Find out the reason for the nightly migration of trillions of zooplankton

Every evening, around the world, trillions of zooplankton - many smaller than a grain of rice - hover a few dozen meters below the surface of the sea, waiting for the sun to disappear to emerge near the surface.

As they emerge, other groups of zooplankton join in, such as flippers, sardines, krill, and fish larvae. The crowd remained near the sea surface all night. However, as soon as the first rays of morning began to appear, they returned to the abyss.

As sunset and sunrise slide from east to west every 24 hours - across the Pacific, then the Indian, South and Atlantic - herd after herd makes the same journey up into the night and withdraws. retreat when daylight returns, Scientific American reports.

Freshwater and marine zooplankton often migrate vertically

Humans are largely unaware of this daily aquatic movement, known as vertical migration. However, it is the most frequent migration of life on Earth.

Current estimates suggest that around 10 billion tons of animals make these excursions every day. Some of them shot up from more than 900m below. That is an amazing feat.

Why do huge numbers of tiny animals make such an arduous journey every day?

The short answer is to eat and avoid being eaten

During the day, zooplankton are vulnerable, hiding from predators such as squid and fish in the dark depths.

When night falls, they rush to the surface to feed on phytoplankton - microscopic aquatic plants that live at a height of several tens of meters of water - under the cover of night.

A new study is revealing the hidden complexities of this mass exodus.

For one thing, this process is tied to what's happening in the sky. When the sun is absent for weeks at a time during the polar winter, some of these animals rearrange their migrations according to the cycles of the Moon.

Solar eclipses can signal them to begin swimming towards the surface. Deborah Steinberg, dean of biology at the Virginia Institute of Marine Science (USA), realized this during a research trip. Although the light changes on the sea surface were not obvious to her and her colleagues, zooplankton somehow noted subtle changes in light far below the water.

Ms. Kelly J. Benoit-Bird of the Monterey Bay Aquarium Research Institute (MBARI) in California and Mr. Mark Moline of the University of Delaware (USA) brought a 300-meter-long self-propelled underwater vehicle into the Catalina Basin offshore the region. Southern California. This vehicle performs vertical sonar measurements of zooplankton migrations.

The results revealed to them that the zooplankton species were organized into well-defined clusters, clustered closely to each type, and migrated together along careful timelines.

Ms. Steinberg and other scientists also found that quite a lot of carbon disappears from the ocean's surface.

However, after many night scuba diving, living with migratory plankton, Ms. Steinberg and colleagues have an answer to the disappearance of carbon.

On the ocean's surface, phytoplankton draw large amounts of CO2 from the atmosphere, but they release most of it into the atmosphere, usually within a few days. When migratory zooplankton swim up at night and feed on these marine plants, they become a kind of biological conveyor, transporting carbon into the deep sea, where carbon can be sequestered in hundreds or thousands of years.

The sun will continue to rise and set. As such, countless animals will follow the tides of darkness and light underwater, eating, excreting and regulating the balance of the elements on our planet.