‘Dead zone’ the size of Connecticut expected along Louisiana coast, scientists say

By Mark Schleifstein, NOLA.com, The Times-Picayune
24 June 2014

gulf dead zone chart

Size of low-oxygen dead zone measured each July along Louisiana’s coast since 1985, in orange. Estimated size of this year’s low-oxygen area, in red, based on the amount of nitrogen carried by the Mississippi River in May, is more than 5,400 square miles. That’s about average, and the same size as the state of Connecticut. (Dan Swenson, NOLA.com | The Times-Picayune)

The size of the annual summer "dead zone" of low-oxygen water in the Gulf of Mexico along Louisiana’s coast will cover between 4,633 and 5,708 miles, about the size of the state of Connecticut, according to a Tuesday forecast announced by the National Oceanic and Atmospheric Administration.

That’s about average for the size of the low-oxygen area since 1985, but still a significant concern, scientists say. And the prediction means another year when states along the Mississippi River have failed to sufficiently reduce the nutrients that cause the dead zone, as called for in a 6-year-old federal-state dead zone reduction plan.

The dead zone moniker is used to describe water that scientists label as hypoxic, meaning it has oxygen levels below 2 parts per million, or anoxic, meaning it contains no oxygen.

The nutrients feed huge blooms of algae, mostly one-cell plants, that grow quickly, die and sink to the Gulf’s bottom near the shoreline. There, the decomposition of the algae uses up oxygen.

The nutrient-laden freshwater tends to lie above saltier water at the bottom, forming a barrier for oxygen to mix into the bottom water until storms or a hurricane cross the area.

Many fish are able to avoid such areas, but others that can’t are killed, as are organisms that live on the bottom or in the bottom sediment that are the food supply for fish and other organisms.


 "The dead zone in the Gulf of Mexico affects nationally important commercial and recreational fisheries and threatens the region’s economy," says a NOAA news release announcing this year’s estimate.

Scientists at the University of Michigan, Louisiana State University, the Louisiana Universities Marine Consortium (LUMCON), and Texas A&M University collaborate with the U.S. Geological Survey in estimating the size of the low-oxygen area and in measuring its size during a two-week cruise along the Gulf Coast in July.

This year, USGS estimated that the water flowing through the Mississippi River basin carried an average 4,761 metric tons of nitrogen a day. Nitrogen and phosphorus, mostly from runoff from Midwest farmland, are carried by the river’s freshwater to the Gulf of Mexico through the Mississippi’s bird foot delta and through the outlets at the end of the Atchafalaya River in central Louisiana. 

The USGS estimated that 101,000 metric tons of nitrate made it into the northern Gulf in May, which is less than the 182,000 metric tons that reached the Gulf a year ago.

Last year, the dead zone was estimated at 5,840 square miles.

 "These ecological forecasts are good examples of the critical environmental intelligence products and tools that NOAA provides to interagency management bodies such as the Chesapeake Bay Program and Gulf Hypoxia Task Force," NOAA Administrator Kathryn Sullivan said in the news release.  "With this information, we can work collectively on ways to reduce pollution and protect our marine environments for future generations."

However, both last year’s measured dead zone and this year’s estimate are well above the federal-state goal of reducing the five-year average dead zone size to below 5,000 square kilometers, or about 1,930 square miles by 2015. That goal was set in 2008 by the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, whose members include five federal agencies, 12 states and Indian tribes located within the Mississippi and Atchafalaya river basins.

NOAA also released a similar early forecast for hypoxia in the Chesapeake Bay, predicting a slightly larger than average dead zone in the nation’s largest estuary.

That estimate looks at the amount of water held by the bay that is low in oxygen, estimating that 1.97 cubic miles of water will be considered as hypoxic, including an early-summer no-oxygen are of just over a half cubic mile and a late summer no-oxygen area of 0.32 cubic miles.

Researchers use the cubic miles measurements because of the shallow nature of large areas of the Chesapeake estuary, rather than using square miles.

The Chesapeake Bay estimate is based on models developed by NOAA-sponsored researchers at the University of Maryland Center for Environmental Scienceand the University of Michigan, based on nutrient loading estimates provided by the USGS. The USGS estimated that 44,000 metric tons of nitrogen entered the bay from the Susquehanna and Potomac rivers between January and May of 2014, higher than the 36,600 metric tons that entered the bay during the same period in 2013.

Link to webpage