Biology
Diel Vertical Migration and Its Role in Trophic Transfer in Zooplankton
Quick fact
This daily round trip, performed by zooplankton across all oceans, transfers roughly 2–6 billion tonnes of carbon dioxide from the surface to the deep sea each year, playing a crucial role in the ocean's carbon sink.
Why this is interesting
Every night, billions of tiny animals in the ocean rise from the depths to feed near the surface, then sink back down before dawn. This daily commute is called diel vertical migration, and it is one of the largest migrations on Earth.
Read the full explanation
Understanding Diel Vertical Migration and Its Role in Trophic Transfer in Zooplankton
Imagine zooplankton like tiny livestock grazing on phytoplankton near the sunlit surface. But the surface is also where visually hunting predators, like fish and squid, can spot them. To avoid being eaten, these small animals spend the day in darker, deeper water. As dusk falls and light diminishes, they swim upward to feed on the abundant phytoplankton. Before dawn, they swim back down. This daily journey is called diel vertical migration (DVM). DVM is not random; it is a trade-off between the need to feed (which requires light and phytoplankton) and the need to avoid predators (which also use light to see). The migration is vast in scale: it involves countless individuals and occurs in every ocean, making it one of the largest synchronized movements of biomass on the planet.
A deeper explanation
The mechanism driving DVM is a balance between energetic gain and predation risk. Zooplankton must feed to grow and reproduce, but they are most vulnerable when they are in the well-lit surface waters where their predators can see them. By feeding only at night, they reduce their exposure to visual predators. The trigger for the migration is the change in light intensity at dawn and dusk, which zooplankton detect using photoreceptors. This light cue is so reliable that it even overrides hunger: a zooplankton that is very hungry may delay its descent, but the fear of being eaten eventually sends it back down. The migration is not uniform; the depth and timing vary among species and with the abundance of predators. Importantly, when zooplankton feed near the surface and then return to depth, they transport the energy and carbon they consumed in their bodies (and in their fecal pellets) down into the deep ocean. This process is a key component of the biological carbon pump, removing carbon dioxide from the atmosphere and storing it in the deep sea for years to centuries. Without DVM, the ocean's capacity to absorb atmospheric CO2 would be significantly weaker, and the deep-sea food web would lose a critical energy input.