Diurnal vertical migration (DVM) is the vertical movement of zooplankton in the ocean (or lake). They move from the warm surface layers, which provide abundant food, to colder, deeper water with its poor food supply.
Zooplankton remains beneath the photic zone during the day, moving toward the surface after dusk and returning to the depths before dawn.
The photoresponses of these organisms become functionally significant during such migrations, since light is agreed to be the dominant environmental factor controlling DVM.
This strategy is principally thought to be an anti-predator defense (avoiding visually orienting predators). However, recent show that may help optimize metabolic ratio – grazers might gain significant metabolic advantages if they began to feed a couple of hours before sunset.