Mesopelagic zone

The mesopelagic zone (Greek μέσον, middle), also known as the middle pelagic or twilight zone, is the part of the pelagic zone that lies between the photic epipelagic and the aphotic bathypelagic zones. It is defined by light, and begins at the depth where only 1% of incident light reaches and ends where there is no light; depth ranges from approximately below the ocean surface.

The mesopelagic zone occupies about 60% of the planet's surface and about 20% of the ocean's volume, comprising a large portion of the total biosphere.

Physical conditions

thumb|200px|left|Pelagic zones.

The mesopelagic zone includes regions of sharp changes in temperature, salinity and density called the thermocline, halocline, and pycnocline respectively. It is a wave-guided zone where sound waves refract within the layer and propagate long distances. The channel got its name during World War II when the US Navy proposed using it as a life saving tool. Shipwreck survivors could drop a small explosive timed to explode in the SOFAR channel and then listening stations could determine the position of the life raft. During the 1950s, the US Navy tried to use this zone to detect Soviet submarines by creating an array of hydrophones called the Sound Surveillance System (SOSUS.) 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. 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. This pathway of carbon fixation has been estimated to be comparable in rate to the contribution by heterotrophic production in this ocean realm.

Nitrogen

Nitrogen is a limiting nutrient in biological systems which influence productivity in marine environments.The mesopelagic zone, an area of significant respiration and remineralization of organic particles, is generally nutrient-rich. This is in contrast to the overlying euphotic zone, which is often nutrient-limited. The difference in nitrogen availability between the epipelagic zone and the mesopelagic zone is attributed to the biogeochemical processes that mediate sinking particulate organic matter.Areas of low oxygen such as OMZ's are a key area of denitrification by prokaryotes, a heterotrophic pathways in which nitrate is converted into nitrogen gas, resulting in a loss to the ocean reservoir of reactive nitrogen. Evidence for increased nitrogen content in the epipelagic zone is seen in an increase of amino-acids found in dissolved organic matter. Once DOP/POP makes it to the mesopelagic, bacteria remineralize these phosphates in order to acquire nutrients, releasing dissolved inorganic phosphate (DIP) back into the water column. DIP is then brought back to the surface waters through vertical mixing and diel vertical migration, which replenishes euphotic phosphates. Phosphatase is an important enzyme in the process of converting DOP/POP into DIP, and is 37 times more active in the mesopelagic zone (800 meters) than at the surface. During the remineralization process, a proportion of the phosphate becomes sequestered in oceanic sediments along with oxygen (in aerobic conditions), and remains largely inert. This phosphate sediment can, however, release nutrients back into the water column under anoxic conditions.

Sulfur

Similar to phosphorus, sulfur also participates in a cycle of renewal with surface waters. Organic sulfate sinks to the mesopelagic, then becomes remineralized into inorganic sulfate, which enters the euphotic layer once again. This cycle, however, is only one component of a larger process in sulfur cycling, known as cryptic sulfur cycling. Cryptic sulfur cycling occurs in OMZs, and is characterized by the tight coupling of sulfate reduction and subsequent oxidation. Much of this sulfur is supplied to the deep ocean by hydrothermal venting, which releases sulfide into the surrounding water. Additionally, sulfide generated in the mesopelagic contributes to the creation of pyrite, which is formed by its combination with iron II, typically in anoxic environments.

Organic Matter

Cycling

A driver of the ocean biological pump is the sinking, decomposition, and cycling of dissolved and particulate organic matter that creates a flux of nutrients. Organic matter consists of fixed carbon, oxygen, and nitrogen originating from plant, animal, and microbe biomass. The molecular composition of particulate and dissolved matter in the mesopelagic zone is heavily dependent on the biogeochemical environment (element limitation, microbiome, abiotic elements) in the euphotic zone. However, as the ocean becomes deeper, the flux of organic matter loses intensity or magnitude. Sinking rates have been measured in the project VERTIGO (Vertical Transport in the Global Ocean) using settling velocity 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. resulting in very little organic carbon making it to deeper ocean waters. 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.

Many organisms in the mesopelagic zone move into the epipelagic zone at night, and retreat to the mesopelagic zone during the day, known as diel vertical migration.

Estimates of the global biomass of mesopelagic fishes range from 1 gigatonne (Gt) based on net tows to 7–10 Gt based on measurements using active acoustics.

Virus and microbial ecology

Little is known about the microbial community of the mesopelagic zone as a result of the difficulty involved in studying this region of the ocean. Studies using DNA from seawater samples emphasized the importance of viruses and microbes in recycling organic matter from the surface ocean, known as the microbial loop. These microbes can get their energy from different metabolic pathways. Some are autotrophs, heterotrophs, and a 2006 study discovered chemoautotrophs.

Microbial biomass and diversity typically decline exponentially with depth in the mesopelagic zone, tracking the general decline of food from above. Though previously thought to be passive predators drifting through the water column, jellyfish may potentially be more active predators. One study found that helmet jellyfish, Periphylla periphylla, exhibit social behavior and can find each other at depth and form groups. For example, the mesopelagic shrimp-like mysid, Gnathophausia ingens, lives for 6.4 to 8 years, while similar benthic shrimp only live for 2 years. A 1980 study puts the mesopelagic fish biomass at about one billion tons. Later, a 2008 study estimated the world marine fish biomass at between 0.8 and 2 billion tons. A more recent study concluded mesopelagic fish could have a biomass amounting to 10 billion tons, equivalent to about 100 times the annual catch of traditional fisheries of about 100 million metric tons. However, there is a lot of uncertainty in this biomass estimate. This ocean realm could contain the largest fishery in the world and there is active development for this zone to become a commercial fishery. One dominant fish in the mesopelagic zone are lanternfish (Myctophidae), which include 245 species distributed among 33 different genera.

Mesopelagic fish are difficult to study due to their unique anatomy. 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 made progress on mesopelagic fish sampling by developing a submersible chamber that can keep fish alive on the way up to the surface under a controlled atmosphere and pressure. Cold water species are especially vulnerable to decline. Other notable mammals include beaked whales, elephant seals, Risso's dolphins, fin whales, and humpback whales. Mesopelagic cetaceans range across nearly all oceans, with the sperm whale alone ranging from Alaskan to Antarctic waters. No estimate currently exists for mammalian biomass in the mesopelagic due to the difficulty involved in distinguishing large fish from cetaceans when using acoustic sampling.

The most prevalent mammalian species to inhabit the mesopelagic is likely the sperm whale, which is estimated to consume as many cephalopods as all human fisheries globally each year. Sperm whales are capable of diving to the bottom of the mesopelagic and deeper, spending anywhere from five minutes to almost an hour hunting in this range. In addition to sperm whales, other whales, such as the common rorqual, are known to target mesopelagic nekton as a main source of food.

Human impacts

Pollution

Marine debris

thumb|Plastic pellets are a common form of 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. Many mesopelagic fish species migrate to the surface waters to feast on their main prey species, zooplankton and phytoplankton, which are mixed with microplastics in the surface waters. Additionally, research has shown that even zooplankton are consuming the microplastics themselves. Mesopelagic fish play a key role in energy dynamics, meaning they provide food to a number of predators including birds, larger fish and marine mammals. The concentration of these plastics has the potential to increase, so more economically important species could become contaminated as well. Concentration of plastic debris in mesopelagic populations can vary depending on geographic location and the concentration of marine debris located there. In 2018, approximately 73% of approximately 200 fish sampled in the North Atlantic had consumed plastic.

Bioaccumulation

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 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 because its concentration in the mesopelagic zone is increasing faster than in surface waters. Inorganic mercury occurs in anthropogenic atmospheric emissions in its gaseous elemental form, which then oxidizes and can be deposited in the ocean. Once there, the oxidized form can be converted to methylmercury, which is its organic form. which means we can expect current mercury concentrations in the ocean to keep rising. Mercury is a potent neurotoxin, and poses health risks to the whole food web, beyond the mesopelagic species that consume it. Many of the mesopelagic species, such as myctophids, that make their diel vertical migration to the surface waters, can transfer the neurotoxin when they are consumed by pelagic fish, birds and mammals.

Fishing

thumb|Fishmeal Powder.

Historically, there have been few examples of efforts to commercialize the mesopelagic zone due to low economic value, technical feasibility and environmental impacts. 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-1980s due to processing difficulties from the high oil content of fish.

As the biomass in the mesopelagic is so abundant, there has been an increased interest to determine whether these populations could be of economic use in sectors 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.

Climate Change

The mesopelagic region plays an important role in the global carbon cycle, as it is the area where most of the surface organic matter is respired. 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 increase due to higher trophic efficiency and increased temperature-driven metabolism. However, because ocean warming will not be uniform throughout the global mesopelagic zone, it is predicted that some areas may actually decrease in fish biomass, while others increase. 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, while declines in other areas, especially moving farther from the equator. 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 pelagic ecosystem. They have developed a device named Deep-See weighing approximately 700 kg, which is designed to be towed behind a research vessel.

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. 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.