Diel vertical migration
Diel vertical migration, also known as diurnal vertical migration, is a pattern of movement used by some organisms, such as Copepods, living in the ocean and in lakes. The migration occurs when organisms move up to the epipelagic zone at night and return to the mesopelagic zone of the oceans or to the hypolimnion zone of lakes during the day. The word diel comes from the Latin dies day, and means a 24-hour period. It is referred to as the greatest migration in the world in terms of biomass.
Discovery
During World War II the U.S. Navy was taking sonar readings of the ocean when they discovered the deep scattering layer (DSL). The DSL was caused by large groupings of organisms that scattered the sonar to create a false or second bottom. The false bottom was shallower during the night and deeper during the day; this was the first recording of diel vertical migration.
Once scientists started to do more research on what was causing the DSL, it was discovered that a large range of organisms were vertically migrating. Most types of plankton and some types of nekton have exhibited some type of vertical migration, although it is not always diel. These migrations may have substantial effects on mesopredators and apex predators by modulating the concentration and accessibility of their prey (i.e. impacts on the foraging behavior of pinnipeds).
