A recent first-of-its-kind mission to the Great Lakes included the deployment of a long-range autonomous underwater vehicle (AUV) to help scientists better understand one of the world’s largest and most complex freshwater environments.
The five Great Lakes (Erie, Huron, Michigan, Ontario and Superior) were carved out by glaciers thousands of years ago and today stretch across an expansive 246,000 square kilometers, holding an estimated 20 percent of the world’s freshwater. The Lakes’ vital role within the surrounding environmental and economic landscape cannot be overstated. For scientists, however, studying this huge yet mysterious ecosystem has often been challenging.
In hopes of better understanding this complex freshwater system, the U.S. Geological Survey (USGS) Great Lakes Science Center – which studies the Lakes’ living resources and their habitats – recently teamed up with the Monterey Bay Aquarium Research Institute (MBARI) to observe plankton communities in a project that seeks to improve upon traditional research methods by, for the first time in Lake Michigan, adding an AUV to the mix.
Conventional sampling technology for this type of study has typically involved some kind of vessel with nets, explained Dr. David Warner, a research fisheries biologist with the USGS Great Lakes Science Center, who said traditional methods are not only less effective, but slower and costlier than sampling with an AUV.
That’s where MBARI’s long-range AUV Tethys comes in. In August 2016 Tethys spent nearly a month traversing the entire body of Lake Michigan to gather data for the study of the lake’s planktonic food webs and provide valuable insight for fisheries management and climate change research.
“The USGS is tasked with trying to understand the entire ecosystem of the Great Lakes, which is a big challenge,” said MBARI mechanical engineer, Brett Hobson. “Because they needed to make basin-scale observations and were interested in employing adaptive sampling strategies to find hot spots of productivity, MBARI’s long-range AUV [Tethys] was a good fit.”
“Tethys is able to sample 24 hours a day, seven days a week, for as much as a couple of weeks at a time, and even longer if you minimize the kind of sampling you do,” Warner said. “That kind of presence gives us a number of samples and amount of data that is orders of magnitude greater than what we can get from the ships. It can sample constantly. Combine the sampling speed of our ships and the expense to cover the kind of ground that the AUV can cover, and it becomes absolutely impossible to match what the AUV can do.”
Tethys is designed first and foremost for travelling long distances. At 30.5 cm in diameter, 230 cm long and weighing 120 kg, the AUV can support an 8 W sensor payload for distances up to 2,000 km at 1 m/s; and this range can be extended to several thousand kilometers when operating at a speed of 0.5 m/s with minimal sensors. By way of a buoyancy engine, Tethys is also capable of trimming to neutral buoyancy and drifting in a low power mode.
But, as the deployment was Tethys’ first in freshwater, MBARI said the project presented several unique challenges to Hobson and principal investigator, Brian Kieft. The vehicle needed to be retrofitted for work in the less buoyant salt-free environment by adjusting ballast and adding extra flotation devises. Another challenge: Tethys had to be disassembled in order to be transported by more than 4,000 kilometers by land from California to Charlevoix, Mich.
Upon arrival to Lake Michigan, the AUV was reassembled and put through a series of sea trials before being packed up again and transported to Muskegon to begin the first leg – the southern portion – of its expedition. In its first week of deployment, Tetheys, outfitted with a suite of biogeochemical sensing equipment, travelled 400 kilometers between Muskegon, Chicago and Milwaukee. The team then transported the AUV over land to run a second and longer deployment through the north end of the lake.
“Our long-range AUV is unique because it is able to run 1,000 km long missions with a science payload to map the distribution of chlorophyll as a function of location and temperature as well as run adaptive missions to focus sampling along a front or vertical layer where hot spots of productivity usually occur,” Hobson said. “The LRAUV we sent to Lake Michigan also measures optical backscatter, PAR and salinity, though that wasn’t very useful in the fresh water lake. In the future we’d probably employ a bio-acoustics sonar for zooplankton imaging and/or our G3 ESP microbial sampler.”
Warner said, “[The AUV] measured connectivity, temperature, depth . . . it measured salinity, it measured relative fluorescence – that’s a way estimating how much chlorophyll is in the water, or more succinctly how much phytoplankton is in the water. It also used acoustic sampling in the form of acoustic Doppler profiler that we’re going to try to use to estimate zooplankton biomass. We’re going to try and do that by using comparative sampling data where we sampled side by side with our ship and the AUV. That comparative sampling should give us the ability to try and better understand some of the variables measured by Tethys actually are in the form of zooplankton abundances and chlorophyll distribution.”
Tethys, used in conjunction with surface satellite imagery and vessel-based research, essentially expands the USGS datasets by providing a more accurate depiction of plankton from the surface down.
According to Warner, the primary difference between data gathered by the AUV and more conventional means is the spatial and temporal resolution and scope. “[With the AUV], we have samples every few meters horizontally and less than every meter vertically . . . The Tethys data we have ended up being somewhere in the order of 150,000 data points compared to what we collected using similar equipment from the ship that amounted to in the thousands of data points, not hundreds of thousands.”
“We’re trying to transform the way we go about sampling in the Great Lakes by utilizing these, what I would call, complimentary tools like the AUV to simply have capabilities well beyond what we already have,” Warner said.
“What we hope to accomplish in the end is to be able to do surveys and research projects where we utilize ships to take advantage of their strengths where they exist – things like actually capturing fish with nets – as well as taking advantage of things like the Tethys AUV to utilize their strengths to do things ships can’t do, like study lower trophic level organisms like phytoplankton and zooplankton. Combining these gears should give us the best set of tools we have for providing a good understanding for how the food webs in these lakes function, how they can support fish which are important for people, and how they store and process carbon, so that helps us understand the role of climate,” Warner said. “Ultimately, this multi-tool approach to this kind of research is critically important and has been shown to be really effective in oceans, but hasn’t been used to as much degree in the Great Lakes or other fresh water systems.”
The research team is now in the early stages of data analysis which will ultimately lead to a number of papers in 2017.
As for future AUV deployments in the Great Lakes, “We don’t have solid plans [at this point],” Warner said. “We would like to do this type of sampling in other lakes, and we especially want to bring Tethys back if fisheries acoustic gear is integrated into the AUV. That’s one of the things the AUV was capable of deploying, and at this point in time the folks at MBARI are working on making that happen. And we would be very attracted to deploy [Tethys] again – potentially even somewhere else – if it can also measure fish abundance.”