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Mesopelagic Zone
The mesopelagic (Greek μέσον, middle) (also known as the middle pelagic or twilight zone) is the part of the pelagic zone that begins at the depth where 1% of incident light at the water surface. It lies between the photic epipelagic above and the aphotic bathypelagic below, where there is no light at all. The depth of this zone is approximately 200 to 1000 meters (~660 to 3300 feet) below the ocean surface. It hosts a diverse biological community that includes bristlemouths, blobfish, bioluminescent jellyfish, giant squid, and a myriad of other unique organisms adapted to live in a low-light environment. It has long captivated the imagination of scientist, artists and writers; deep sea creatures are prominent in popular culture, particularly as horror movie villains.

Physical Conditions
The mesopelagic zone includes the region of sharp changes in temperature, salinity and density called the thermocline, halocline, and pycnocline. The temperature varies from over 20 °C (68 °F) at the top to around 4 °C (39 °F) at the boundary with the bathyal zone. The variation in salinity is smaller typically between 34.5 and 35 psu. The density range is 1023 to 1027 g/kg of seawater. These changes in temperature, salinity, and density induce stratification which create ocean layers that affect gradients and mixing of nutrients and dissolved gasses. The Sound Fixing and Ranging channel (SOFAR) is located at the base of the mesopelagic zone at about 600-1200m. Sound travels the slowest in this channel due to the salinity and temperature variations. It is a wave-guided zone where sound waves reflect within the layer and propagate long distances. Mode water is a water mass that is typically defined by its vertically mixed properties. It often forms as deep mixed layers at the depth of the thermocline. The mode water in the mesopelagic has residency time s on decadal or century scales. The longer overturning times contrast with the daily and shorter scales that a variety of animals move vertically through the zone and sinking of various debris.

Carbon
The mesopelagic zone plays a key role in the ocean's biological pump, which contributes to the oceanic carbon cycle. In the biological pump, organic carbon is produced in the surface euphotic zone where light promotes photosynthesis, and a fraction of this production is exported out of the surface mixed layer and into the mesopelagic zone. One pathway for carbon export from the euphotic layer is through sinking of particles, which can be accelerated through repackaging of organic matter in zooplankton fecal pellets, ballasted particles, and aggregates.

In the mesopelagic zone, the biological pump is key to carbon cycling, as this zone is largely dominated by remineralization of particulate organic carbon (POC). When a fraction of POC is exported from the euphotic zone, an estimated 90% of that POC is respired in the mesopelagic zone. This is due to the microbial organisms that respire organic matter and remineralize the nutrients, while mesopelagic fish also package organic matter into quick-sinking parcels for deeper export.

Another key process occurring in this zone is the diel vertical migration of certain species, which move between the euphotic zone and mesopelagic zone and actively transport particulate organic matter to the deep. In one study in the Equatorial Pacific, myctophids in the mesopelagic zone were estimated to actively transport 15-28% of the passive POC sinking to the deep, while a study near the Canary Islands estimated 53% of vertical carbon flux was due to active transport from a combination of zooplankton and micronekton. When primary productivity is high, the contribution of active transport by vertical migration has been estimated to be comparable to sinking particle export.

Particle Packaging and sinking
Mean particle sinking rates are 10 to 100 m/day. Sinking rates have been measured in the project VERTIGO (Vertical Transport in the Global Ocean) using neutrally buoyant sediment traps. The variability in sinking rates is due to differences in ballast, water temperature, food web structure and the types of phyto- and zooplankton in different areas of the ocean. If the material sinks faster, then it gets respired less by bacteria, transporting more carbon from the surface layer to the deep ocean. Larger fecal pellets sink faster due to gravity. More viscous waters could slow the sinking rate of particles.

Oxygen
Dissolved oxygen is a requirement for aerobic respiration, and while the surface ocean is usually oxygen-rich due to atmospheric gas exchange, the mesopelagic zone is not in contact with the atmosphere and is controlled by other processes. Organic matter is exported to the mesopelagic zone from the overlying euphotic layer, while the minimal light in the mesopelagic zone limits photosynthesis. This results in oxygen consumption through respiration of most of the sinking organic matter, depleting oxygen and often creating an oxygen minimum zone (OMZ). The mesopelagic OMZ is particularly severe in the eastern tropical Pacific Ocean and tropical Indian Ocean due to poor ventilation. Oxygen concentrations are occasionally depleted to suboxic concentrations due to remineralization, which can make aerobic respiration difficult for organisms. In these anoxic regions, chemosynthesis is more likely to occur in which CO2 and reduced compounds such as sulfide or ammonia are taken up to form organic carbon, contributing to the carbon reservoir in the mesopelagic. This pathway of carbon fixation has been estimated to be comparable in rate to the contribution by heterotrophic production.

Nitrogen
The mesopelagic zone, an area of significant respiration and remineralization of particles, is generally nutrient-rich. This is in contrast to the overlying euphotic zone, which is usually nutrient-limited. Areas of low oxygen such as OMZ’s are a key area of denitrification by prokaryotes, where nitrate is converted into nitrogen gas, a loss to the ocean reservoir of nitrogen. At the suboxic interface that occurs at the edge of the OMZ, nitrite and ammonium can be coupled to produce nitrogen gas through anammox, removing nitrogen from the biologically available pool.

Biology
Although some light penetrates the mesopelagic zone, it is insufficient for photosynthesis. This is a very efficient ecosystem with many organisms recycling the organic matter descending from the epipelagic zone. The general types of life forms found are daytime-visiting herbivores, detritivores feeding on dead organisms and fecal pellets, and carnivores feeding on those detritivores. Phytoplankton do not inhabit the mesopelagic layer, due to lack of light.[3]  The biological community of the mesopelagic zone has adapted to a low-light, low-nutrient environment.

Virus and microbial ecology
Very little is known about the microbial community of the mesopelagic zone because it is a difficult area to study, but recent work has used DNA from seawater samples. Recent research highlights the importance of viruses and microbes role in recycling organic matter from the surface ocean, known as the biological pump.

These many microbes can get their energy from different pathways. Some are autotrophs, heterotrophs, and a 2006 study even discovered chemoautotrophs. This Archaea crenarchaeon Candidatus can oxidize ammonia as their food source without oxygen. They could be a major impact on the nitrogen and carbon cycles. One study estimates these nitrite-oxidizing bacteria, which are only 5% of the microbial population, can annually capture 1.1 Gt of carbon.

Overall, biomass and diversity typically decline exponentially with depth in the mesopelagic zone. The community composition changes at different depths in the mesopelagic as different organisms are evolved for different light conditions. Microbial biomass in the mesopelagic is greater at higher latitudes, decreasing towards the tropics, likely linked to the productivity levels in the surface waters. Viruses are very abundant in the mesopelagic, around 1010 - 1012 every cubic meter and are fairly uniform throughout the mesopelagic zone.

Zooplankton ecology
The mesopelagic zone hosts a diverse zooplankton community. Many inhabitants of the mesopelagic zone move between the epipelagic zone during the night, and retreat to the mesopelagic zone during the day, a process known as diel vertical migration. These migrators can avoid predation during the day and feed at night, while their predators migrate up at night to follow the prey. This is such a massive migration that sonar operators in World War II would regularly misinterpret the signal returned by this thick layer of plankton as a false sea floor.

Gelatinous organisms are thought to play an important role in the ecology of the mesopelagic and are common predators. One study found that the helmet jellyfish Periphylla periphylla exhibit social behavior and can find each other at depth and form groups. Scientists speculate this could be a feeding strategy to allow a group to hunt together.

Mesopelagic zooplankton have unique adaptations for the low light. Bioluminescence is very common, which is used for a variety of functions like  communication, prey attraction and mating. Many jellyfish are bioluminescent. Other organisms, like shrimp, have highly developed eyes to pick up the limited available light. Light organs, common in krill and shrimp, are another adaptation. Some octopus and krill even have tubular eyes that look upwards. Most life processes, like growth rates and reproductive rates, are slower in the mesopelagic. Metabolic activity has been shown to decrease with depth and with temperature in colder-water environments. For example, the mesopelagic shrimp-like mysid, Gnathophausia ingens, lives for 6.4 to 8 years, while benthic shrimp only live for 2 years.

Fish ecology
Mesopelagic fish have a global distribution, except for the Arctic Ocean. The mesopelagic is home to a significant portion of the world's total fish biomass; one study estimated mesopelagic fish could be 95% of the total fish biomass. Another estimate puts mesopelagic fish biomass at 1 billion tons. It could be the largest fishery in the world and there is active development for this zone to become a commercial fishery. There are thirty families of mesopelagic fish. One dominant fish in the mesopelagic zone are lanternfish (Myctophidae.) They have prominent photophores along their ventral side. The Myctophid group includes 245 species distributed among 33 different genera. The Gonostomatidae, or bristlemouth, are also common mesopelagic fish. The bristlemouth could be the Earth's most abundant vertebrate, numbering in the hundreds of trillions to quadrillions.

Mesopelagic fish are difficult to study. Many of these fish have swim bladders to help them control their buoyancy, which makes them hard to sample because those gas-filled chambers typically burst as the fish come up in nets and the fish die. Scientists in California have developed a submersible chamber that can keep fish alive on the way up to the surface in a controlled atmosphere. Another way to estimate mesopelagic fish numbers is with echosounders and finding the 'deep scattering layer' through the backscatter received from these acoustic sounders. Some areas have shown a recent decline in abundance of mesopelagic fish, like in Southern California over a long-term study dating back to the 1970s. Cold water species were especially vulnerable to decline. Mesopelagic fish are adapted to a low-light environment. Many fish are black or red, because these colors appear dark due to the limited light penetration at depth. Many fish also have rows of photophores, which are small light-producing organs, mirrors on their sides angled to reflect the surrounding ocean and protect the fish, or countershading, with light colors on ventral side and dark colors on the dorsal side.

Food is often limited and patchy, leading to dietary adaptions. Many fish have sensitive eyes, huge jaws and small bodies, reducing the effort put into building muscles. Adaptations include jaws that can unhinge, elastic throats, and massive, long teeth. Other predators develop bioluminescent lures, like the angler fish, that can attract prey. Many fish also respond to chemical cues instead of light cues.

Marine Debris
Marine debris, specifically in the plastic form, have been found in every ocean basin and have a wide range of impacts on the marine world One of the most critical issues is ingestion of plastic debris, specifically microplastics. Mesopelagic species migrate to the surface waters to feast on their main prey species, Zooplankton, who are mixed with microplastics in the surface waters. Additionally, research has shown that even Zooplankton are consuming the microplastics themselves. These fish play a key role in energy dynamics, meaning they provide food to a number of predators including birds, fish and marine mammals. The concentration of these plastics has the potential to increase, meaning more economically important species could be contaminated as well. Concentration of plastic debris in mesopelagic populations can vary depending on geographical location and the concentration of marine debris located there. In 2018, approximately 73% of over 200 fish sampled in the North Atlantic had consumed plastic.

Bioaccumulation
The process of bioaccumulation (a buildup of a certain substance in the adipose tissue) and biomagnification (the process in which the concentration of the substance grows higher as you rise through the food chain) are growing issues in the mesopelagic zone. Mercury in fish, which can be traced back to a combination of anthropological factors (such as coal mining) in addition to natural factors. Mercury is a particularly important bioaccumulation contaminant, as it’s concentration in the mesopelagic zone is increasing faster than in surface waters. Inorganic mercury occurs in anthropogenic atmospheric emissions as it’s gaseous elemental form, which can be oxidized and then deposited in the ocean. Once there, the oxidized form can be converted to methylmercury, which is its organic form. Research suggests that current levels anthropogenic emissions will not equilibrate between the atmosphere and ocean for a period of decades to centuries, which means we can expect current mercury concentrations in the ocean to keep rising. Mercury is a potent neurotoxin, and poses health risks to not only the mesopelagic species that consume it. Many of the mesopelagic species, such as myctophids, make their diurnal migration to the surface waters where they are consumed by pelagic fish, birds and mammals.

Fishing
Historically, there have been few examples of efforts to commercialize the mesopelagic zone, due to low economic value, technical feasibility and environmental impacts. While the biomass may be abundant, fish species here generally smaller in size and slower to reproduce. Fishing with large trawl nets poses threats to a high percentage of bycatch as well as poses potential impacts to the carbon cycling process. Additionally, getting to the mesopelagic requires fairly long journeys offshore. In 1977, a Soviet fishery opened but closed less than 20 years later due to low commercial profits, while a South African purse seine fishery closed in the mid-1980's due to processing difficulties from the high oil content of fish. As the biomass in the mesopelagic is so abundant, there has been an increase in interest to whether or not these populations could be of economic use in areas other than direct human consumption. For example, it has been suggested that the high abundance of fish in this zone could potentially satisfy a demand for fishmeal and nutraceuticals.

With a growing population, the demand for fishmeal in support of a growing aquaculture industry is high. There is a potential for economically viable harvest, for example, 5 billion tons of mesopelagic biomass could result in the production of circa 1.25 billion tons of food for human consumption. Additionally, the demand for nutraceuticals is also rapidly growing, stemming from the consumption of Omega-3 Fatty Acids in addition to the aquaculture industry requiring a specific marine oil for feed material. Lanternfish are of much interest to the aquaculture market, as they are especially high in fatty acids.

Climate Change
The mesopelagic region plays an important role in the Global Carbon Cycle, as it is the area where most of the organic matter that is travelling down the water column is respired. Mesopelagic species acquire carbon during their migration to feed in surface waters, and transport that carbon into the deep sea when they die. It is estimated that the mesopelagic cycles between 5 and 12 billion tons of carbon dioxide from the atmosphere per year, and until recently, this estimate was not included in many climate models. It is difficult to quantify the effects of climate change on the mesopelagic zone as a whole, as climate change does not have uniform impacts geographically. Research suggests that in warming waters, as long as there are adequate nutrients and food for fish, then mesopelagic biomass could actually be higher due to increases in trophic efficiency and temperature driven metabolism. However, because warming will not be uniform throughout the global mesopelagic zone, it is predicted that some areas may actually decrease in fish production, while others increase. Also, with warming waters there is a higher likelihood of increased stratification. This increase reduces the input of nutrients to the euphotic zone through an increase in nutrient limitation, and can lead to decreases in both net primary production and sinking particulate matter. Additional research suggests shifts in the geographical range of many species could also occur with warming, with many of them shifting away from the equator. The combination of these factors could potentially mean that as global ocean basins continue to warm, there could be areas in the mesopelagic that increase in biodiversity and species richness, especially moving farther from the equator.

Research and Exploration
There is a dearth of knowledge about the mesopelagic zone so researchers have begun to develop new technology to explore and sample this area. The Woods Hole Oceanographic Institution (WHOI), NASA, and the Norwegian Institute of Marine Research are all working on projects to gain a better understanding of this zone in the ocean and its influence on the global carbon cycle. Traditional sampling nets have proved to be inadequate due to scaring off creatures from the pressure wave formed by the towed net and from the light produced by the bioluminescent species caught in the net. Mesopelagic activity was first investigated by use of sonar. However, there are many challenges with acoustic survey methods and previous research has estimated errors in measured amounts of biomass of up to three orders of magnitude. This is due to inaccurate incorporation of depth, species size distribution, and acoustic properties of the species. Norway’s Institute of Marine Research has launched a research vessel named Dr. Fridtjof Nansen to investigate mesopelagic activity using sonar with their focus being on the sustainability of fishing operations. To overcome the challenges faced with acoustic sampling, WHOI is developing robots (Deep-See, Mesobot, and Snowclops) that are capable of studying this zone more precisely.

Discovery and Detection
The deep scattering layer often characterizes the mesopelagic due to the high amount of biomass that exists in the region. Acoustic sound sent into the ocean bounces off particles and organisms in the water column and return a strong signal. The region was initially discovered by American researchers during World War II in 1942 during anti-submarine research with sonar. Sonar at the time could not penetrate below this depth due to the large number of creatures obstructing sound waves. It is uncommon to detect deep scattering layers below 1000m. Until recently, sonar has been the predominate method for studying the mesopelagic.

The Malaspina Circumnavigation Expedition was a spanish led scientific quest to gain a better understanding of the state of the ocean in 2011 and the diversity in the deep oceans. The data collected, particularly through sonar observations showed that the biomass estimation in the mesopelagic was lower than previously thought.

Deep-See
WHOI is currently working on a project to characterize and document the mesopelagic ecosystem. They have developed a device named Deep-See weighing approximately 700 kg, which is designed to be towed behind a research vessel. The Deep-See is capable of reaching depths up to 2000 m and can estimate the amount of biomass and biodiversity in this mesopelagic ecosystem. Deep-See is equipped with cameras, sonars, sensors, water sample collection devices, and a real-time data transmission system.

Mesobot
WHOI is collaborating with the Monterey Bay Aquarium Research Institute (MBARI), Stanford University, and the University of Texas Rio Grande Valley to develop a small autonomous robot, Mesobot, weighing approximately 75 kg. Mesobot is equipped with high-definition cameras to track and record mesopelagic species on their daily migration over extended periods of time. The robot's thrusters were designed so that they do not disturb the life in the mesopelagic that it is observing. Traditional sample collection devices fail to preserve organisms captured in the mesopelagic due to the large pressure change associated with surfacing. The mesobot also has a unique sampling mechanism that is capable of keeping the organisms alive during their ascent. The first sea trial of this device is expected to be in 2019.

Snowclops
Another mesopelagic robot developed by WHOI is the Snowclops. This device descends down the water column and measures the amount of marine snow at various depths. These tiny particles are a food source for other organisms so it is important to monitor the different levels of marine snow to characterize the carbon exchange between the surface ocean and the mesopelagic.

SPLAT cam
The Harbor Branch Oceanographic Institute has developed the Spatial PLankton Analysis Technique (SPLAT) to identify and map distribution patterns of bioluminescent plankton. The various bioluminescent species produce a unique flash that allows the the SPLAT to distinguish each specie's flash characteristic and then map their 3-dimensional distribution patterns. Its intended use was not for investigating the mesopelagic zone, although it is capable of tracking movement patterns of bioluminescent species during their vertical migrations. It would be interesting to apply this mapping technique in the mesopelagic to obtain more information about the diurnal vertical migrations that occur in this zone of the ocean.