NOAA scientists are expecting a very large “dead zone” in the Gulf of Mexico this year following a relatively small one by recent standards in 2012 due to the worst drought conditions in 100 years affecting the approximately 41% of the land area of the conterminous United States, ranging as far west as Idaho, north to Canada, and east to Massachusetts, drained by the Mississippi River system, based on several NOAA-supported forecast models.
According to findings of a Texas A&M University researcher just back from studying the region, the NOAA’s predictions appear to be right on target.
TAMU professor of oceanography Steve DiMarco, one of the world’s leading experts on the Gulf dead zone, says he and a Texas A&M team surveyed areas off the Texas-Louisiana coast last week, finding large areas of oxygen-depleted water -– an area covering roughly 3,100 square miles, or about the size of Delaware and Rhode Island combined.NOAA-supported modelers at the University of Michigan, Louisiana State University, and the Louisiana Universities Marine Consortium are forecasting that this year’s Gulf of Mexico hypoxic “dead” zone will be between 7,286 and 8,561 square miles, which could place it among the ten largest recorded, ranging from an area the size of Connecticut, Rhode Island and the District of Columbia combined on the low-end to the New Jersey on the upper end. The high estimate would exceed the largest ever reported: 8,481 square miles in 2002.
The NOAA’s Gulf estimate is based on the assumption of no significant tropical storms in the two weeks preceding or during the official measurement survey cruise scheduled from July 25-August 3 2013. If a storm does occur, the size estimate could drop to a low of 5344 square miles, slightly smaller than the size of Connecticut. This year’s prediction for the Gulf reflect flood conditions in the U.S. Midwest that caused large amounts of nutrients to be transported from the Mississippi watershed to the Gulf. Last year’s Gulf of Mexico dead zone was the fourth smallest on record due to aforementioned drought conditions, and covered an area of approximately 2,889 square miles — slightly larger than the state of Delaware (TAMU estimates that the 2012 the dead zone measured only 1,580 square miles). The overall average between 1995-2012 is 5,960 square miles, an area about the size of Connecticut.
“We found hypoxia (oxygen-depleted water) just about everywhere we looked,” DiMarco reports in a TAMU release. “The most intense area is where you would expect it – off the Louisiana coast south of Atchafalaya Bay and Grande Isle, La. But we also found significant amounts off High Island and near Galveston. The farther south we went, the less we found hypoxia in the water column, but we still found plenty of depleted oxygen waters up to just west of Freeport. There is no doubt there is a lot of hypoxia in the Gulf this year.”
Hypoxic (very low oxygen) and anoxic (no oxygen) zones are caused by excessive nutrient pollution, often from human activities such as agriculture, which results in insufficient oxygen to support most marine life in near-bottom waters. Aspects of weather, including wind speed, wind direction, precipitation and temperature, also impact the size of dead zones.
According to the Louisiana Universities Marine Consortium’s Gulf Hypoxia Net hypoxia in the northern Gulf of Mexico is defined as a concentration of dissolved oxygen less than 2 mg/L (2 ppm). This figure is based on observational data that fish and shrimp species normally present on the sea floor are not captured in bottom-dragging trawls at oxygen levels < 2mg/L. In other oceans of the world, the upper limit for hypoxia may be as high as 3-5 mg/L.
Hypoxic and anoxic (no oxygen) waters have existed throughout geologic time, but their occurrence in shallow coastal and estuarine areas appears to be increasing as a result of human activities (Diaz and Rosenberg, 1995). The largest hypoxic zone currently affecting the United States, and the second largest hypoxic zone worldwide, occurs in the northern Gulf of Mexico adjacent to the Mississippi River on the Louisiana/Texas continental shelf. The maximum areal extent of this hypoxic zone was measured at 22,000 km2 during the summer of 2002; this is approximately the same size as the state of Massachusetts. The size of the zone has been shown to be influenced by the nutrient runoff, volume of freshwater discharged, and prevailing winds, which controls the freshwater river plume’s movement.
The Gulf Hypoxia Net cites four major factors contributing to formation of hypoxia in the Gulf of Mexico:
1. Freshwater discharge and nutrient loading of the Mississippi River
2. Nutrient-enhanced primary production, or eutrophication
3. Decomposition of biomass by bacteria on the ocean floor
4. Depletion of oxygen due to stratification
You can view a Flash animation describing the nutrient process that leads to hypoxia in the northern waters of the Gulf of Mexico here:
The Mississippi River system accounts for almost 90 percent of freshwater runoff into the Gulf of Mexico, and is the dominant source of nutrients to the northern Gulf of Mexico, its discharge controlled so that 30 percent of its volume flows seaward through the Atchafalaya River delta, while 70 percent flows through the Mississippi River birdfoot delta. About 53 percent of the Mississippi River delta discharge flows westward onto the Louisiana shelf.
Consequently, the Gulf Hypoxia Net notes that Mississippi River nutrient concentrations and loading to the adjacent continental shelf have greatly changed in the last half of the 20th century, with marked increases in concentrations of nitrogen and phosphorous in the Lower Mississippi River. This increase has been attributed to the increased use of nitrogen and phosphorous fertilizers, nitrogen fixation by leguminous crops, and atmospheric deposition of oxidized nitrogen from the combustion of fossil fuels. Nitrogen and phosphorous occur in four inorganic forms in the river: nitrate (NO3-), nitrite (NO2-), ammonium (NH4+), and orthophosphate (PO4-3). Many of these nutrients enter the river from non-point sources like runoff, which are much more difficult and complex to control and monitor than point sources of pollution.
Eutrophication follows when ocean systems are over enriched with nutrients beyond natural levels, causing significant increases in primary production, or growth of algae in marine systems. In the same way that nitrogen and phosphorous fertilize human crops, they also fertilize plants in the ocean. The spring delivery of nutrients initiates a seasonal progression of biological processes that ultimately leads to the depletion of oxygen in the bottom water. In the northern Gulf of Mexico, eutrophication initiates a massive growth of phytoplankton on the water’s surface — well beyond the natural capacity of predators or consumers to graze it down to a balanced level. Phytoplankton have a relatively short life span, and after dying sink down to the bottom waters where they await decomposition by bacteria. In the warmer months, the water column is also stratified, meaning that environmental factors like temperature and salinity are not uniform from top to bottom. Freshwater flowing from the river, and seasonally warmed surface water, has low density and forms a layer above saltier, cooler and more dense water masses near the bottom, leaving the bottom layer isolated from the surface layer and cut off from a normal resupply of oxygen from the atmosphere.
Nutrient-based Hypoxia Formation (Gulf Hypoxia Net):
1) Nutrient-rich water flows in, 2) Algae grow, feed, and die, 3) Zooplankton eat the algae, 4) Bacteria feed on fecal pellets and dead algae, 5) Bacteria deplete the water of oxygen, 6) Marine life flees or dies.
The dead zone in the Gulf of Mexico affects nationally important commercial and recreational fisheries, and threatens the region’s economy. Hypoxia, which was first documented in the northern Gulf of Mexico off the Louisiana coast in 1972, can persist several months until strong mixing of ocean waters caused by hurricanes or cold fronts occur in the fall and winter. TAMU’s Dr. Steve DiMarco says the size of the dead zone off coastal Louisiana has been routinely monitored since 1985, and that previous research has also shown that nitrogen levels in the Gulf related to human activities have tripled over the past 50 years.
The NOAA” reports that during May 2013, stream flows in the Mississippi and Atchafalaya rivers were above normal resulting in more nutrients flowing into the Gulf. According to USGS estimates, 153,000 metric tons of nutrients flowed down the rivers to the northern Gulf of Mexico in May, an increase of 94,900 metric tons over last years 58,100 metric tons, when the region was suffering through drought. The 2013 input is an increase of 16 percent above the average nutrient load estimated over the past 34 years.
The confirmed size of the 2013 Gulf hypoxic zone will be released in August, following a monitoring survey led by the Louisiana Universities Marine Consortium beginning in late July, and the result will be used to improve future forecasts.
Ironically a substantial proportion of the increase in agricultural runoff pollution has been due to increased production of corn in order to supply purportedly “sustainable” biofuels demand. An Action Plan developed by a Mississippi River/Gulf of Mexico Nutrient Task Force in 2008 outlined eleven key action items that provide a framework for reducing inputs of nitrogen and phosphorus to the Gulf. The success of this strategy was to be reviewed after five years (2013) to reassess nitrogen and phosphorus load reductions, the response of the hypoxic zone, changes in water quality throughout the Mississippi/Atchafalaya River Basin, and the economic and social effects, including changes in land use and management, of the reductions in terms of the goals of this Action Plan, to evaluate and how current policies and programs affect the management decisions made by industrial and agricultural producers, analyze lessons learned, and determine appropriate actions to continue to implement or, if necessary, revise this strategy.
Given the forecast of a possible record-large dead zone in the Gulf this year, it would appear that revision is in order. The NOAA observes that Despite the Mississippi River/Gulf of Mexico Nutrient Task Force’s goal to reduce the dead zone to less than 2,000 square miles, it has averaged 5,600 square miles over the last five years. Demonstrating the link between the dead zone and nutrients from the Mississippi River, this annual forecast continues to provide guidance to federal and state agencies as they work on the implementation actions outlined by the Task Force in 2008 for mitigating nutrient pollution.
TAMU’s Dr. DiMarco’s research on the dead zone is supported by the NOAA as part of its long-term commitment to advancing the science to inform management practices aimed at mitigating the hypoxic zone. “While we await additional data from the entire summer, these early findings start to validate our prediction that we could see one of the largest dead zones ever in the Gulf of Mexico this July,” Robert Magnien, Ph.D., center director at NOAA’s National Centers for Coastal Ocean Science is cited observing in the TAMU release. “This is further confirmation of the link between upstream nutrient management decisions and the critical habitats and living resources in the Gulf.”
Dr. DiMarco has made 28 research trips to investigate the dead zone since 2003. His cruise this year carried 10 investigators from Texas A&M and Texas A&M at Galveston and included two research scientists, Matthew Howard and Ruth Perry, five graduate students, Laura Harred, Jordan Young, Yan Zhao, Heather Zimmerle, and Nicole Zuck, and two marine technicians, Eddie Webb and Andrew Dancer (Geochemical and Environmental Research Group). On shore investigators include Lisa Campbell, Wilford Gardner, Shari Yvon-Lewis, and Ethan Grossman , all from Texas A&M, and Antonietta Quigg from Texas A&M-Galveston.