Diel vertical migration
Diel vertical migration (DVM), 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 the greatest migration in the world in terms of biomass. This form of migration is not restricted to any one taxa as examples are known from crustaceans (copepods), molluscs (squid), and ray-finned fishes (trout). There are various stimuli responsible for this phenomenon, the most prominent being response to changes in light intensity, though evidence suggests that biological clocks are an underlying stimulus as well. There are a number of potential reasons to explain the phenomenon in nature, though it is most typically to access food and avoid predators. While this mass migration is typically nocturnal, with the animals ascending from the depths at nightfall and descending at sunrise, novel types of vertical migration can occur in response to the different cues and stimuli that trigger migration. Some novel events include the absence of DVM during midnight sun in Arctic regions and the sudden onset of migration due to a solar eclipse.
During World War II the U.S. Navy was taking sonar readings of the ocean when they discovered the deep scattering layer (DSL). While performing sound propagation experiments the University of California's Division of War Research (UCDWR), consistently had results of the echo-sounder that showed a distinct reverberation that they attributed to mid-water layer scattering agents, there was speculation these readings may be attributed to enemy submarines. By collaborating with biologists from Scripps Institution and the UCDWR, they 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 (e.g., impacts on the foraging behavior of pinnipeds).
Types of vertical migration
- 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 epipelegic zone and deeper mesopelagic zone of the ocean or hypolimnion zone of lakes.: Furthermore, there are three recognized types of diel vertical migration. This includes nocturnal vertical migration, the most common form where organisms ascend to the surface around dusk remaining at the surface for the night then migrating to depth again around dawn. The second form is reverse migration, which 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. The third form is twilight diel vertical migration that 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.
- 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 occur 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 symbiotes.
- 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 associated risk of visual predators, like fish, as being larger makes them more noticeable.
Vertical migration stimuli
There are two different factors that are known to play a role in vertical migration, endogenous and exogenous. Endogenous factors originate from the organism itself; sex, age, biological rhythms, etc. Exogenous factors are environmental factors acting on the organism such as light, gravity, oxygen, temperature, predator-prey interactions, etc.
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. Thus suggestions that some type of internal response was causing the migration.
Clock gene expression
Many organisms including the copepod C. finmarchicus, has genetic material devoted to maintaining its biological clock. The expression of these gene varies temporally with the expression significantly increasing following dawn and dusk at times of greatest vertical migration seen in this species. These finding may indicate they work as a molecular stimulus for vertical migration.
The relative body size of an organism has been found to effect DVM. Bull trout express daily and seasonal vertical migrations with smaller individuals always staying at a deeper layer than the larger individuals. This is most likely due to a predation risk but is dependent on the individuals own size such that smaller animals may be more inclined to remain at depth.
Light is the most common and critical cue for vertical migration. Organisms want to find an optimum light intensity (isolume). Whether it is no light or a large amount of light, an organism will travel to where it is most comfortable. Studies have shown that during a full moon organisms will not migrate up as far or during an eclipse they will start to migrate.
Organisms will migrate to a water depth with suitable temperatures that best suit the organisms needs, for example some fish species migrate to warmer surface waters in order to aid digestion. Temperature changes can influence swimming behavior of some copepods. In the presence of a strong thermocline some zooplankton may be inclined to pass through it, and migrate to the surface waters, though this can be very variable even in a single species. The marine copepod, Calanus finmarchicus, will migrate through gradients with temperature differences of 6 °C over George's Bank; whereas, in the North Sea they are observed to remain below the gradient.
Changes in salinity may promote organism to seek out more suitable waters if they happen to be stenohaline or unequipped to handle regulating their osmotic pressure. Areas that are impacted by tidal cycles accompanied by salinity changes, estuaries for example, may see vertical migration in some species of zooplankton. In areas such as the Arctic, melting ice causes a layer of freshwater which organisms cannot cross.
Pressure changes have been found to produce differential responses that result in vertical migration. many zooplankton will react to increased pressure with positive phototaxis, a negative geotaxis, and/or a kinetic response that results in ascending in the water column. Likewise, when there is a decrease in pressure, the zoo plankton respond by passively sinking or active downward swimming to descend in the water column.
A predator might release a chemical cue which could cause its prey to vertically migrate away. This may stimulate the prey to vertically migrate to avoid said predator. The introduction of a potential predator species, like a fish, to the habitat of diel vertical migrating zooplankton has been shown to influence the distribution patterns seen in their migration. For example, a study used Daphnia and a fish that was too small to prey of them (Lebistus reticulatus), found that with the introduction of the fish tot the system the Daphnia remained below the thermocline, where the fish was not present. This demonstrates the effects of kairomones on Daphnia DVM.
Some organisms have been found to move with the tidal cycle. A study looked at the abundance of a species of small shrimp, Acetes sibogae, and found that they tended to move further higher in the water column and in higher numbers during flood tides than during ebb tides experiences at the mouth of an estuary. It is possible that varying factors with the tides may be the true trigger for the migration rather than the movement of the water itself, like the salinity or minute pressure changes.
Reasons for vertical migration
There are many hypotheses as to why organisms would vertically migrate, and several may be valid at any given time.
- Predator avoidance
- Light-dependent predation by fish is a common pressure that causes DVM behavior in zooplankton. A given body of water may be viewed as a risk gradient whereby the surface layers are riskier to reside in during the day than deep water, and as such promotes varied longevity among zooplankton that settle at different daytime depths. Indeed, in many instances it is advantageous for zooplankton to migrate to deep waters during the day to avoid predation and come up to the surface at night to feed.
- Metabolic advantages
- By feeding in the warm surface waters at night and residing in the cooler deep waters during the day they can conserve energy. Alternatively, organisms feeding on the bottom in cold water during the day may migrate to surface waters at night in order to digest their meal at warmer temperatures.
- Dispersal and transport
- Organisms can use deep and shallow currents to find food patches or to maintain a geographical location.
- Avoid UV damage
- The sunlight can penetrate into the water column. If an organism, especially something small like a microbe, is too close to the surface the UV can damage them. So they would want to avoid getting too close to the surface, especially during daylight.
A recent theory of DVM, termed the Transparency Regulator Hypothesis, argues that water transparency is the ultimate variable that determines the exogenous factor (or combination of factors) that causes DVM behavior in a given environment. In less transparent waters, where fish are present and more food is available, fish tend to be the main driver of DVM. In more transparent bodies of water, where fish are less numerous and food quality improves in deeper waters, UV light can travel farther, thus functioning as the main driver of DVM in such cases.
Due to the particular types of stimuli and cues used to initiate vertical migration, there can be sudden anomalies that change the pattern drastically.
For example, the occurrence of midnight sun in the Arctic induces changes to planktonic life that would normally perform DVM with a 24-hour night and day cycle. In the summers of the Arctic the Earth's north pole is directed toward the sun creating longer days and at the high latitude continuous day light for more than 24-hours. Species of foraminifera found in the ocean cease their DVM pattern, and rather remain at the surface in favor of feeding on the phytoplankton, for example Neogloboquadrina pachyderma, and for those species that contain symbionts, like Turborotalita quinqueloba, remain in sunlight to aid photosynthesis.
There is also evidence of changes to vertical migration patterns during solar eclipse events. In the moments that the sun is obscured during normal day light hours, there is a sudden dramatic decrease in light intensity. The decreased light intensity, replicates the typical lighting experienced at night time that stimulate the planktonic organisms to migrate. During an eclipse, some copepod species distribution is concentrated near the surface, for example Calanus finmarchicus displays a classic diurnal migration pattern but on a much shorter time scale during an eclipse.
Importance for the biological pump
The biological pump is the conversion of CO2 and inorganic nutrients by plant photosynthesis into particulate organic matter in the euphotic zone and transference to the deeper ocean. This is a major process in the ocean and without vertical migration it wouldn’t be nearly as efficient. The deep ocean gets most of its nutrients from the higher water column when they sink down in the form of marine snow. This is made up of dead or dying animals and microbes, fecal matter, sand and other inorganic material.
Organisms migrate up to feed at night so when they migrate back to depth during the day they defecate large sinking fecal pellets. Whilst some larger fecal pellets can sink quite fast, the speed that organisms move back to depth is still faster. At night organisms are in the top 100 metres of the water column, but during the day they move down to between 800–1000 meters. If organisms were to defecate at the surface it would take the fecal pellets days to reach the depth that they reach in a matter of hours. Therefore, by releasing fecal pellets at depth they have almost 1000 metres less to travel to get to the deep ocean. This is something known as active transport. The organisms are playing a more active role in moving organic matter down to depths. Because a large majority of the deep sea, especially marine microbes, depends on nutrients falling down, the quicker they can reach the ocean floor the better.
Zooplankton and salps play a large role in the active transport of fecal pellets. 15–50% of zooplankton biomass is estimated to migrate, accounting for the transport of 5–45% of particulate organic nitrogen to depth. Salps are large gelatinous plankton that can vertically migrate 800 meters and eat large amounts of food at the surface. They have a very long gut retention time, so fecal pellets usually are released at maximum depth. Salps are also known for having some of the largest fecal pellets. Because of this they have a very fast sinking rate, small detritus particles are known to aggregate on them. This makes them sink that much faster. So while currently there is still much research being done on why organisms vertically migrate, it is clear that vertical migration plays a large role in the active transport of dissolved organic matter to depth.
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