From Submersibles to Satellites, Advanced Tools Illuminate a Changing Gulf of Maine

Most visitors to Bigelow Laboratory have seen the bright yellow contraptions that could be confused for drones or torpedoes. But Henry and Grampus, as they’re affectionately known, are autonomous underwater gliders — named for the oceanographer and institute namesake Henry Bigelow and his first research schooner. For over a decade, they’ve glided through the Gulf of Maine, collecting valuable data as part of the Gulf of Maine North Atlantic Time Series to help illuminate the depths of this rapidly changing body of water.

GNATS began in 1998 to monitor long-term environmental change in the Gulf of Maine and validate NASA satellite observations. Since then, a team of Bigelow Laboratory scientists has traversed the Gulf of Maine over 200 times, following a roughly 300-mile round trip transect between Portland, Maine, and Yarmouth, Nova Scotia. Since the first glider launched in 2008, Henry and Grampus have collectively spent over 500 days gathering data along the same transect line.

“They’re very slow but very efficient, which allows them to stay at sea for long periods of time,” said Dave Drapeau, a senior research associate who has been involved with GNATS since the beginning and likens the movement of the gliders to swimming. “Because of how they travel, they’re giving us this really high-resolution depth data, which is a useful addition to the time series.”

While the GNATS cruises enable intensive data collection at the surface, the gliders can collect high-resolution profiles of the salinity, temperature, and oxygen content — down to almost 300 meters in the case of Grampus. They also provide information on various features of the light reaching different depths.

One vital property the gliders measure is colored dissolved organic matter, the assorted mixture of molecules leached from decaying plant material, algae, and bacteria, which can influence the amount of light penetrating below the surface. Pure water absorbs red light, reflecting back blue light, which gives the ocean its blue color. Dissolved organic matter, though, absorbs light on the blue end of the spectrum, giving the water a brown or yellow color. Light sensors on the gliders measure these colors, which can then be translated using mathematical algorithms into an amount of organic material. Scientists use “optical proxies'' like this to estimate biological activity throughout the whole water column.

Using those proxies confidently requires complex modeling to quantify the relationship between these light data and corresponding biological information and understand how that relationship changes in different parts of the ocean.

Just recently, Senior Research Scientist Catherine Mitchell, who manages the GNATS program, published a paper on how to deal with one such challenge, called fluorescence quenching, that makes it harder to interpret glider data.

When a photon of light hits a phytoplankton cell, it can be used for photosynthesis, dissipated as heat, or absorbed and released at a different wavelength, which is called fluorescence. When fluorescence is higher, the assumption is there are more phytoplankton cells. But, when there’s a lot of light coming in — like in the middle of the day — cells will dissipate most of that light as heat, instead of fluorescing it. The resulting decrease in fluorescence, which could be interpreted as less biomass, is just the cells protecting themselves from excessive light. Mitchell and Bigelow Laboratory colleagues developed a method to account for this when estimating biomass from fluorescence data, a method that doesn’t just work in the open ocean, but also in a dynamic, highly variable system like the Gulf of Maine.

“Satellites are measuring the surface ocean, so we have this information on what is happening to phytoplankton in the Gulf of Maine that’s all based on that top layer,” Mitchell said. “But we also have all this glider data, which we can use to better understand the relationship between the surface layer the satellite sees and what’s happening below.”

This is vital for GNATS, which has two primary aims. The first is to validate ocean color observations from NASA satellites with ship-based measurements of surface waters across the Gulf, collecting on-the-water data as satellites pass overhead.

The other goal is to keep a pulse on the Gulf of Maine, which is considered one of the most rapidly warming parts of the ocean. In 2022, a Bigelow Laboratory team led by Senior Research Scientist Emeritus Barney Balch, and including Mitchell and Drapeau, published a paper summarizing 20 years of data from the GNATS time series.

“The Gulf of Maine is this rapidly changing ecosystem, and if we want to monitor that rapid change, we have to be out there and doing it regularly,” Mitchell said. “These kinds of time series become increasingly valuable the longer they go on.”

But the 2022 study also found that the warming trend in the Gulf wasn’t consistent across depths and seasons, highlighting the importance of collecting data from below the surface, with tools like the autonomous gliders, to get a complete picture.

Mitchell recently secured funding for a new series of GNATS cruises to collect the ship-based data from 2025 to 2027 aboard the R/V Bowditch, Bigelow Laboratory’s research vessel. As GNATS enters its next era, Mitchell and Drapeau are exploring collaborations with other organizations in the state to share the institute’s gliders and knowledge and potentially facilitate additional surveys. Meanwhile, the last several decades of GNATS and glider data are publicly available on NASA’s data archive to enable ongoing research into using optical proxies to measure biological activity.

“I think it’s important in oceanography to realize that every tool has its pros and cons,” she said. “Because of the resolution of data they provide, the gliders are a really powerful complementary tool.”