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Biology

Seeing in the dark: zooplankton in Arctic winter.

The paper:

Cohen, J. H., Berge, J., Moline, M. a., Sørensen, A. J., Last, K., Falk-Petersen, S., … Johnsen, G. (2015). Is Ambient Light during the High Arctic Polar Night Sufficient to Act as a Visual Cue for Zooplankton? Plos One, 10(6), e0126247. doi:10.1371/journal.pone.0126247

Introduction

Krill in the lab.

Krill in the lab.

As it changes over a day and over seasons, light drives many biological processes in the oceans. Light fuels photosynthesis in the phytoplankton and mediates predator-prey interactions by determining when animals are best equipped to hide or hunt. Zooplankton (small animals drifting through the ocean) rely on light changes over the day to regulate daily migrations up and down in the water column to feed, find mates or avoid predators.

High latitudes, near the poles, experience extreme light changes with the seasons. Summer brings constant daylight whereas winter is dominated by darkness. Night lasting over 24 hours is known as polar night and only occurs within the polar circle (>66°N or S). This study measured the amount of light available during polar nights and explored how zooplankton may be able to see using that light.

The researchers worked in Kongsfjorden, Spitsbergen (78°55’N) where the polar night lasts 129 days per year. As a representative animal the study focused on a particular zooplankton species, krill, known widely as a common whale snack.

 

Methods

Ambient light measurements

Measurements were taken in January of 2014 and 2015. All nearby artificial lights were put out or covered and atmospheric light intensity was measured using an irradiance sensor. These measurements were used to model the underwater light field up to 70 meters in depth. Sky images were taken every 30 minutes for three days (Shown in figure below).

Spectral sensitivity of krill eyes

The scientists captured krill near the light measurement location. In the lab they examined the visual abilities of the tiny krill eyes. Electrophysiological measurements were made to determine the visual spectrum for krill (what light they can see and what they can’t). The measurements of ambient light include light along the spectrum that zooplankton cannot see. “Krill utilized photons” were defined a where the visual spectrum for krill overlapped the light available underwater.

 

Results and Significance

The light measurements show a change throughout the day with the greatest light at noon, even in polar night. Krill show greatest sensitivity for blue light, which aligns with deep water at the site. Krill appear to be able to see in depths down to over 20 meters.

Along with atmospheric light- bioluminescence, northern lights, and starlight contribute to the light under the surface. The same ability of krill to see in the top 20 meters may help in the detection of bioluminescence given off by prey or predators at greater depths. The figure below shows the difference between available light over depth and the light available to krill eyes.

Fig 3. Modelled underwater spectral light field in Kongsfjorden at midday under clear sky conditions. Contours show the ambient underwater light as scalar irradiance (Ambient Light, left panel) and krill-utilized photons (Utilized Light, right panel). For both panels, light is expressed in units of μmol photons m-2 s-1 nm-1, derived from a radiative transfer model as described in the Materials and Methods.

Fig 3. Modelled underwater spectral light field in Kongsfjorden at midday under clear sky conditions.
Contours show the ambient underwater light and krill-utilized photons (Utilized Light, right panel).

Fig 1. All-sky pictures from Ny-Ålesund 21st and 22nd of January 2014.

Fig 1. All-sky pictures from Ny-Ålesund 21st and 22nd of January 2014.

 

This study presents the first look at what light in these regions means for zooplankton behavior. The light regime, like so many things, will be altered as climate change decreases the ice cover, leading to more light. Understanding how the ecosystem functions will help us understand changes in the future.

What other marine processes may be impacted by changes in the light regime? How would 129 days of darkness change your daily life? Let us know in the comment section below!

Discussion

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