Eclipse seen from Adventfjorden. Photo: Jørgen BergeBillions were fooled by nature’s early 1st of April joke this year. During a field campaign on Svalbard in March this year, we studied how marine zooplankton responded to the total solar eclipse that occurred around noon on the 20th of March. Would they react to the declining light and be fooled into believing that it was night, or are their internal clocks so strong that they would not be bothered about nature’s little prank? To cut a long story short - they were fooled! In their billions!

Diel vertical migration (DVM) of zooplankton is a characteristic feature of the world’s oceans and lakes, and has been claimed to be the largest synchronized movement of biomass on the planet. Since the phenomenon was first detected almost two centuries ago (in 1817), there have been numerous studies into both the adaptive significance of this behaviour and its ecosystem consequence.  A migration of animals to the surface layer at night allows zooplankton to feed in food-rich waters with reduced likelihood of detection by visual predators (predator-avoidance hypothesis), whereas during daytime they seek refuge in the darkness of the deep. The period around equinox, when the day and night are equally long are known to be a peak period for this type of migration, as it holds a significant advantage to each individual to be able to hide away down in the deep during the bright day and to migrate up in the surface waters during the night to feed on the many small algae and smaller zooplankton that live there. Predators in the pelagic generally use two main feeding modes; they either search for prey using vision (visual predators, e.g. many fish, birds and large zooplankton) or they search for prey by sensing vibrations and movements (tactile predators, many zooplankton). As a result, the prey encounter of visually searching predators is tightly bound to the light regime and prey encounter will be a function of day and night, time of the year and latitude. The extreme seasonality of high latitudes, including the polar night, creates a unique research laboratory for our endeavours to understand the relative roles of different prey encounter modes and for the functioning and constraints of visual predators in the north. To read more about the results, see these recent articles (Norwegian/Icelandic) , or wait for the story to appear in a scientific journal or other media soon..!

See an amazing video of the eclipse by Witek Kaszkin from the Polish polar station at Hornsund:

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The Mare Incognitum web pages are maintained by Marine Night technician Daniel Vogedes, UiT.

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