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. It is largely nutrient-rich in comparison to the euphotic zone, making the biogeochemical processes of the mesopelagic particularly important to nutrient cycling in the ocean.
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. It hosts a diverse biological community that includes bristlemouths, blobfish, bioluminescent jellyfish, giant squid, and myriad other unique organisms adapted to live in a low-light environment. It features prominently in art and literature through depictions of deep-sea creatures.
Physical conditions
The mesopelagic zone includes regions of sharp changes in temperature, salinity and density called the thermocline, halocline, and pycnocline respectively. The temperature variations are large; from over at the upper layers to around at the boundary with the bathyal zone. The variation in salinity is smaller, typically between 34.5 and 35 psu. The density ranges from 1023 to 1027 g/L of seawater. These changes in temperature, salinity, and density induce stratification which creates ocean layers. These different water masses affect gradients and mixing of nutrients and dissolved gasses. This makes it a dynamic zone.
The mesopelagic zone has some unique acoustic features. The Sound Fixing and Ranging (SOFAR) channel, where sound travels the slowest due to salinity and temperature variations, is located at the base of the mesopelagic zone at about 600–1,200m. 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). Oceanographers later used this underwater surveillance system to determine the speed and direction of deep ocean currents by dropping SOFAR floats that could be detected with the SOSUS array.
The mesopelagic zone is important for water mass formation, such as mode water. 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 times 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.
Biogeochemistry
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. 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. Organic carbon that is not remineralized is buried into ocean sediments or remain as refractory dissolved organic matter.
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.
Oxygen Dissolved oxygen is a requirement for aerobic respiration, and while the surface ocean is usually oxygen-rich due to atmospheric gas exchange and photosynthesis, the mesopelagic zone is not in direct contact with the atmosphere, due to stratification at the base of the surface mixed layer. Organic matter is exported to the mesopelagic zone from the overlying euphotic layer, while the minimal light in the mesopelagic zone limits photosynthesis. The oxygen consumption due to respiration of most of the sinking organic matter and lack of gas exchange, often creates an oxygen minimum zone (OMZ) in the mesopelagic. The mesopelagic OMZ is particularly severe in the eastern tropical Pacific Ocean and tropical Indian Ocean due to poor ventilation and high rates of organic carbon export to the mesopelagic. Oxygen concentrations in the mesopelagic are occasionally result in suboxic concentrations, making aerobic respiration difficult for organisms. In these anoxic regions, chemosynthesis may occur in which CO2 and reduced compounds such as sulfide or ammonia are taken up to form organic carbon, contributing to the organic carbon reservoir in the mesopelagic. 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. At the suboxic interface that occurs at the edge of the OMZ, nitrite and ammonium can be coupled to produce nitrogen gas through anammox, also removing nitrogen from the biologically available pool. Nitrogen availability is also dependent on the region of the globe. The limitation of nitrogen as a nutrient is typically found in subtropical gyres and is often co-limited with iron. A flux of nitrogen-rich particulate organic matter in deeper parts of the ocean is created from the microbial diagenesis of organic matter that begins in the epipelagic zone. Evidence for increased nitrogen content in the epipelagic zone is seen in an increase of amino-acids found in dissolved organic matter.
Phosphorus Phosphorus is a vital component of nutrient cycling in the mesopelagic. Phytoplankton in the euphotic zone use available phosphate for growth and deposit dissolved organic phosphate (DOP) and particulate organic phosphate (POP) through fecal secretion or dead material. 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. The main limitation to sinking organic matter found in the mesopelagic zone is because of the variability of nitrogen across the globe. In other words, the molecular composition of organic matter that enters the mesopelagic zone varies spatially but is not dependent on the depth of the ocean. However, as the ocean becomes deeper, the flux of organic matter loses intensity or magnitude. This reduction of flux, or attenuation, of organic matter is estimated to be about 70-85% lower. Oxygen in the epipelagic zone is consumed through the removal of dissolved organic matter as well as the remineralization of nitrogen.
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 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. 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 lower friction-surface/mass ratio. More viscous waters could slow the sinking rate of particles. The binding to fecal pellets also creates a physical barrier for microbes slowing the consumption of particulate organic matter. The decreased consumption of particulate matter suggests that dissolved organic matter is removed quicker by microbes than particulate matter is removed. It is also important to note that microbial digestion and respiration adapts as depth of the ocean increases which affects spatial molecular compositions.The microbial activity in the ocean also explains the organic matter flux attenuation caused by remineralization or packaging of sinking particles.
Biology