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 adjective "diel" comes from, and refers to a 24-hour period. The migration occurs when organisms move up to the uppermost layer of the water at night and return to the bottom of the daylight zone of the oceans or to the dense, bottom layer of lakes during the day. DVM is important to the functioning of deep-sea food webs and the biologically-driven sequestration of carbon.
In terms of biomass, DVM is the largest synchronous migration in the world. It is not restricted to any one taxon, as examples are known from crustaceans, molluscs, and ray-finned fishes.
The phenomenon may be advantageous for a number of reasons, most typically to access food and to avoid predators.
It is triggered by various stimuli, the most prominent being changes in light-intensity, though evidence suggests that biological clocks are an underlying stimulus as well. While this mass migration is generally nocturnal, with the animals ascending from the depths at nightfall and descending at sunrise, the timing can alter in response to the different cues and stimuli that trigger it. Some unusual events impact vertical migration: DVM can be absent during the midnight sun in Arctic regions and vertical migration can occur suddenly during a solar eclipse. The phenomenon also demonstrates cloud-driven variations.
The common swift is an exception among birds in that it ascends and descends into high altitudes at dusk and dawn, similar to the vertical migration of aquatic lifeforms.
Discovery
The phenomenon was first documented by French naturalist Georges Cuvier in 1817. He noted that daphnia, a type of plankton, appeared and disappeared according to a diurnal pattern.During World War II the U.S. Navy was taking sonar readings of the ocean when they discovered the deep scattering layer. While performing sound propagation experiments, the University of California's Division of War Research consistently had results of the echo-sounder that showed a distinct reverberation that they attributed to mid-water layer scattering agents. At the time, there was speculation that these readings may be attributed to enemy submarines.
Martin W. Johnson of Scripps Institution of Oceanography proposed a possible explanation. Working with the UCDWR, the Scripps researchers were able to confirm that the observed reverberations from the echo-sounder were in fact related to the diel vertical migration of marine animals. The DSL was caused by large, dense groupings of organisms, like zooplankton, that scattered the sonar to create a false or second bottom.
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.
Types of vertical migration
Diel
This is the most common form of vertical migration. Organisms migrate on a daily basis through different depths in the water column. Migration usually occurs between shallow surface waters of the epipelagic zone and deeper mesopelagic zone of the ocean or hypolimnion zone of lakes.There are three recognized types of diel vertical migration:
Nocturnal vertical migration
In the most common form, nocturnal vertical migration, organisms ascend to the surface around dusk, remaining at the surface for the night, then migrating to depth again around dawn.Reverse migration
Reverse migration occurs with organisms ascending to the surface at sunrise and remaining high in the water column throughout the day until descending with the setting sun.Twilight diel vertical migration
Twilight diel vertical migration involves two separate migrations in a single 24-hour period, with the first ascent at dusk followed by a descent at midnight, often known as the "midnight sink". The second ascent to the surface and descent to the depths occurs at sunrise.Seasonal
Organisms are found at different depths depending on what season it is. Seasonal changes to the environment may influence changes to migration patterns. Normal diel vertical migration occurs in species of foraminifera throughout the year in the polar regions; however, during the midnight sun, no differential light cues exist so they remain at the surface to feed upon the abundant phytoplankton, or to facilitate photosynthesis by their symbionts. This is not true for all species at all times, however. Zooplankton have been observed to resynchronize their migrations with the light of the moon during periods when the sun is not visible, and to stay in deeper waters when the moon is full.Larger seasonally-migrating zooplankton such as overwintering copepods have been shown to transport a substantial amount of carbon to the deep ocean through a process known as the lipid pump. The lipid pump is a process that sequesters carbon out of the surface ocean via the descent of copepods to the deep during autumn. These copepods accumulate these lipids during late summer and autumn before descending to the deep to overwinter in response to reduced primary production and harsh conditions at the surface. Furthermore, they rely on these lipid reserves that are metabolized for energy to survive through winter before ascending back to the surface in the spring, typically at the onset of a spring bloom.
Ontogenetic
Organisms spend different stages of their life cycle at different depths. There are often pronounced differences in migration patterns of adult female copepods, like Eurytemora affinis, which stay at depth with only a small upward movement at night, compared to the rest of its life stages which migrate over 10 meters. In addition, there is a trend seen in other copepods, like Acartia spp. that have an increasing amplitude of their DVM seen with their progressive life stages. This is possibly due to increasing body size of the copepods and the associated risk of visual predators, like fish, as being larger makes them more noticeable.Vertical migration stimuli
There are two different types of factors that are known to play a role in vertical migration, endogenous and exogenous. Endogenous factors originate from the organism itself; sex, age, size, biological rhythms, etc. Exogenous factors are environmental factors acting on the organism such as light, gravity, oxygen, temperature, predator-prey interactions, etc.Endogenous factors
Endogenous rhythm
Biological clocks are an ancient and adaptive sense of time innate to an organism that allows them to anticipate environmental changes and cycles so they are able to physiologically and behaviorally respond to the expected change.Evidence of circadian rhythms controlling DVM, metabolism, and even gene expression have been found in copepod species, Calanus finmarchicus. These copepods were shown to continue to exhibit these daily rhythms of vertical migration in the laboratory setting even in constant darkness, after being captured from an actively migrating wild population.
An experiment was done at the Scripps Institution of Oceanography which kept organisms in column tanks with light/dark cycles. A few days later the light was changed to a constant low light and the organisms still displayed diel vertical migration. This suggests that some type of internal response was causing the migration.