In the ocean’s twilight zone, where the reach of the Sun fades to nothing, the world’s largest migration begins every time night falls. It could also have an outsized effect on our climate.
During World War Two sonar technicians made an extraordinary discovery. The pings from their echo sounders reflected off what they thought must be the ocean floor. But the sea was much shallower than they had expected and – even more puzzlingly – the seabed seemed to move up and down throughout the day.
This wasn’t the undulating ocean floor, however, but the many inhabitants of the twilight zone making their nightly migration to the surface to feed. This concentrated “deep scattering layer” of marine organisms, suspended in the water column, was so extraordinarily high that it scattered the sound, reflecting the sonar pings as if it was a solid object.
Beneath the waves, the twilight zone – or mesopelagic zone – starts at a depth of 200m (656ft), where the ocean is bathed in perpetual twilight. Sink deeper and the reach of the Sun’s rays fades rapidly. The last remnants of sunlight vanish completely around 1,000m (3,280ft). There, the only light is the eerie glint of bioluminescence, produced by creatures that glow in the dark.
This vast layer of water spans the globe and is teeming with an astonishing diversity of life. It is home to an estimated 95% of all fish biomass, and around 10,000 million tonnes of fish.
Every night trillions of zooplankton that inhabit this zone rise from the deep to feed under the cover of darkness. This phenomenon, known as diel vertical migration (DVM), is the largest natural migration of animals on the planet, with an estimated biomass of 10 billion tonnes. As the Earth spins on its axis, diel vertical migration takes place throughout the world’s oceans. “I like to think of it as a Mexican wave,” says Laura Hobbs, lecturer in Arctic Marine Science at the Scottish Association for Marine Science, describing the swell of animals rising and falling, following the night around the globe. “Zooplankton go to the surface to feed because that’s where the phytoplankton is,” says Hobbs.
“Zooplankton”, she explains, is an umbrella term for many different species of tiny animals that live in the ocean. “These critters are just millimetres long, they’re tiny. And they’re swimming hundreds of metres every single day, up and down again. It would be like running multiple marathons.” (Read about Jailing Cai’s incredible experience of photographing animals of the twilight zone.)
Phytoplankton, meanwhile, are the plant-plankton. “Phytoplankton need the sunlight [to grow]. That’s why they’re restricted to the surface layers.”
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“As the Sun rises,” says Hobbs, “the zooplankton become threatened by visual predators, bigger zooplankton or fish [that can see them now in the light]. So, they migrate back down into darker waters and stay there to digest. Then they excrete their waste, get hungry again and, as the Sun sets, they come back up for more.” One of the first direct observations of DVM, says Jon Copley, professor of ocean exploration at Southampton University, UK, was made in 1966 by the legendary ocean explorer, Jacques Cousteau when diving in his UFO-shaped “diving saucer” submersible. Almost half a century later, Copley himself was also lucky enough to see the phenomenon up close.
Copley has explored the deep ocean all over the world and the big realisation, he says, is how much of the Earth rests in perpetual darkness. “We’re told it’s an ocean planet, a blue planet. Well, 71% of the surface is blue – but actually the blue comes from the sunlight reflecting off the sunlit upper layer. Most of [the ocean] is beyond the reach of the Sun’s rays. It’s not only an ocean planet, it’s a deep, dark ocean planet.” (BBC)