A smart AUV maps phytoplankton to help seabird populations

An artist’s visualization of Harald in the ocean, detecting and measuring chlorophyll a as an indication of phytoplankton amounts and locations. Image: David Fierstein and Arild Hareide

Phytoplankton form the base of the marine food chain but are notoriously difficult for scientists to account for — a little like trying to identify and count motes of dust in the air. A truly independent underwater vehicle shows it can do the job.

Trygve Olav Fossum watched an orange, torpedo-shaped instrument slide off the R/V Gunnerus and plop into the coastal waters near the island called Runde. It was June 2017 and Fossum, a PhD candidate at NTNU, was part of a team of researchers trying to find answers to a vexing problem.

Runde, a triangle-shaped island off the mid-Norwegian coast, is known for its large seabird populations, including Atlantic puffins and Northern Gannets.

In recent years, bird numbers here and in much of the North Atlantic have dropped precipitously. No one knows quite why.

As a first step in their search for clues, NTNU researchers had assembled an interdisciplinary team of geologists, biologists, mathematicians, computer scientists and engineers, like Fossum, whose two metre-long autonomous underwater vehicle (AUV) would contribute to one of the most unusual pieces of information on the Gunnerus’s week-long survey.

Fossum’s AUV, named after the Norwegian oceanographer Harald Sverdrup, would collect information that allowed scientists to make a 3-D map of hot spots of phytoplankton. These are the tiny single-celled algal cells at the base of the food chain. Their microscopic size and tendency to collect in patches have made this information nearly impossible for biologists to gather in the past.

The AUV was programmed to think on the go — “seeing” where the phytoplankton were, choosing its own course to zoom in on patches in an area to get a better sample. Scientists call this “adaptive sampling.” The 3-D maps, in turn, could provide important clues as to why bird populations around Runde were plummeting.

Zooplankton eat phytoplankton. Little fish eat zooplankton. Bigger fish eat the smaller fish. Finally, seabirds like puffins feast on these patches of fish. If something was changing phytoplankton amounts or distribution, it could set off a chain reaction that could affect the birds.

Having a smart AUV that can be programmed to seek out phytoplankton patches “is a complete game-changer,” says Geir Johnsen, an NTNU biologist is collaborating on the project. The results from Harald’s tour in the waters off Runde were recently reported in Science Robotics.

Large areas of unknown, and concentrated patches of fecundity

Marine biologists face a fundamental problem. The ocean is deep, broad and generally poorly understood. Some areas are more interesting than others, especially the small, concentrated areas that teem with life, such as coastal waters or the places where currents meet. To do their job, biologists need to understand what factors make some patches of ocean fertile while others are not.

Biologists describe this situation as, well, “patchiness,” Fossum said. The patchiness of phytoplankton is related to a number of different biophysical interactions, such as currents, turbulence and mixing, and biological processes, like how many other creatures are eating the phytoplankton.

“That means it’s a very hard question to figure out what controls the patchiness of these organisms in the ocean,” Fossum said.

Even if you are in a place that’s known to be a hot spot, patchiness can make it difficult to accurately quantify marine organisms in the area, especially if you are taking samples from a research boat, says Glaucia Fragoso, a postdoc at NTNU’s Department of Biology who was on the cruise with Fossum.

“If we drop our sampler in the wrong spot, we may undersample and underestimate phytoplankton numbers,” she said. “Or if we drop our sampler right in the middle of a patch, we can overestimate.”

Why patches are where they are

That’s what makes the adaptive sampling of Harald, the AUV, so unique, Fragoso said. Given an area to explore, it can make a 3-D map of phytoplankton patches. And knowing where patches are allows scientists to study other characteristics of that area so they better understand why the patches are where they are.

“Is the (phytoplankton) concentration there because of salinity?” said Fossum. “Maybe the phytoplankton are concentrated along a temperature or salinity layer, or maybe there is some other physical effect that is keeping them where they are?”

Knowing where and why phytoplankton aggregate and cluster in different ways can help answer questions about creatures that depend on the ocean for food, like the seabirds at Runde.

Seabirds typically nest in areas where they have easy access to food, since they have to feed themselves and their chicks, too. So figuring out phytoplankton amounts and where they are, in combination with other measurements, may help explain larger trends in seabird populations.

Adaptive sampling for greater detail

Harald was programmed with a sophisticated brain and equipped with a special measuring device called an ECOpuck nestled in its backside. When Fossum released it into the water that June day, Harald would roam the ocean’s depths in an area bounded by a 700×700 metre box, collecting information to make a 3-D map of phytoplankton.