Diel vertical migration refers to a pattern of movement that some organisms living in the ocean's photic zone undertake each day. The organisms that exhibit this pattern of behaviour range in size from microscopic plankton through to much larger nekton such as fish. Organisms generally undergo diel vertical migration in response to ecological gradients (resources, prey, predators) in the pelagic zone. In the case of autotrophic phytoplankton, while light for photosynthesis declines exponentially from the ocean's surface, the availability of nutrients typically rises down the water column (because of transport by the biological pump). Consequently, some phytoplankton migrate down the water column at night to obtain nutrients, but return to the surface during daylight. By contrast, heterotrophic zooplankton and larger animals do not usually require light for growth (though some mixotrophs possess endosymbionts that do require light). However, in well-lit surface waters (or even the "twilight" conditions of the mesopelagic zone) where their prey are most abundant, they may be vulnerable to larger predators that hunt by sight. As a result, some undertake the opposite pattern of diel vertical migration, travelling to the surface during night to feed, then descending to darker, safer depths during the day. Aside from avoidance of predators, deeper waters are also cooler. During periods when food is relatively scarce (i.e. away from spring blooms), visits to these lower temperature waters slows the metabolism of organisms, allowing them to subsist on a lower dietary intake.
