Polar Moondance
Decades of Acoustic Data Reveal Zooplankton Migration in the Arctic Winter
Themisto libellula
Credit: Daniel Vogedes | The Arctic University of Norway, UiT
It’s a new take on lunar effect. New research shows an unexpected link between moonlight and zooplankton activity in the Arctic winter. This news alters previous conceptions of marine animal activity in the polar night and will have implications as the Arctic continues to warm.
Researcher Dr. Kim S. Last of the Scottish Association for Marine Science and his international team published today their findings in Current Biology.
The focus is on the migration patterns of zooplankton, tiny, water-dwelling animals that form a critical component of the ocean food chain. Billions of these organisms complete a daily, synchronized journey known as diurnal vertical migration in which they drop to lower depths during the day and return to the surface at night.
Zooplankton are a key food source for visually-hunting, near-surface predators including fish and whales.
While their daily migration may be an effort to avoid predators in the daylight, it also plays an important role in ocean health. The migration is credited with helping transport carbon away from the surface and to the ocean interior and having a cascading effect on predator migration patterns within the pelagic community.
Last wondered what happened to migrations of zooplankton in environments with no day-night cycles as in the case of the Arctic winter. “Do they still exist?” he said in his video abstract on the topic.
The answer turned out to be yes, but the story is quite different than what they expected.
Decades of data available from several acoustic Doppler current profiler (ACDP) moorings across the Arctic Ocean allowed the researchers to visualize zooplankton migrations. The data, cumulatively spanning 50 years, was “massive” Last said. “When we mined this data we found something which was hidden and this was very exciting,” he said in the video.
What Last’s group saw was a pattern of migration tied to the unique sunless conditions of the Arctic winter. “A shift from solar-day (24-hr period) to lunar day (24.8-hr period) vertical migration takes place in winter when the moon rises above the horizon. Further, mass sinking of zooplankton from the surface waters and accumulation at a depth of ~50m occurs every 29.5 days in winter, coincident with periods of full moon,” he said in the study.
“What was even more exciting,” he said in the video, “was that once the full moon had gotten high enough and bright enough in the sky every month for about 6 days there would be a massive migration of zooplankton down to about 50 meters.” As with sunlight, bright moonlight enables increased predator activity.
Methodological approach to determine the lunar influence of zooplankton migrations. Image from Last et al.
There are many important implications of these findings. They provide a new understanding of patterns of migration and predation in the Arctic winter now and in the future as Arctic ice retreats and opens the ocean to more light, even in the winter.
This article originally appeared in Meta Science News January 2016