Gliders Return to Action

After a year of shark attacks, leaking instruments, and a hiatus resulting from the global COVID-19 pandemic, BIOS’s gliders are back to work in the waters offshore Bermuda.

This February marked the return of the BIOS glider fleet for their first missions of 2021, dedicated to the Biological Production and Exports (BPE) experiment. This 20-month research program combines gliders and ship-based observations to investigate the local biological carbon pump, a process that globally produces about half of all the oxygen in Earth’s atmosphere via photosynthesis by marine plankton. Later this spring, the gliders will also be looped into an ongoing microbial oceanography study, BIOS-SCOPE (Bermuda Institute of Ocean Sciences – Simons Collaboration on Ocean Processes and Ecology).

The glider program at BIOS began in 2014, with the arrival of gliders Anna, Jack, and Minnie, named for family members of the donors who funded their purchase. Over the subsequent six years, the battery-powered gliders each traveled up to 15 miles (24 kilometers) daily in the Sargasso Sea on their missions in the upper reaches of the water column, repeatedly diving and climbing to depths of 3,000 feet (about 900 meters).

The gliders carry a payload of science sensors mounted on their hulls to collect and measure ocean properties, including temperature, salinity, oxygen, chlorophyll, nutrients, turbulent mixing, light penetration, and currents. Their uses are varied: during hurricane season, the glider fleet sends daily measurements of upper ocean temperatures to weather-observing and forecasting groups. The fleet has also directly measured the ocean’s response to five tropical cyclones, and in each winter-to-spring season has acquired detailed observations of the annual phytoplankton bloom.

The gliders had begun the BPE experiment in March 2019, focused intensely on a tiny area of the Sargasso Sea, about 7 square miles (20 square kilometers) in size, when leaks and saltwater corrosion on an instrument package attached to one of the gliders, called a MicroRider, began dogging scientists. Attempted fixes failed to correct the source of the problem, hampering the data collection plan, which called for two gliders to simultaneously survey the study area over the course of the experiment.

In March 2020 the COVID-19 pandemic completely shut down oceanographic ship operations (including the BPE experiment), as the world turned its attention to reducing spread of the infection. For the glider program, it made sense to halt operations and regroup, said physical oceanographer Ruth Curry, manager of the Mid-Atlantic Glider Initiative and Collaboration (MAGIC) program at BIOS.

In the months before then, the glider fleet had experienced a spate of shark attacks, which repeatedly charged the crew on the BIOS-operated research vessel Atlantic Explorer to retrieve and repair damaged gliders. In the past, the predators regarded the yellow and black, airplane-shaped gliders as a potential food source; Jack was the first of the three to be attacked in July 2017. But the real trouble began in 2019, when over seven months the three gliders were attacked nine times.

While at first it was interesting to think about predators patrolling the depths and interacting with the gliders, “it soon became annoying, then ultimately costly and destructive,” said Curry, who uses her laptop to monitor the gliders via satellite when they are out on missions. The encounters resulted in interruptions to the data collection, and in one case, the total loss of the glider and its science payload.

Watching their trajectories on her screen, she would see the pitch and roll suddenly change after a shark attacked, the result of a chewed-on tail or a completely severed wing. She could then see a glider begin to drift off its programmed course, or develop a tilt to one side.

“It was just, bang, here we go again” she said. She would then relay instructions to the glider to return to the surface and wait for help from the shipboard equivalent of a pit crew on Atlantic Explorer. They would recover the vehicle, fix it if possible, and either redeploy, or return it to shore.

“It got to the point where we routinely put a recovery cart and glider replacement parts onto the ship each time it went out, just in case,” she said.

On November 7, 2019, the glider Anna fell out of contact and was never found. The cost was more than just monetary (about $250,000 of equipment). “All the data that had been painstakingly collected over the previous two months was lost, too, and that really hurt,” Curry said.

Though she doesn’t know with certainty, Curry theorizes that a shark bite punctured the long, tube-like nutrient sensor mounted on Anna’s back, which then flooded and sank the glider. The culprit, she said, could be one of several species of sharks living in the waters around Bermuda, including great whites, makos, tiger sharks, porbeagles, and sixgills. Following another attack in the spring, four months after losing Anna and just as the pandemic began, the glider Minnie was retrieved with multiple tooth fragments wedged in the tail. Curry plans to send them to colleagues in Florida for identification.

Surprisingly, Curry said, most of the attacks occurred at depths below 1,600 feet (500 meters). “I would not have guessed that predators are hunting in those deep, dark regions,” she said. She theorizes that the sharks were drawn to the gliders’ optical sensors that flash lights every few seconds to measure chlorophyll, nitrate, and oxygen.

This spring, the glider fleet returns to the water in top form. An insurance policy enabled the purchase of a new glider, named Anna2. The leaky MicroRider was replaced with a new unit, then tested over four weeks in October 2020, confirming its reliability. Minnie and Jack are sporting new tails to replace the parts damaged by shark attacks and “we learned a few lessons,” Curry said, like turning off the optical sensors below 1,300 feet (400 meters). Anna2 and the other two gliders are now programmed to stop flashing at depth, in an attempt to prevent future shark attacks.

“Hopefully, we won’t be ringing the dinner bell down there anymore,” Curry said.