San Francisco -- Although polar bears and seals have become the poster children for vanishing sea ice in the Arctic, they have thrived for a long time. The bears eat the seals, but what do the seals eat? Maybe fish, although in many parts of the Arctic fish are few in number. Even then, what do the fish eat? Researchers have not had a way to fully investigate the food web, but now they do: A new robot, called Nereid, that can cruise on its own just underneath the ice for kilometers at a time and return with data and video to a ship at the ice’s edge.

The Nereid Under Ice vehicle, built and operated by a consortium led by the Woods Hole Oceanographic Institute, completed four dives during its first Arctic mission in July. Three leading researchers from the mission described the dives yesterday during a press conference here at the American Geophysical Union fall meeting in San Francisco. My colleague Rich Monastersky at Nature described the dives in an article he posted yesterday afternoon (Scientific American is part of the Nature Publishing Group).

The researchers also released some intriguing scientific results from the dives. The most surprising is the realization that there is far more life floating around just under the Arctic sea ice, and that there is a much more complex food web, than scientists had thought. Before Nereid, scientists had a difficult time trying to probe or even see undisturbed life under the ice. But Nereid showed extensive communities of dark algae clinging to the underside of the ice. It, in turn, was being consumed by all sorts of critters, many tiny but some larger.

I caught up with Antje Boetius, a marine biologist at the Helmholtz Center for Polar and Marine Research in Bremerhaven, Germany, and chief scientist of the expedition, later on at the meeting. I wanted to know what she and her colleagues saw, and which life forms she thinks are eating which other life forms. The paragraphs below describe what she told me. Have a look, then watch the video, here, which the robot took during its dives, and you should be able to see the various life forms Boetius describes.

Because the ice is thinning from global warming, more sunlight is getting through to the underside, and a form of algae known as diatoms is growing, sometimes in spots and sometimes in large collections. In the video, these appear like dark splotches on the ice. Some of the diatoms attach to the ice, but others attach to those diatoms and float alongside. The plankton produce a kind of slime that helps them all stick together. Some also trap oxygen bubbles, which the diatoms produce, to help the extended mass float right up against the underside of the ice, allowing the community to get the maximum amount of sunlight for photosynthesis.

Tiny crustaceans called copepods float freely in the water and eat the algae. They are a main component of what looks like the “dust” you can see floating around in the water in the video. But because nutrients in the water dwindle or even disappear during the dark months of winter, when the algae’s photosynthesis basically stops, the copepods create and store lipids inside their bodies, which they live off of during those dark months.

Now for the really cool critters: ctenophores and larvaceans. These are bigger living blobs that look somewhat like small jelly fish. In the video, starting about halfway through, you can see them hovering and flapping their ghostly bodies. They eat the copepods.

But what eats the ctenophores in the under-ice region? “That we don’t know,” Boetius told me. “We saw no fish.” In warmer waters tiny fish such as anchovies and larger ones such as cod would eat these gelatinous animals, and seals and polar bears would eat the fish. Boetius is eager to return for future missions to try to complete the food web, which is obviously critical to the survival of all these animals large and small, and which in turn could affect other food webs that link the Arctic region to warmer regions at lower latitudes. Ultimately, Boetius said, “learning more will tell us what ice loss means for life on earth.”

Photo and video of Nereid mission courtesy of Woods Hole Oceanographic Institution