louisiana Universities Marine Consortium, NOAA and other universities will travel along the coasts of Louisiana and Texas during the last week of July and first week of August to see if the estimate holds up.
But LSU marine biologist Nancy Rabalais, who has overseen the dead zone mapping cruises since they began in 1985, said there’s a good chance the estimate could be high for a second straight year. The reason: Less rainfall in the Midwest is expected to again keep the flow of the Mississippi and Atchafalaya rivers’ freshwater below normal levels well into the summer, The Times-Picayune/The New Orleans Advocate reported.
And while less river water means less nutrients that cause the dead zone, the low river flows are also likely to disrupt the process that causes the low-oxygen conditions.
The Gulf dead zone is largely created by urban and agricultural runoff and discharges of nitrogen and phosphorus to the Mississippi River, which drains 41% of the continental United States. In the Gulf, the nutrients feed an overgrowth of algae, which die and sink to the bottom, using up oxygen from the ocean floor as they decompose. Fish, shrimp and crabs can swim away. Animals that are slower or fixed to the bottom cannot.
But Rabalais said the smaller flow of freshwater this year means there’s a good chance more oxygen will mix from the atmosphere through the thinner freshwater layer into the lower, saltier water, meaning less hypoxia.
This is the sixth year NOAA has produced its own dead zone forecast, using models jointly developed by the agency and by researchers at LSU, University of Michigan, the College of William and Mary’s Virginia Institute of Marine Science, North Carolina State University and Dalhousie University in Nova Scotia, Canada.
If this year’s estimate is accurate, the dead zone would still be more than two times bigger than a goal set by a task force of federal agencies and states along the river to reduce its size to a five-year average of no more than 1,900 square miles (4.920 square kilometers) by 2035.
In 2001, the task force called for that goal to be met by 2015. But by 2016, it was clear that efforts to get Midwest farmers to reduce their fertilizer use was not working, and the task force pushed the goal date back to 2035. It also added a new goal to reduce the amount of phosphorus and nitrates in the river by 20% by 2025.
“The action plan to reduce the size of the Gulf of Mexico dead zone has been in place for over two decades, but each year the size of the dead zone varies around the long-term average (of 5,364 square miles). That average is almost three times the goal set in 2001,” said Don Scavia, a University of Michigan professor of environment and sustainability who leads one of several research teams partnering with the federal government on the annual forecast.
But Scavia said reductions in nutrients identified by United States Geological Survey in May also don’t represent long-term reductions in total nutrients carried by the river year-round, as called for in the task force goals. And critics of the present voluntary system of projects aimed at reducing fertilizer use argue that only a move to set federal limits on nutrients in runoff from farmland will result in long-term reductions in the Mississippi River.
Still, federal officials remain hopeful that present efforts to install nutrient-reduction projects in the Midwest and elsewhere along the river will succeed, especially with recent funding made available by Congress.