Zooplankton migrate to deeper waters when the sun comes up. But what happens if the sun sinks below the horizon for weeks at a time?
This is the question scientists at the Scottish Association for Marine Science (SAMS) are hoping they have solved. In a new study, they argue that in the absence of sunlight, zooplankton move up and down the water column using the light of the moon. The study shows that even under a layer of ice, during the darkest days of the polar winter, there is a significant zooplankton response to what might be considered very subtle changes in illumination.
‘The findings were incredibly exciting,’ says Laura Hobbs, Arctic researcher at SAMS and co-author of the study. ‘The fact that some of the zooplankton population remains active throughout the Polar Night (instead of entering a hibernation phase) has been suggested for some years. However, up until now, we had little understanding of what they were responding to.’ The data was collected from instruments deployed all across the Arctic and according to the findings, the lunar response is ubiquitous across the region – in fjords, ice shelves and the open sea.
The vertical migration of zooplankton takes place in all water bodies. It is seen as the world’s greatest migration according to biomass. Thought to be avoiding being seen and eaten, the zooplankton migrate on a 24-hour cycle – to the deep in the day and the shallows at night. According to the study, the so-called ‘werewolf’ behaviour of Arctic zooplankton means adjusting their daily migrations to the 24.8-hour lunar day, when the moon has a greater impact than the partial sun, with a time of transition in spring and autumn. When the full moon is bright, they sink to depths of around 50 metres.
Understanding the Arctic-wide movement of zooplankton could reveal much about the Polar ecosystem at large. ‘The existence of a vertical migration (whether it be on a 24 or 24.8-hour cycle) throughout winter suggests that active predation is a continued threat,’ says Hobbs. She believes it could also be creating natural patterns: ‘a dense layer of zooplankton at 50 metres, during the full moon, could actually lead to higher rates of predation, through more chance encounters between predator and prey.’