Aquatic Robotics: Underwater Glider Helps Monitor Great Lakes Water Quality
By Tom Hollenhorst and Paul McKinney
It’s always exciting to be on a boat heading out under the Duluth lift bridge towards the middle of Lake Superior, but last month’s trip was especially thrilling. Our mission was to rendezvous with EPA’s autonomous Slocum glider, the Nokomis.
The glider was returning to the Duluth area after a nearly 40-day deployment in which it travelled over 1000 kilometers across Lake Superior. Acquired in 2014, the glider complements the EPA’s Great Lakes science initiatives by providing high resolution observations of temperature and concentrations of chlorophyll-a, colored dissolved organic matter, and suspended matter. These are important measurements because they tell us about the relative health and productivity of the lake. These types of data are especially useful if they are collected continuously over a period of time across an area of interest, like the data collected by gliders. And even more useful if the measurements are made in conjunction with other monitoring efforts and data (including remote sensing data). In addition to continuously collecting data every half second, the gliders can also be out in the lake during storms and adverse conditions, when we wouldn’t want to put lives at risk.
Named after Joshua Slocum, the first person to single-handedly sail around the world, the glider propels itself by changing its buoyancy and adjusting the position of its forward battery pack. The buoyancy changes cause it to rise and fall, and its wings turn the vertical motion into forward motion. This method of propulsion is very battery efficient, allowing the glider to perform extraordinarily long missions. In fact, a Slocum Glider piloted by students at Rutgers University crossed the Atlantic Ocean in 2009. That trip took 220 days. As a result of its unique saw-toothed path, our glider, Nokomis completed over 7000 vertical profiles as it made its way back towards Duluth this summer.
Throughout its mission, Nokomis regularly sent in snippets of the data it was collecting while receiving updated instructions via the satellite phone in its tail. The regular contact provided our team opportunities to pilot the glider towards areas of interest that we had observed in satellite images of the lake’s surface. By combining the remotely sensed data with the high resolution glider data, we expect to increase our understanding of exchange processes between nearshore and offshore areas of the lake. The work is a collaboration with EPA’s Great Lakes National Program Office and is part of its collaborative science monitoring initiative.
To learn more about our glider work and the recent post-mission recovery, check out the Duluth News Tribune article Gliders provide in-depth scientific data on Lake Superior.
About the Authors:
Tom Hollenhorst is an Ecologist at EPA’s Mid-Continent Ecology Division. He’s been studying the landscapes and watershed in and around the Great lakes for nearly two decades. He’s especially interested in understanding watershed-nearshore-offshore connections and the transfer of energy and nutrients between them.
Paul McKinney is a National Research Council (NRC) postdoctoral research associate based at EPA’s Mid-Continent Ecology Division. His research is focused on understanding the processes linking nearshore and offshore areas of the Great Lakes.
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