Near record low-oxygen ‘dead zone’ predicted for Gulf
This map by researchers at Dalhousie University, in Halifax, Nova Scotia, predicts the coverage of Gulf Coast waters by low-oxygen conditions on July 27. The thick black line shows the limits for the most probable areas showing hypoxic conditions. The university’s forecast actually calls for the hypoxia zone to expand to more than 10,000 miles in August, if no tropical weather causes mixing.
The summertime low-oxygen “dead zone” on the floor of the Gulf of Mexico off the coasts of Louisiana and Texas is likely to be close to record size, at 7,829 square miles, or roughly the size of Massachusetts, the National Oceanic and Atmospheric Administration announced Monday.
And NOAA researchers warned that this year’s increased size, estimated by modeling of nutrients carried to the Gulf by the record-setting high Mississippi River, could be repeated in the future because of the effects on rainfall of climate change.
Federal report warns of up to 4 feet sea level rise, intense rainfall events, heat risks, mosquito-borne diseases.
“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,” said Steve Thur, director of NOAA’s National Centers for Coastal Ocean Science, in a news release announcing the prediction. “The assessment predicts an increase in the frequency of very heavy precipitation events in the Midwest, Great Plains, and Southeast regions, which would impact nutrient input to the northern Gulf of Mexico and the size of the hypoxic zone.”
Water levels at the spillway are not high enough to require its opening, based on Mississippi River forecasts, Army Corps of Engineers says.
When the freshwater carrying the nutrients flows into the Gulf from the mouths of the Mississippi and Atchafalaya rivers, it creates a layer atop the saltier Gulf waters. The nutrients trigger the growth of huge blooms of algae that eventually die and sink to the sea floor, where it decomposes, using up the oxygen in the deeper, saltier water.
Oxygen only returns to the deeper water when the freshwater above is mixed into it, which often doesn’t happen in the summer until a tropical storm or hurricane crosses the area, or when thunderstorms produced by fall-like frontal systems cause mixing.
The oxygen levels -- between zero and 2 parts per million of water -- are low enough to kill bottom-living organisms and to force life forms that can escape – including many commercial fishing species -- into deeper water.
Between 6,714 square miles and 8,717 square miles of the Gulf’s shelf waters along the two states will be affected by hypoxia – the scientific name for water containing no to only 2 parts per million of oxygen – according to models conducted for NOAA by teams of scientists at five universities and the U.S. Geological Survey.
The NOAA estimate is its compilation of the model results. But one model conducted by researchers at Dalhousie University in Halifax, Nova Scotia, predicted that the low-oxygen zone would expand to as much as 10,000 square miles in early August. The record measurement of a dead zone was 8,776 square miles, set in July 2017, according to NOAA.
A team of scientists led by Louisiana State University marine scientist Nancy Rabalais will sample oxygen levels along the Louisiana and Mississippi coasts between July 22 and July 31, with the results to be compared to the modeling estimates.
Rabalais, with her husband and wetlands scientist R. Eugene Turner, authored the LSU/Louisiana Universities Marine Consortium pre-cruise modeling effort, which included the largest dead zone estimate of 8,717 square miles, an area about the size of New Hampshire and 67 percent larger than the average of 5,226 square miles measured during 34 years of coastal cruises.
In their modeling report, they also point out that the estimate means the dead zone will be 4 ½ times as large as the national goal for reducing the dead zone to a five-year average area no larger than 5,000 square kilometers – 1,930.5 square miles.
That goal, set by the national Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, was originally supposed to be reached by 2015, but is now set for 2035. The 5-year average through 2018 is 5,770 square miles.
The task force includes representatives of federal agencies and officials representing 12 states located in the Mississippi watershed. Established in 1998, it has repeatedly delayed deadlines for meeting its dead zone reduction goal as it relies on largely voluntary conservation and regulatory efforts to reduce the flow of nutrients into rivers and streams.
In flood years such as this, using natural storage areas to slow the flow of water downstream can help in allowing the capture of nutrients by wetlands in those areas, said Doug Daigle, coordinator of the Lower Mississippi River Sub-Basin Committee, which works on hypoxia reduction efforts.
“… (B)ut clearly more ecosystem restoration and reconnection of natural floodplains needs to happen, and combined with nutrient reduction efforts, can help alleviate both problems,” Daigle said.
A key problem is that while the amount of nutrients reaching the Gulf vary from year to year because of natural swings in the amount of rainfall and its flow south, the USGS has found continued increases in the amount of nutrients reaching the Gulf from the Mississippi.
“Long-term monitoring of the country's streams and rivers by the 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.”
The 2018 cruise measured only 2,720 square miles, the fourth smallest since monitoring began in 1985 and much smaller than the pre-cruise modeling estimate of about 6,620 square miles. Rabalais said at the time that strong winds out of the west and northwest in the weeks just before last year’s July monitoring cruise likely pushed the largest concentrations of low-oxygen water into the eastern part of the Louisiana shelf.
Rabalais and Turner said that a significant tropical storm during the two weeks before this year’s cruise also could reduce the size of the dead zone, compared to the pre-cruise estimate.
The large dead zone estimate is driven by repetitive rainfall events dating back to last November along the Mississippi River watershed, which encompasses parts of 31 states and two Canadian provinces, and includes the Missouri, Arkansas, and Ohio rivers.
The rain has washed nitrogen, phosphorus and other nutrients off farmland throughout the basin, but especially in the Midwest. Similar nutrients have been released into the river by sewage systems and septic tanks in urban and rural areas along the river’s watershed, which includes parts of 31 states and two Canadian provinces.
“This past May, discharge in the Mississippi and Atchafalaya rivers was about 67% above the long-term average between 1980 and 2018,” said a NOAA news release announcing the dead zone estimates. “USGS estimates that this larger-than-average river discharge carried 156,000 metric tons of nitrate and 25,300 metric tons of phosphorus into the Gulf of Mexico in May alone.
“These nitrate loads were about 18% above the long-term average, and phosphorus loads were about 49% above the long-term average,” the release said.
In May, the Iowa Department of Natural Resources confirmed that there were more than 70 cases of communities releasing untreated sewage wastewater into rivers and streams during heavy rainfall events, due to sewers and storm drains unable to deal with more than 9 inches of rain.
Mark Schleifstein covers the environment and is a leader of the Louisiana Coastal Reporting Team for NOLA.com | The Times-Picayune. Email: email@example.com. Facebook: Mark Schleifstein and Louisiana Coastal Watch. Twitter: MSchleifstein.