Mapping phytoplankton in norwegian waters with a smart AUV
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. 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 ocea
The ECOpuck doesn’t actually measure phytoplankton itself, but something called chlorophyll a fluorescence. Phytoplankton use chlorophyll a pigments in the process of photosynthesis, and the substance fluoresces red when exposed to light. The ECOpuck detects the fluorescence, which can indicate how much phytoplankton biomass is found in the water. At the start of the AUV’s journey, it takes measurements on the sides of the box and then gradually zooms into the area outlined by the box as it detects the region that seems to have the most chlorophyll a, Fossum says. In spite of the success of the AUV, Fossum and others explain that biologists still need to gather information from other sources — like research cruises aboard the R/V Gunnerus. “Oceanography is moving towards combined efforts to collect data, where robotic sampling is an essential part, providing capabilities and resolution that were previously unattainable with traditional methods,” Fossum said. “The ultimate goal is to effectively measure the impact of climate change in the ecosystem, for example.” Fossum says there’s a need for much more persistent monitoring of Norway’s coasts, marine protected areas, and fragile habitats. “The goal is to eventually automate much more of this work, but we are not aiming to replace ships, they are still vital in this endeavour,” he said.