What Is Diel Vertical Migration and Why Does It Matter?

Diel vertical migration (DVM) is a daily, synchronized movement of aquatic organisms between the surface and deeper waters of oceans and lakes. In terms of sheer biomass, it is the largest migration on Earth, involving countless small animals from fish to tiny crustaceans called copepods. This rhythmic movement of life plays a part in the structure and function of aquatic ecosystems.

The Daily Pattern of Movement

The most widespread pattern of this migration is nocturnal, directly tied to the daily cycle of the sun. Organisms ascend toward the food-rich surface waters at dusk and descend back into the dark, deeper layers at dawn. This movement is precisely timed, with the animals often following a specific level of light, known as an isolume. The change in light intensity is the primary trigger for this journey.

While nocturnal migration is the most common, other patterns exist. Some species exhibit reverse migration, moving toward the surface during the day and descending at night. Another variation is twilight migration, which involves two ascents and two descents within a 24-hour period. In this pattern, organisms rise at dusk, sink around midnight, and rise again at sunrise before their final descent. Secondary cues, such as changes in water pressure and chemical signals from predators, can also influence these movements.

Driving Forces Behind Vertical Migration

The primary reason for this journey is a trade-off between feeding and safety. The surface waters are abundant with food sources, but they are also brightly lit during the day, making small organisms easy targets for visual predators. By feeding at the surface only under the cover of darkness and retreating to the dark depths during the day to hide, these animals reduce their risk of being eaten.

Beyond predator avoidance, this migration offers metabolic advantages. The deep waters are colder than the surface, and this low temperature slows down an organism’s metabolic rate. By spending the day in these cold, deep layers, animals conserve precious energy, allowing them to make the most of the food consumed at the surface.

Another factor driving this behavior is the avoidance of harmful ultraviolet (UV) radiation. In clear waters, UV rays from the sun can penetrate the surface layers and damage the DNA and tissues of aquatic organisms. Migrating to deeper, darker waters during the day provides an effective shield against this damaging radiation.

Ecological Significance of the Migration

The daily movement of organisms has major consequences for the planet, particularly through its role in the “biological pump.” Migrating animals feed on carbon-rich organisms near the surface at night. When they descend into the deep ocean, they bring that carbon with them. Through respiration and excretion, they release this carbon at depth, effectively transporting it from the surface and preventing it from immediately returning to the atmosphere. This process helps sequester carbon in the deep ocean.

This vertical movement also serves as a link in aquatic food webs, connecting the productive surface ecosystem with deep-sea communities. The daily influx of migrating organisms provides a dependable food source for deeper-dwelling predators. This transports energy from the sunlit zone to the dark, food-scarce depths. Without this connection, many deep-sea ecosystems would not be able to support the life that they do.