Light in total darkness

Light in total darkness

Top image: The lab set-up, where a video camera using infrared light recorded the zooplankton responses to light adjustments. Photo: Anna Båtnes/UNIS.  

Marine organisms in the Arctic Ocean have a daily rhythm cued on by light – even in the dark season when it is pitch black outside and the ocean has a thick ice cover preventing light from penetrating into the water. But how light sensitive are these organisms – and what light sources affect their behavior? Experiments at UNIS is part of a PhD project aiming at finding out just that.

11 May 2011
Text: Eva Therese Jenssen/UNIS

How come these tiny marine organisms react to even the smallest amount of light – when it is, to human eyes, totally dark? How light sensitive are these small animals – so important to the Arctic life?

That’s what Anna Båtnes (28), PhD student at NTNU and UNIS, wants to find out. Her research project is to find out exactly that. A better understanding of how light sensitive these animals are will identify which light factors they react to.

Predators and prey: Controlled by light
Zooplankton is the preferred diet for fish species in the Arctic. The fish are visual predators – they use their eyes to hunt for food. Thus, the zooplankton‘s strategy to avoid being eaten by predators in the Arctic ocean is to feed when it is dark.

Zooplankton migrate up to the food rich surface waters to feed at night-time, and descend to deeper waters during daytime to avoid predators. This phenomenon is called Diel Vertical Migration (DVM), and it is the largest synchronized movement of biomass on the planet.

A change in light intensity is the cue for these migrations. Until a few years ago it was assumed that the biological processes, including DVM, in the Arctic Ocean came to a stop during the dark season. In 2008 scientists from UNIS, The Scottish Marine Association for Marine Sciences, Norwegian Polar Institute, the university college in Bodø, NTNU and University of St. Andrews proved that it is not so. They presented evidence of active DVM during the most intense polar night. For humans it is pitch black darkness, but not for the tiny organisms living in the depths in the Arctic Ocean. The new research even showed that the DVM continued in ice covered waters.

Sun, moon and northern lights
So these animals obviously respond to extremely low levels of light. In the polar night there are three obvious light sources at very low intensity levels, visible and not visible to the human eye: background radiation from the sun (which is below the horizon); moon light and northern light (Aurora Borealis).

How important are each of these three light sources for DVM? In order to find out that, one must find out how low intensity of light can zooplankton detect in order to follow its daily migration pattern. And that’s what Anna Båtnes is investigating. In January she came to UNIS in order to do lab experiments on copepods (zooplankton) from Svalbard. The aim is to investigate the copepods’ responses to light.

-I am looking at threshold values, that is, how low light intensities do these animals see and respond to, of both white light and different colours, Båtnes explains. – Together with colleague Cecilie Miljeteig, I used copepods of the species Calanus finmarchicus and Calanus glacialis from Adventfjorden, and did light response experiments in the lab at UNIS.

Calanus glacialis

The copedpod Calanus glacialis was used in the lab experiment at UNIS. Photo: Janne Søreide/UNIS.

Dark, then light
The experimental setup allowed for detailed adjustments in light intensities, and the response of the copepods was captured by a video camera using infrared light.

The experiment started with complete darkness in the lab. Light was gradually introduced by marginally increasing the light intensity with 10 minutes intervals. At a certain light intensity the animals started moving away from the light source. The preliminary results indicate a high sensitivity to light in these species, which corresponds to research showing that they perform DVM during the polar night.

– These animals have a fantastic sight and they are extremely sensitive to light, Båtnes explains.

– Their response to light is to move away from the light source, which fits well with their behaviour while performing DVM: moving to deeper and darker waters during daytime. They are highly sensitive to white light as well as blue and green light, but have lower sensitivity to red light. This is probably an adaptation to that blue and green light penetrate deep into the water column, while red light disappears relatively quickly, she says.

The experimental setup used in Longyearbyen is developed by PhD student Cecilie Miljeteig and a team of supervisors from NTNU, Sintef and BioTrix in Trondheim, where it is used for research on Calanus finmarchicus from their lab culture. Their tests show the same response pattern as Båtnes found in her lab work at UNIS.

These experiments are only part of the Båtnes PhD project. She will also investigate which of the different light sources in the Polar night influence the DVM the most. The PhD will be completed in 2013.

Anna Båtnes

Anna Båtnes, PhD student at NTNU and UNIS, with the lab set-up allowing her to study the light sensitivity of zooplankton. Photo: Eva Therese Jenssen/UNIS.

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