Life in the dark

Life in the dark

Top image: A clione under the sea ice. New research by UNIS scientists and colleagues show that there is a lot of activity in the Arctic Ocean during the dark season. Photo: Geir Johnsen/UNIS. 

Until now scientists have believed that the marine life in the Polar Ocean is hibernating through the long, Arctic night. Recent research shows evidence to the contrary. – This knowledge may even impact our understanding of the exchange of climate gasses between the ocean and the atmosphere, says UNIS Professor Jørgen Berge.

31 October 2008
Text: Eva Therese Jenssen

It has until now been a universal assumption that the marine life in the Arctic Ocean goes into a lethargic state when the sun disappears below the horizon and the dark season begins.

But now recent research, published by UNIS staff and colleagues, show that this assumption must be re-evaluated, as the activity in the Arctic Ocean is more extensive than previously documented. The new scientific evidence concludes that some marine organisms continue a surprisingly high activity level throughout the winter, even in areas covered by sea ice.

– These findings are important for our understanding of the Arctic ecosystem in wintertime, but these processes can also have a great impact on the exchange of climate gasses between the ocean and the atmosphere, says Jørgen Berge, UNIS professor in Marine biology and lead author of an article describing the new findings published in Biology Letters last week.

New knowledge about the fundament of Arctic life
There are extreme differences in physical and biological activities in the Arctic, depending on the variations in the amount of sun light. These differences also affect the building blocks of all life in the Arctic: the billions of micro organisms living in the Polar Ocean.

Scientists from UNIS, The Scottish Marine Association for Marine Sciences, Norwegian Polar Institute, the university college in Bodø, NTNU and University of St. Andrews have over the past two winters done extensive research on the biological diversity in the Arctic waters, and studied the processes that take place during the long Arctic winter.

Migration as survival strategy
Marine predators, such as the Polar cod, feed on zooplankton, such as the Calanus spp. Most predators depend on visual contact in order to identify their prey. Thus, many zooplankton species have developed a survival strategy where they ascend into the food-rich surface waters during darkness and retreat to deeper waters during day. This strategy is called Diel Vertical Migration (DVM).

Continues despite “complete” darkness
It is presumed that changes in light conditions are the main factor determining the DVM pattern of the zooplankton. Humans perceive darkness as complete when the sun is more than 12 degrees below the horizon, and it has hence been generally assumed that the DVM ceases during this time.

The scientists used acoustic data from two localities in Svalbard; Kongsfjorden on the west coast of Spitsbergen (79ºN) and Rijpfjorden on the northern coast of Nordaustlandet, to test the hypothesis that there is no DVM behavior during the polar night.

The data was collected over a 24-hour periodicity for each depth layer for defined periods throughout the polar night. From this data it is clear that the zooplankton at both locations do perform DVM through most of the Arctic winter, even in those periods of the winter when there is seemingly no changes in the light conditions. The zooplankton migrates towards the surface during “daytime” – even when we humans perceive only complete darkness.

Kongsfjorden, where there’s been no sea ice over the past couple of winters, the DVM signal seems stronger than in Rijpfjorden, which is covered by sea ice most of the year.

However, even if the sea ice – up to 1 meter thick – blocks out most of the light in the ocean as the sum returns, the zooplankton in Rijpfjorden keeps performing DVM.

Follows the lunar cycle
This activity even in the dark season shows that there is a lot we still do not know or understand about the Arctic marine ecosystems.

The new research shows that the ecosystems go through the same processes in complete darkness as it does in the Arctic springtime when the difference between day and night are clearly defined.

The analysis also shows that when there is a full moon in the winter, the zooplankton migration patterns shift from a 24-hour solar cycle to a 25-hour lunar cycle.

– In other words, the circadian rhythm expands with one hour when there is a full moon, which definitely indicate that light is the prime factor in the marine processes even at the darkest time during the polar night, Berge says.

Thus, zooplankton reacts even to the slightest changes in the lighting conditions, even in the dark season when humans perceive only blackness.

Consequences for carbon emissions
The zooplankton feed in the surface waters and digest organic carbon before they retreat down into the deep ocean. As DVM is a critical component of the “biological pump” that draws organic carbon and atmospheric CO2 into the ocean interior, the results imply that an increased activity of the biological pump in Arctic waters could be expected in response to the predicted reduction of winter sea ice cover, Berge says.

– It is important to note that the DVM process is only one small component in the complex climate puzzle. Other processes, such as the ocean temperature, will most likely have the opposite effect and cause an increase in the CO2 emissions. However, it is very important to uncover and understand all these intricate components in order to get a comprehensive picture of the Arctic climate system, Berge concludes.

Berge, J., F.R. Cottier, K. Last, Ø. Varpe, E. Leu, J. Søreide, K. Eiane, S. Falk-Petersen, K. Willis, H. Nygård, D. Vogedes, C. Griffiths, G. Johnsen, D. Lorentzen, A. Brierley (2008), Diel vertical migration of Arctic zooplankton during the polar night, Biology Letters, Oct. 23, 2008, doi: 10.1098/rsbl.2008.0484

polar cod and zooplankton

Marine predators, such as the Polar cod feed on zooplankton species like Themisto spp. (picture) and Calanus spp. Photo: Geir Johnsen/UNIS.

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