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| Photic | |
| Epipelagic | |
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| Mesopelagic | |
| Bathyalpelagic | |
| Abyssopelagic | |
| Hadopelagic | |
| Demersal | |
| Benthic | |
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| Pycnocline | |
| Isopycnal | |
| Chemocline | |
| Halocline | |
| Thermocline | |
| Thermohaline | |
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A pycnocline is the cline or layer where the density gradient (∂ρ/∂z) is greatest within a body of water. An ocean current is generated by the forces such as breaking waves, terms of temperature and salinity differences, wind, Coriolis effect, and tides caused by the gravitational pull of the Moon and the Sun. In addition, the physical properties in pycnocline driven by density gradients also affect the flows and vertical profiles in the ocean. These changes can be connected to the transport of heat, salt, and nutrients through the ocean, and the pycnocline diffusion controls upwelling[1].
Below the mixed layer, a density gradient (or pycnocline) separates the upper and lower water resulting in a barrier (the so-called pycnocline barrier) to vertical mixing[2]. This separation has important biological effects on the ocean and the marine living organisms.
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Turbulent mixing produced by winds and waves transfers heat downward from the surface. In low and mid-latitudes, this creates a surface mixed layer of water of almost uniform temperature which may be a few meters deep to several hundred meters deep. Below this mixed layer, at depths of 200-300 m in the open ocean, the temperature begins to decrease rapidly down to about 1000 m. The water layer within which the temperature gradient is steepest is known as the permanent thermocline[3]. The temperature difference through this layer may be as large as 20℃. The permanent thermocline coincides with a change in water density between the warmer, low-density surface waters and the underlying cold dense bottom waters. The region of rapid density change is known as the pycnocline, and it acts as a barrier to vertical water circulation; thus it also affects the vertical distribution of certain chemicals which play a role in the biology of the seas. The sharp gradients in temperature and density also may act as a restriction to vertical movements of animals[4].
The growth rate of phytoplankton is controlled by the nutrient concentration and the regeneration of nutrients in the sea is very important part of the interaction between higher and lower trophic levels. The separation due to the pycnocline formation prevents the supply of nutrients from the lower layer into the upper layer. Nutrient fluxes through the pycnocline are lower than at other surface layers[5].
The microbial loop is a trophic pathway in the marine microbial food web. The term, microbial loop is coined by Azam et al. (1983) to describe the role of microbes playing in the marine ecosystem carbon and nutrients cycles where dissolved organic carbon (DOC) is returned to higher trophic levels via the incorporation into bacterial biomass, and also coupled with the classic food chain formed by phytoplankton-zooplankton-nekton.
At the end of phytoplankton bloom, when the algae enter a senescent stage, there is an accumulation of phytodetritus and an increased release of dissolved metabolites. It is particularly at this time that the bacteria can utilize these energy sources to multiply and produce a sharp pulse (or bloom) that follows the phytoplankton bloom. The same relationship between phytoplankton and bacteria influences the vertical distribution of bacterioplankton. Maximum numbers of bacteria generally occur at the pycnocline, where phytodetritus accumulates by sinking from the overlying euphotic zone. There, decomposition by bacteria contributes to the formation of oxygen minimum layers in stable waters[6].
One of the most characteristic behavioural features of plankton is a vertical migration that occurs with a 24-hour periodicity. This has often been referred to as diurnal or diel vertical migration. The vertical distance travelled over 24 hours varies, generally being greater among larger species and better swimmers. But even small copepods may migrate several hundred meters twice in a 24-hour period, and stronger swimmers like euphausiids and pelagic shrimp may travel 800 m or more[7]. The depth range of migration may be inhibited by the presence of a thermocline or pycnocline. However, phytoplankton and zooplankton capable of diel vertical migration are often concentrated in the pycnocline[8]. Furthermore, those marine organisms with swimming skills through thermocline or pycnocline may experience strong temperature and density gradients, as well as considerable pressure changes during the migration.
The water in the pycnocline is very stable than in a water column less stability, which means that it takes much more energy to displace a particle of water up and down through the pycnocline. As previously mentioned, the pycnocline is a barrier to the vertical transport of water and water properties and we can identify the pycnocline based on the energy fluctuation or the stability of water layers with depths. In order for us to determine the stability in the water columns, the Brunt-Väisälä frequency or buoyancy frequency can be used[9].
The changes in pycnocline depth or properties can be simulated from some computer program models. The simple approach for those models is to examine the Ekman pumping model based on the ocean general circulation model (OCGM)[10].