How marine creatures react to long winter nights

SCUBA welcomes new science columnist Karen Boswarva, who begins her tenure by turning her eyes to the sky, then the sea.

Has anyone else noticed that the nights are slowly drawing in? The shorter days are signalling that summer is coming to a close, where hibernating in a woolly jumper with a hot brew sounds more appealing than the mandatory torches and icy mask clears that winter tends to bring. These dark evenings have at least been offering us something quite spectacular above sea level.

We’ve been very lucky that on at least two occasions this year (so far), the whole of the UK from Shetland to the Scillies was treated to one of the greatest natural light shows on planet earth (unless you’re one of the few that slept in and are therefore still kicking yourself for doing so). The aurora borealis, or northern lights, usually reserved for those with the darkest skies in the far north of Scotland. What an incredible sight it was, as the night sky raved its way into dawn 90s-style, a blaze of flashing green and purple rays of light.

It certainly made us all, including me, get outside and look up. As I stared up in awe, it made me wonder how might its presence be felt underwater? Do marine animals make use of the light emitted from these solar flares? Many of the marine animals we come across on a dive are sensitive to light; you recall the effect when turning a torch on and catching some creature in the light. Some will flee, very quickly mind, while others (both predator and prey) will surround you, using you to their advantage in their otherwise darkened world.

Meet Karen Boswarva

Hello! I’m based in Orkney where I work as a marine habitat mapper and volunteer as Seasearch coordinator for Orkney and Shetland. I learned to dive with BSAC in 2010, progressing to Instructor, Diving Officer, Regional Training Coordinator, and environment sub-committee member. I’m an absolute geek for nature, science and technology, especially the underwater kind. You’ll find me anywhere on or under the sea.

The behaviours and physiology of marine animals such as zooplankton and fish attune to even the smallest changes in light, ensuring that ecosystems remain active and functioning all year, from day to night, spring to winter. 

Zooplankton are vital to the health of the seas, A biodiverse rich soup that most larger animals (including us) rely upon for food.

Throughout the summer when there’s plenty of daylight hours, these tiny marine animals travel up and down the water column in a daily cycle that follows a circadian rhythm while also adjusting to small changes in ambient light. This is called Diel Vertical Migration (DVM). At night they will travel towards the surface to feed, returning to the depths during the daytime to try and hide from predators.

During winter when there are fewer sunlight hours, our seas don’t simply lie dormant hiding under a blanket for summer to arrive. Marine animals will make use of any available light to keep to their daily rhythm. An extreme example of this is the polar night, the last remaining undisturbed and pristine dark environment on the planet. To the human eye there is no difference in illumination between night and day. During a polar night, zooplankton will harness light from the moon and stars, in much the same way as organisms generate energy from sunlight, using specialised light detecting cells called photoreceptors.

The available light emitted from the sun, moon, stars, and aurora borealis is called ambient irradiance. Scientists tested the light sensitivity of a type of zooplankton called Calanus sp. Calanus is a fat filled copepod – basking sharks find them irresistible – that performs DVM during the polar night. The test consisted of gradually increasing the ambient irradiance of light in the white, blue, green, and red wavebands, recording the Calanus response using a specialised video camera. The Calanus showed high sensitivity to low levels of blue and green light and responded negatively, diving deeper from ambient light levels that mimicked the night sky at depths of 70-80m, moonlight at 120-170 m and aurora borealis down to 80-120 m depth. 

Now there’s something to ponder next time you’re searching blindly for the light switch.

 Article ‘Rave 'till dawn’ by Karen Boswarva first published in SCUBA magazine, Issue 150 November 2024.