We evaluated the use of gliders to improve Gulf of Mexico hypoxic zone monitoring. The efficacy of this technology was determined and we developed a comprehensive plan that integrates glider capabilities into a range of ongoing and planned Gulf of Mexico hypoxic zone monitoring efforts.
Why We Care
The northern portion of the Gulf of Mexico experiences an annual summer hypoxia event when over-enrichment of nutrients in the water causes very low levels of dissolved oxygen. Ships have monitored this hypoxic zone since 1985, and despite decades of research, fully coupled hydrodynamic water quality models cannot yet accurately forecast the hypoxic zone. Using innovative monitoring technologies—such as autonomous underwater gliders—contributes data to enhance models, identify the size of the hypoxic zone (dead zone), and validate data collected during ship survey cruises.
What We Are Doing
During the 2014 hypoxia season, NCCOS and the U.S. Integrated Ocean Observing System demonstrated how gliders deployed from July to September monitor hypoxia in the Northern Gulf of Mexico. During the same period, Dr. Steven DiMarco of the Texas A&M University Geochemical and Environmental Research Group led a research cruise that took shipboard hypoxia measurements. The coordinated mission enabled efficient and effective comparisons between the glider- and ship-collected data in a highly stratified, shallow, and heavily ship-trafficked region of the northern Gulf of Mexico.
The project scientists successfully operated two gliders in the Gulf of Mexico dead zone during the testing period. Three missions lasted about 100 days total and focused primarily on 20-meter depth contours. The gliders measured temperature, salinity, depth, chlorophyll, and dissolved oxygen to monitor hypoxia. The gliders consistently came within 1.6 meters of the bottom during the three missions. Data from the project went near real-time on the Gulf of Mexico Coastal Ocean Observing System Data Portal and on the Gulf Gliders Experiment map that tracks the gliders.
Benefits of Our Work
The glider-hypoxia demonstration credits the efforts of many organizations cooperating and collaborating on the project, showing the gliders efficiency for detecting and assessing hypoxia. The project demonstrated that gliders can effectively profile the entire depth of the water column, take measurements almost at seafloor, and hover within one to two meters from the bottom.
Following the successful demonstration, this project supported a pilot study to further expand and test the application of gliders toward improving Gulf of Mexico hypoxic zone monitoring, with the ultimate goal of contributing to the development of a robust, comprehensive, and sustainable (operational) monitoring program. The results of the project will determine the efficacy of future glider applications to Gulf of Mexico hypoxic zone monitoring, and if gliders will be key to a comprehensive operational monitoring program for the northern Gulf of Mexico hypoxic zone.
Using gliders for efficient and cost-effective hypoxia monitoring is a core system requirement in the Gulf of Mexico Hypoxia Monitoring Implementation Plan (2009, revised in 2012). The NCCOS-led Gulf Hypoxia Glider Application Meeting (held in April 2013) explored this as part of the Forum for Gulf of Mexico Hypoxia Research Coordination and Advancement. The forum acknowledged the potential application of gliders for operational monitoring of hypoxia and then developed a plan for implementation—the Glider Implementation Plan for Hypoxia Monitoring in the Gulf of Mexico.
Specifically, the project conducted two targeted field campaigns in the summers of 2016 and 2017 using ocean gliders in the Gulf of Mexico hypoxic zone. The goal was to demonstrate their capability and operational robustness to obtain dissolved oxygen data as a comprehensive approach to hypoxic zone monitoring. The project applied past research and testing to evaluate the effectiveness of gliders for routine hypoxic zone characterization in terms of efficiency (time, cost), accuracy, resolution, breadth (e.g., closeness to bottom, areal and volumetric expanse), telemetric capabilities (e.g., near real-time), and sensor response time. The project developed a scalable glider monitoring implementation plan for the hypoxic zone for transitioning glider applications to operations. The principal activities of this study build upon previously funded (2003–2014) research of 33 major oceanographic cruises, including advanced technologies such as gliders and undulating towed vehicles, and the development of a near-operational coupled physical-biogeochemical numerical model. Research scientists coordinated observational activities with relevant and existing federal and state funded operational and regional efforts and other regional investigators through active data exchange and participation on appropriate and available cruises of opportunity.
Among the Key findings found in the report:
- Suitably ballasted gliders can come within 2 meters of bottom 95 percent of the time (average 1.4 meters)
- Only 1-2 gliders simultaneously in water is not sufficient enough to create a map of region due to speed (around 0.5 nautical mph) and current (strong eastward current preventing movement onshore or offshore)
- A single glider is consistently unable to repeat lines, repeat transects, make smooth, reliable turns
- Density gradients are still a limiting factor (buoyancy gliders limited use of thrusters and shallow/shelf buoyancy pumps)
- Explore use of Autonomous Surface Vehicles
- Buoyancy Gliders can:
- Reliably sample within 2 meters of bottom (if environmental conditions allow)
- Resolve spatial and temporal scales of dissolved oxygen concentration, temperature, and salinity
- Buoyancy Gliders CANNOT:
- Reliably map the coastal environment (due to slow forward speed and challenges in navigating in shallow, highly stratified, strong and variable coastal current conditions
- Recommendation: hybrid approach that uses combination of buoyancy gliders and autonomous vehicles