Diel vertical migration

Diel vertical migration is a daily change in the depth occupied by organisms, rather than a horizontal journey. It is widespread among plankton and other small pelagic animals, and is also observed in some fish and gelatinous organisms. In the best-known nocturnal form, migrating animals rise toward upper waters around dusk and descend again around dawn. The depth travelled, timing, and proportion of a population that migrates vary among species, life stages and environments; some organisms remain near the surface or at depth, and reverse patterns also occur.

There is no single explanation for every migration. Many animals feed in productive surface waters at night, when darkness may reduce their visibility to predators that hunt by sight, and retreat to deeper water during daylight. Light is an important cue, but temperature, prey availability, food and predation risk also affect movements. The migration reflects a trade-off: reaching food has benefits, but movement costs energy and places organisms in different thermal and predation conditions. Research modelling plankton communities indicates that interactions among predators and prey can help produce a diversity of migration patterns.

By changing where and when organisms feed, DVM influences encounters between predators and prey and the grazing pressure on plankton. Migrators also transport material across depth boundaries: after feeding near the surface, they may release waste or respire carbon at depth. This active transport complements the sinking of particles and contributes to the biological carbon pump, although the amount varies with location and community. Migration and associated animal movement can also affect the distribution of nutrients and oxygen in the water column.

Scientists observe vertical movements using methods that include acoustic measurements, which can reveal layers of organisms changing depth, and direct sampling. The fine-scale timing and depth of migration are not uniform: species, size, food-web structure, light conditions and local oceanography all matter. Researchers continue to investigate how these factors combine and how migration affects carbon cycling. The magnitude and direction of any climate-related change in DVM are not yet established across regions; NOAA identifies this as an area where further research is needed.