The largest low oxygen (hypoxic) zone affecting the United States is in the northern Gulf of Mexico, adjacent to the Mississippi River. This “dead zone” in the Gulf of Mexico affects nationally important commercial and recreational fisheries and threatens the region’s economy. Since 1990, we annually map the size of the dead zone, and since 2005, we annually develop hypoxia forecasts in support of the Mississippi River Gulf of Mexico Watershed Nutrient (Hypoxia) Task Force.
Why We Care
The northern portion of the Gulf of Mexico ecosystem contains almost half of the nation’s coastal wetlands and supports commercial and recreational fisheries that generate billions of dollars annually. This ecosystem has undergone profound changes due to nutrient enrichment of Mississippi River water from land-based sources. This over-enrichment of nutrients stimulates the development of seasonal hypoxia (very low oxygen concentration) in waters over the Louisiana/Texas continental shelf in summer and results in the largest recurring hypoxic zone in the United States. In 2008, the hypoxic zone (or “dead zone”) was the second largest on record, encompassing more than 8,000 mi2, an area roughly the size of Massachusetts. Hypoxic waters can cause habitat loss, stress, and even death to marine organisms, affecting commercial harvests and the health of impacted ecosystems.
What We Are Doing
We are integrating modeling, observations, and experiments to:
- better define the processes that drive and maintain hypoxia,
- describe how hypoxia has changed over time, and
- determine how hypoxia will likely change in response to changes in watershed land use, differences in physical structure, and the biological responses within these contexts.
Our main objectives are to advance the science of hypoxia forecasting for the northern Gulf of Mexico and serve management needs. This is accomplished by integrating observations, hypoxia research, data acquisition, and modeling into a planned Gulf of Mexico Hypoxia Forecasting System.
Currently, the project team has developed at least four Gulf of Mexico models related to the Gulf of Mexico dead zone. These models, in addition to others developed by EPA, have been used to determine the percent reduction in nutrient loads needed to reduce the size of the dead zone to the target size (5,000 km2 or ~2,000 mi2). Since 2003, several of these models have successfully forecasted the size of the dead zone. This prediction helps managers, policy makers, and the public better understand what causes the dead zone.
Every July since 1985, the Louisiana Universities Marine Consortium (LUMCON) has mapped and measured the size of the dead zone. Monitoring the size and geographic coordinates of the dead zone provides critical information both for measuring effectiveness of management strategies to reduce it, but also as input to models being developed to explain the causes and forecast its ebb and flow over the course of the season. We are using three different methods to collect data on hypoxia:
- long-term deployment of instruments on stationary moorings,
- monthly cruises of fixed offshore transects, and
- an annual shelf-wide cruise to map the widest extent of hypoxia during each summer.
What We Found
The hypoxic zone in the northern Gulf of Mexico (adjacent to the Mississippi River on the Louisiana/Texas continental shelf) is the largest human-caused hypoxic zone currently affecting the United States and the second largest hypoxic zone worldwide. The maximum areal extent of this hypoxic zone reached 22,000 km2during the summer of 2002, approximately the size of the state of Massachusetts. The average size of the hypoxic zone in the northern Gulf of Mexico over the past 30 years (1985–2014) is about 13,650 km2 (or 5,300 mi2). For comparison, the entire surface area of the Chesapeake Bay and its major tributaries measures about 11,000 km2.
Benefits of Our Work
Since 1990 NCCOS has supported measuring the size of the dead zone, and since 2005 has provided a forecast a few weeks prior to the measurement. These yearly efforts have steadily improved the ability to measure and forecast the dead zone. Most recently the use of remote underwater gliders is being tested to more accurately measure the size of the dead zone. A recently released white paper assesses the status of several empirical and deterministic models capable of characterizing Gulf hypoxia. “Modeling Approaches for Scenario Forecasts for Gulf of Mexico Hypoxia” presents conclusions from the 2013 joint NOAA and Northern Gulf Institute Forum for Gulf of Mexico Research Coordination and Advancement. The paper concludes that several empirically based models are ready for transition to operational use in scenario forecasts of nutrient reduction goals required for hypoxia mitigation.
Our project provides the scientific basis for implementing goals and developing progress reports by the Mississippi River Gulf of Mexico Watershed Nutrient (Hypoxia) Task Force to reduce and control hypoxia in the Gulf of Mexico. Based on the project mapping and forecasts, the Task Force reports on progress in dead zone reduction and may provide new measures and actions to control nutrient runoff to further decrease the size of the dead zone.
Twelve states that serve on the Hypoxia Task Force have used our project findings and forecasts to devise new strategies to speed up reduction of nutrient levels in waterways in the Mississippi/Atchafalaya River Basin. Each state has outlined specific actions it will take to reduce nitrogen and phosphorus in the Mississippi/Atchafalaya River Basin from wastewater plants, industries, agriculture, and storm water runoff. In 2015, the Task Force decided to extend the target date for shrinking the dead zone from its current average size of almost 6,000 mi2to about 2,000 mi2 from 2015 to 2035. Progress has been made in certain watersheds within the region, but the project science shows a 45 percent reduction is needed in nitrogen and phosphorus entering the Gulf of Mexico. In order to track progress and spur action, the Task Force is also aiming at a 20 percent reduction in nutrient loads by 2025. Project forecasts and mapping are needed to confirm progress.
This project is led by Louisiana State University and Louisiana Universities Marine Consortium (LUMCON), with co-partners at the University of Michigan and NOAA’s National Centers for Coastal Ocean Science. The project is part of the NCCOS Gulf of Mexico Ecosystems & Hypoxia Assessment (NGOMEX) program.