The bathypelagic zone is one of five vertical ecological zones into which the deep ocean is commonly divided. It is found immediately above the abyssopelagic zone and is directly below the mesopelagic zone (Figure 1). The upper limit of this zone begins at 100 to 700 meters (330 to 2300 ft) below the ocean surface and coincides with a sea water temperature of 10° Celsius (°C). The lower limit of the bathypelagic is found at a depth between 2000 to 4000 m (6600 to 13,200 ft) where the ocean temperature is 4°C.
Because of its depth and associated light deprivation, the bathypelagic zone is also known as the midnight zone (Figure 2). Virtually no sunlight penetrates into the ocean at these depths. As such, the animals that live here are bioluminescent. Animals living in the bathypelagic zone also tend to have a poor ability to swim. This adaptation is the result of low predator density and reduced visibility. The increased weight of the water above this zone results in a pressure force greater than 5800 pounds per square inch (psi). For reference, the average pressure at the Earth’s surface is 14.7 psi.
Figure 1: Vertical ecological zones in the deep ocean as organized by ocean depth below the surface and seawater temperature
Figure 2: Common names for the ecological zones found in the deep ocean. Also, relationship of these zones to physiographic
Ecology
Without any appreciable sunlight, the bathypelagic zone lacks photosynthetic plants and primary productivity.
Species present in this zone 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;
(b) Resident carnivores; or
(c) other resident scavengers.
Unlike the mesopelagic zone, there is little vertical migration of species to take advantage of bioproductivity at shallower depths. This is largely due to the great vertical distances and high energy expenditures associated with long distance travel. Some of the organisms are predator specialists that lie in waiting and lure their prey. This feeding strategy is advantageous in an environment where movement involves costly energy expenditure. Correspondingly, many of the organisms are listless and lack any real muscle development due to their inactive life style.
Most bathypelagic fishes are darker in color than their mesopelagic counterparts. Also, swim bladders are almost always absent in fishes of the bathypelagic zone in contrast to species in the mesopelagic zone. For bathypelagic creatures of a larger size, there is often significant storage of lipids (fats) to achieve neutral buoyancy under the great pressure.
Gigantism of life forms is quite common in the bathypelagic. This outcome is often related to fluid bloating of the organism's body and not a result of increase in dry weight compared to comparable species at shallower depths. Bioluminescence is so common that bioluminescent light levels can often be measured in the bathypelagic.
Diurnal vertical migrations of great range are much more uncommon in bathypelagic realms compared to the Mesopelagic zone. However, there are a number of fish species found in the Bathypelagic zone whose eggs or larval forms reside in the Epipelagic zone. This survival adaptation results in at least two lifetime extensive vertical migrations.
Circulation, chemistry and respiration
Water residence time in the Bathypelagic zone is approximately 900 years or roughly nine times as long as what occurs in the mesopelagic zone. Despite this slow turnover rate, there is very active downward movement of carbon into the bathypelagic zone. This movement is driven by gravity, organism migration, concentration gradients of dissolved organic carbon and convective forces.
The deep oceans are a huge carbon reservoir on our planet. The total deep ocean carbon storage is estimated at about 38,000 gigatons of carbon (chiefly as bicarbonate ion), or roughly 50 times what is stored in the atmosphere. This estimate includes the mesopelagic, bathypelagic and deeper ocean waters. Dissolved organic carbon, while a smaller element of deep ocean carbon storage, is comparable to all of atmospheric carbon.
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 (Figure 2 and see Physiography of the ocean basins) 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 bathypelagic is a continuing carbon sink with respect to DOC as well as POC.
Respiration rates within the Bathypelagic zone are not dissimilar to the Mesopelagic 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.
Relation to outer continental shelf oil drilling
Some oil drilling exploration and production are conducted from deep sea platforms on the continental slope within the bathypelagic zone. The Gulf of Mexico is a notable such region for some of its crude oil extraction. Due to the extreme pressures and temperatures of the bathypelagic zone, there are a number of technical challenges to oil drilling. For example, the explosion and crude oil release from the Deepwater Horizon oil spill have underscored the difficulties of managing drilling technology at these depths. Considerable water pollution can result from major ocean oil spills, and particularly from dispersants that are sometimes used to achieve cosmetic cleanup; for example, the dispersant authorized for use by the U.S. EPA in the Deepwater Horizon oil spill has acute toxicity to large numbers of marine organisms.
