The denizens of the deep ocean may seem as strange as aliens from another planet but these creatures are products of the same biochemistry that impels other animals. Driven by the need for food and to avoid predators, deep-sea marine animals – especially tiny free-floating zooplanktons – swim up to the surface at night. This synchronised movement is called diel vertical migration (DVM), and it makes for a captivating display of nature’s ingenuity. It is also a crucial player in the earth’s carbon cycle.
At DVM’s heart lies a delicate balance between the need to feed and to avoid becoming prey. Zooplankton in particular are constantly wary: they can stay in dangerous waters during the day, looking for food, or swim down to the twilight zone and risk starvation. DVM is a way to cut past these terrible options and live the best of both worlds.
As the sun dips below the horizon, deep-sea organisms rise from the mesopelagic layer (200-1,000 m deep) to the epipelagic layer, where darkness provides a cloak of safety. This nightly ascent allows them to graze on the microscopic phytoplankton while skirting the haunts of their daytime predators. The timing of this migration is tuned to the natural rhythms of sunrise and sunset, and the migration itself is the largest on the planet by biomass. And it occurs daily in all oceans.
DVM is also more than a survival strategy: it plays a pivotal role in sequestering carbon, a process vital for regulating the earth’s climate. Animals dwelling in the mesopelagic layer actively remove substantial amounts of carbon from the upper ocean as they feed on surface-dwelling plankton. When these organisms return to deeper waters, they carry the carbon with them.
Even within the twilight zone, some migratory animals become part of the food chain, passing on the carbon they have consumed to their predators. The carbon-rich waste produced by the predators then sinks to the ocean floor, where it remains trapped for millennia. This natural process is an important carbon sink that helps regulate the concentration of carbon dioxide in the atmosphere.
