The mesopelagic zone is one of the five major vertical divisions of the oceans. The nominal depth delineation of this zone is between 200 and 1000 meters below the ocean surface. Due to the limited sunlight penetration to the depths of this zone, it is frequently termed the twilight zone. The ocean zone above the mesopelagic is known as the epipelagic zone and the vertical region immediately below is the bathypelagic zone. The mesopelagic zone is significant in the diurnal vertical migration of numerous species of small organisms upward to the epipelagic zone; it is also notable for significant carbon storage and vertical movement of large carbon stores downward into the deeper oceans.
Circulation, chemistry and respiration
Water residence time in the mesopelagic zone is approximately 100 years or roughly one ninth that of the bathypelagic zone. Even though this description makes the mesopelagic zone seem hydrologically slow moving, there are very active diurnal animal migrations and vertical carbon fluxes moving downward into the bathypelagic driven by gravity, organism migration, concentration gradients of dissolved organic carbon and convective forces.
The deeper oceans are a huge carbon reservoir, although it is difficult to disaggregate the component deep sea layers. For example, the total deep ocean carbon storage is estimated at about 38,000 Gigatons of carbon (chiefly as bicarbonate ion), or roughly 50 times the atmospheric reservoir. This estimate includes the mesopelagic, bathypelagic and deeper oceans, although the mesopelagic and bathypelagic zones are known to be appreciable components. Dissolved organic carbon is also present in appreciable quantities.
There appears to be a pronounced downward vertical migration of both participate organic carbon (POC) and dissolved organic carbon (DOC). Worldwide downward movement of POC has been estimated as high as one petamole C/yr, as reckoned at the upper edge of the mesopelagic. The pronounced vertical flux of POC continues by detritus snow (including organism corpses, mucus sheets and fecal pellets) into the bathypelagic, with the sediment traps on the continental slope of the bathypelagic being particularly effective carbon traps.
DOC vertical migration is driven by concentration gradient as well as spatially variant convective fluxes. There is also a downward bias to these carbon fluxes, such that the mesopelagic and bathypelagic are continuing carbon sinks with respect to DOC as well as POC.
Respiration rates within the mesopelagic zone are not dissimilar to the bathypelagic zone; in fact, in many world regions the measured respiration rates actually increase with depth from the mesopelagic to the bathypelagic, before declining with depth in the abyssopelagic zone.
With some amount of sunlight penetration, the mesopelagic zone incurs some primary productivity, even though at lesser rates than the epipelagic zone above. Species present are limited to: (a) Detrivores who feed on the downward drizzle of moulted exoskeletons, mucus sheets, fecal pellets, organism corpses and other organic debris falling from the mesopelagic zone above; (b) Herbivores that occupy mesopelagic depths, but migrate diurnally upward at night; or (c) Carnivores that are either resident in the mesopelagic or migrate upward to the epipelagic by night.
Most mesopelagic fishes are lighter in color than their bathypelagic counterparts; swim bladders are commonly present in fishes of the mesopelagic in contrast to bathypelagic zone denizens, who lack those structures.
Diurnal vertical migrations of great range are common in mesopelagic realms, following the 24 hour light cycle. In addition, there are a number of fish species whose eggs or larval forms reside in the epipelagic zone, necessitating additional vertical migrations for those species.
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