A few hundred miles from the North Pole, the winter sky is dark all day long, and the ocean depths are even darker. Aquatic creatures, it was long believed, went into a hibernation-like state until new food sources — plankton — bloomed in the spring with the return of sunlight.
It turns out the icy waters are not so dark and dormant after all.
University of Delaware scientists have discovered a vibrant winter community of light-emitting organisms hundreds of feet beneath the surface.
The team, which described its findings recently in the journal Scientific Reports, found these creatures in a fjord in Norway's Svalbard islands. The scientists measured the organisms' bioluminescence — their emissions of eerie, bluish light — by lowering a boxy light-measuring device deep into the water and sucking up quantities of water at periodic intervals.
It is not entirely clear what role the light plays in helping the various organisms survive, though there are several intriguing theories, one a defense mechanism called the burglar-alarm hypothesis, that the scientists plan to examine.
Another unanswered question is what happens when more light comes from above. That is a growing concern with the increased melting of ice, attributed to human-induced climate change, allowing small but measurable amounts of natural light to penetrate the water.
If an organism uses light in some subtle way to gather food or to avoid being eaten, the addition of more natural light could upset that balance, or at the very least drive such creatures deeper into the sea, the scientists say.
The fjord where the Delaware team conducted its research, called Kongsfjord, used to freeze over every winter but has been ice-free for years, said study coauthor Jonathan H. Cohen, an assistant professor in Delaware's College of Earth, Ocean, and Environment. That phenomenon could be due both to warmer air temperatures and to changing currents bringing warmer waters from the south, he said.
Coauthor Mark Moline, director of Delaware's School of Marine Science and Policy, said it was too soon to say how added light changes the picture.
"It likely will favor certain organisms over others," said Moline, who also is known for using scientific techniques in the hunt for missing World War II aircraft. "How that plays out in the food web is not known at all, as we are just beginning to find out how light influences perception and interaction of these animals."
One scientist not involved with the study, Edith Widder of the nonprofit Ocean Research and Conservation Association in Fort Pierce, Fla., said the research provided valuable clues. The addition of more light to a near-dark environment is cause for concern and requires further study, she said.
"It's going to change the dynamics of the ecosystem profoundly," Widder said.
The lead author of the study was Heather A. Cronin, a former graduate student at Delaware who has since gone to work in the private sector. She, Cohen, and Moline were joined in the work by Norwegian scientists Jørgen Berge and Geir Johnsen.
The team conducted its measurements in January 2014 and 2015, spending several weeks at a research station on the fjord.
In the upper part of the water column, they found that most of the light came from the sun, even though it remains well below the horizon at midday in January, yielding a deep twilight sky. A small amount of light in these upper reaches of the water came from algae-like organisms.
Then, as the scientists took samples at greater depths, they found more of the light came from small crustaceans called copepods and from jellyfish-like ctenophores.
But this light is not "on" all the time — only when the creatures are reacting to their environment in some way.
In the burglar-alarm hypothesis, the idea is that if some kind of algae senses the presence of a predator, such as a copepod, it then emits light to attract an even larger predator, such as a fish, to eat the smaller predator — like summoning a police officer to apprehend a burglar.
Cohen, who plans to go back to the fjord in January for more research, said much remains to be discovered. One thing is clear: Life does not go on hiatus during the annual months of darkness known as polar night.
"While there is this traditional view of the polar night period as the time of sort of dormancy, there's more to it than that," he said. "The story is much more rich than that."