Mesopelagic zone
[1] The mesopelagic zone occupies about 60% of the planet's surface and about 20% of the ocean's volume, amounting to a large part of the total biosphere.
[2] 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.
[3] It has long captivated the imagination of scientists, artists and writers; deep sea creatures are prominent in popular culture.
[4] The mesopelagic zone includes the region of sharp changes in temperature, salinity and density called the thermocline, halocline, and pycnocline respectively.
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.
[7] 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.
)[7] Oceanographers later used this underwater surveillance system to figure out the speed and direction of deep ocean currents by dropping SOFAR floats that could be detected with the SOSUS array.
[5] 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.
[11] When primary productivity is high, the contribution of active transport by vertical migration has been estimated to be comparable to sinking particle export.
[12] Sinking rates have been measured in the project VERTIGO (Vertical Transport in the Global Ocean) using settling velocity sediment traps.
There is so much biomass in this 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.
Recent work using DNA from seawater samples emphasized the importance of viruses and microbes role in recycling organic matter from the surface ocean, known as the microbial loop.
[21] This chemoautotrophic Archaea crenarchaeon Candidatus can oxidize ammonium as their energy source without oxygen, which could significantly impact the nitrogen and carbon cycles.
[8] Microbial biomass in the mesopelagic is greater at higher latitudes and decreases towards the tropics, which is likely linked to the differing productivity levels in the surface waters.
[31] A passive method to estimate mesopelagic fish abundance is by echosounding to locate the 'deep scattering layer' through the backscatter received from these acoustic sounders.
[16] Some predators develop bioluminescent lures, like the tasselled anglerfish, which can attract prey, while others respond to pressure or chemical cues instead of relying on vision.
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.
[42] Historically, there have been few examples of efforts to commercialize the mesopelagic zone due to low economic value, technical feasibility and environmental impacts.
[25] Fishing with large trawl nets poses threats to a high percentage of bycatch as well as potential impacts to the carbon cycling processes.
[44] 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.
[45] 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.
[46] 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.
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.
Deep-See is equipped with cameras, sonars, sensors, water sample collection devices, and a real-time data transmission system.
[3][51] Mesobot is equipped with high-definition cameras to track and record mesopelagic species on their daily migration over extended periods of time.
[3] Traditional sample collection devices fail to preserve organisms captured in the mesopelagic due to the large pressure change associated with surfacing.
This device descends down the water column and takes images of the amount and size distribution 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 cycling processes between the surface ocean and the mesopelagic.
[52] 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.
