Diel vertical migration is a daily pattern of movement in which organisms living in oceans and lakes rise towards the surface at night and descend to deeper waters during the day. The term “diel” derives from the Latin for day and refers to a 24-hour cycle.
In terms of biomass, this is the largest synchronous migration on Earth. It occurs across multiple taxonomic groups, including copepods, squid and trout. Most organisms make the ascent at nightfall and return to depth at sunrise, though the timing can shift in response to environmental cues. Changes in light intensity are the primary trigger, but biological clocks also play a role. The pattern can be disrupted by unusual events: it may be absent during the Arctic midnight sun, occur suddenly during a solar eclipse, or vary with cloud cover. The behaviour is thought to offer advantages in both accessing food and avoiding predators. Because it moves such vast quantities of organisms vertically through the water column each day, diel vertical migration plays an important role in deep-sea food webs and the biological sequestration of carbon.
