By Mark Schleifstein, NOLA.com | The Times-Picayune
on June 09, 2016 at 12:59 PM, updated June 09, 2016 at 6:15 PM
Low-oxygen area of water along Louisiana’s Gulf Coast can kill bottom-living organisms and cause commercial fish and shrimp to leave the area.
The U.S. Environmental Protection Agency has set a goal of reducing nitrogen pollution along the Mississippi River watershed by 45 percent reduction goal, as necessary to reduce the size of the dead zone to less than 1,930 square miles. Federal and state officials along the Mississippi and Ohio river watersheds had implemented mostly voluntary measures to control the amount of fertilizer used on crops, and have urged planting “cover” crops to suck up remaining fertilizer during offseasons.
A 2015 peer-reviewed study found those efforts lacking. But it said the goal could be met through new fertilizer reduction techniques and the strategic restoration of wetlands in key locations in the Midwest to capture nutrients before they end up in the river.
EPA is not meeting goal of 45 percent reduction of nitrogen pollution.
Hypoxia kills bottom-living organisms, known as benthos, and fish and shrimp avoid the low-oxygen areas if they can. Some large fish kills have occurred in the past when low-oxygen water has been pushed towards shorelines by weather or currents.The dead zone is formed when nutrient-rich freshwater from the Mississippi and Atchafalaya rivers flows into the nearshore area of the Gulf of Mexico in the spring and early summer. The nutrients feed blooms of tiny algae that die and sink to the bottom, where they decompose and use up oxygen in the lower water levels. The freshwater tends to stay on top of the saltier Gulf water, creating a barrier for oxygen to mix into the deeper waters until a series of weather frontal systems or tropical storm or hurricane mixes them.
Researchers with the Louisiana Universities Marine Consortium and the Department of Oceanography and Coastal Sciences at Louisiana State University measure oxygen levels in the water during a week-long cruise in late July. According to the new forecast — produced by Eugene Turner, an oceanography professor at LSU; Nancy Rabalais, a marine ecologist and director of the marine consortium; and Don Scavia, an aquatic ecologist at the University of Michigan — this year’s dead zone is on track to be 29 percent larger than the average measured during cruises dating from 1978.
The majority of the Mississippi River nutrients that trigger the algae growth, primarily nitrogen and phosphorus, flow downstream from farmland across the central United States, the result of rainwater runoff during the winter, spring and early summer.
Scientists use computer modeling to compare the nutrient content of the water and the size of the river flow in May to previous years as part of the forecasting process. This year, the Mississippi flow at Tarbert Landing, near Simmesport, above Baton Rouge, averaged 30,000 cubic meters per second, the 20th largest flow during the past 40 years, and about 106 percent of average.
According to U.S. Geological Survey measurements of nitrogen compounds in the river during May, concentrations of nitrate and nitrite at Baton Rouge were above average during the winter, and then near the highest May values since the agency began measuring it in 1997.
“The concentration rose at the end of May and is expected to rise through to the mapping cruise in July,” the scientists said in their forecast. But average river discharge rates have helped keep the May nitrite plus nitrate loading to about 80 percent of the average value since 1985.
The researchers said the size estimate assumes that there will be no significant tropical storms in the two weeks before the monitoring cruise or during the cruise. “If a storm does occur, then the size of the zone is predicted to be 70 percent of the predicted size without the storm,” or about 4,776 square miles. That’s still 29 percent higher than the average size measured from 1985 to 2015.
The federal-state consortium, the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, set a goal of reducing the five-year running average of the size of the hypoxic zone to only 1,950 square miles by 2015. The task force first set the goal in a 2001 action plan, and restated the same goal in 2008 and 2013.
But between 1995 and 2015, the dead zone has average 5,941 square miles.
“Even though the federal-state action plan calling for nutrient-load reductions has been in place since 2001, there has been little systematic reduction in those loads,” said Scavia, who is director of the University of Michigan’s Graham Sustainability Institute.
“And while the latest forecast calls for an average-size dead zone in the Gulf of Mexico, it is important to recognize that these averages are unacceptable. The bottom line is that we will never reach the action plan’s goal of 1,950 square miles until more serious actions are taken to reduce the loss of Midwest fertilizers into the Mississippi River system.”
Scavia is a member of NOAA-funded teams that also produce annual forecasts for the Chesapeake Bay and Lake Erie. The 2016 Chesapeake Bay hypoxia forecast will be released next week, and the Lake Erie harmful algal bloom forecast will be issued in early July.
The Gulf of Mexico hypoxia research is financed by grants from the National Oceanic and Atmospheric Administration to LSU and the marine consortium.