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8.2.7 Autonomous Mapping of the Hypoxic Zone in the Gulf of Mexico


Summary: The hypoxic zone in the northern Gulf of Mexico, a region devoid of life, occurs in an area that was once one of the most fertile fishing grounds in the region. Since the 60s and 70s, nutrient rich water flowing into the Gulf from the vast Mississippi River watershed, has caused an annually recurring hypoxic zone that extends over an area that can approach the size of New Jersey. Hypoxia is generally used to denote waters containing less than 2mg/l of oxygen and which are typically too low to support life, hence the common name of “dead zone” for severe hypoxia areas around the US. The largest dead zone in the US is in the Gulf of Mexico and a major effort to mitigate this dead zone has been undertaken since the early 2000’s by the interagency and multistate Hypoxia Task Force (HTF). Primarily focused on reducing watershed nutrient pollution, NOAA’s responsibilities to the HTF include providing the scientific understanding of the causes 63 of the hypoxic zone and its ecosystem impacts. Over the years, NOAA’s research investment has led to development of quantitative predictive models currently used to establish hypoxia mitigation goals and nutrient reduction targets needed by the States in the Mississippi River watershed to achieve those goals. NOAA’s assessment of HTF progress toward their hypoxia mitigation goal is based on hypoxic zone monitoring, currently very limited in scope despite the national interest in the issue. Adequate monitoring data are a fundamental need for proper calibration and validation of the predictive hypoxia models being used for decision-making. Current hypoxic zone monitoring is limited to one shelfwide survey per year due to limited funding to support the extensive ship surveys and fixed observation systems needed to monitor the area impacted by the dead zone on an annual basis. These issues are further compounded by the large size of the system and logistical constrains with measuring oxygen throughout the water column, especially near the bottom where hypoxia typically occurs. Gliders are widely recognized as an effective and cost-efficient monitoring tool for high spatiotemporal coverage of water quality parameters. They are routinely used for mapping parameters in regions where water column density gradients are low enough to enable adequate buoyancy control. The Gulf of Mexico dead zone, however, is a challenging environment for glider mapping because a large portion of it occurs in relatively shallow, highdensity gradient areas of the shelf or along bottom waters below the halocline. Glider deployments to date have not been able to fully map the dead zone due to the difficulty in controlling buoyancy in these conditions. For gliders to be operationally useful for hypoxia monitoring in the Gulf, there is an urgent need to overcome these multiple challenges in a cost effective manner while also maintaining data coverage and accuracy.


Project Goals: NOAA is seeking autonomous vehicle technologies that can resolve the challenge of mapping the Gulf dead zone effectively and cost-efficiently. The goals are to 1) substantially improve the monitoring capabilities currently implemented for the Gulf hypoxic zone, to 2) provide a costefficient mapping capability that could leverage operational support from interested partners as contribution to the Cooperative Hypoxia Monitoring Program (, currently in development, and, if successful, to 3) extend this technology to other environments where high density gradients in shallow waters have hampered glider applications to monitoring. Integrated, multiple autonomous vehicle platforms (e.g. gliders and autonomous surface vehicles) working synergistically to cover the full range of sampling environments are also encouraged. Current sampling methodologies rely on a shipbased platform to measure the annual dead zone ( Proposed AUV technology should provide a similar capability, in both time and space, and be able to sample over the range of salinity, temperature, and depths encountered during the annual summer survey cruise.

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