Bathypelagic zone
The bathypelagic is also known as the midnight zone because of the lack of sunlight; this feature does not allow for photosynthesis-driven primary production, preventing growth of phytoplankton or aquatic plants.
[2][4] The bathymetry of the bathypelagic zone consists of limited areas where the seafloor is in this depth range along the deepest parts of the continental margins, as well as seamounts and mid-ocean ridges.
[5] The continental slopes are mostly made up of accumulated sediment, while seamounts and mid-ocean ridges contain large areas of hard substrate that provide habitats for bathypelagic fishes and benthic invertebrates.
Other technological challenges, such as measuring microbial activity under the pressure conditions experienced in the bathypelagic zone, have also restricted our knowledge of the region.
This is also the dominant delivery mechanism of food to organisms in the bathypelagic zone because there is no sunlight for photosynthesis, with chemoautotrophy playing a more minor role as far as we know.
Smaller parcels of POM often become aggregated together as they fall, which quickens their descent and prohibits their consumption by other organisms, increasing their likelihood of reaching lower depths.
[19][20] The density of these particles may be increased in some regions where minerals associated with some forms of phytoplankton, such as biogenic silica and calcium carbonate "ballast" resulting in more rapid transport to deeper depth.
[21][23] The vertical mixing of DOC-rich surface waters is also a process that delivers carbon to the bathypelagic zone, however, it constitutes a substantially smaller portion of overall transport than POC delivery.
[15][20] DOC transport occurs most readily in regions with high rates of ventilation or ocean turnover, such as the interior of gyres or deep water formation sites along the thermohaline circulation.
[22] The region in the water column at which calcite dissolution begins to occur rapidly, known as the lysocline, is typically located near the base bathypelagic zone at approximately 3,500 m depth, but varies among ocean basins.
In a supersaturated environment, the tests of calcite-forming organisms are preserved as they sink toward the sea floor, resulting in sediments with relatively high amounts of CaCO3.
The trophic networks in this region rely on particulate organic matter (POM) that sinks from the epipelagic and mesopelagic water, and oxygen inputs from the thermohaline circulation.
A comprehensive understanding of the inputs driving the microbial ecology in the bathypelagic zone is lacking due to limited observational data, but has been improving with advancements in deep-sea technology.
In more recent years there has been an effort to categorize the diversity of the eukaryotic assemblages in the bathypelagic zone using methods to assess the genetic compositions of microbial communities based on supergroups, which is a way to classify organisms that have common ancestry.
Some important groups of bacterial grazers include Rhizaria, Alveolata, Fungi, Stramenopiles, Amoebozoa, and Excavata (listed from most to least abundant), with the remaining composition classified as uncertain or other.
[47] The bathypelagic zone contains sharks, squid, octopuses, and many species of fish, including deep-water anglerfish, gulper eel, amphipods, and dragonfish.
However, this zone is difficult for fish to live in since food is scarce; resulting in species evolving slow metabolic rates in order to conserve energy.
[5] However, more recently locations further offshore and at greater depths, such as ocean ridges and seamounts, are being increasingly studied due to advances in technology and laboratory methods, as well as collaboration with industry.
[53][54][55][52] The first discovery of communities subsisting off of the chemical energy in hydrothermal vents was aboard an expedition in 1977 led by Jack Corliss, an oceanographer from Oregon State University.
