460:
550:. 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. 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. 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.
392:. The temperature difference through this layer may be as large as 20°C, depending on latitude. 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.
310:
451:, the surface waters are much colder year-round due to latitude and much fresher due to the melting of sea and land ice, high precipitation, and freshwater runoff, while deeper waters are fairly consistent across the globe. Due to this, there is no permanent thermocline present, but seasonal thermoclines can occur. In these areas, a permanent halocline exists, and this halocline is the main factor in determining the permanent pycnocline.
407:
In the summer, warmer temperatures, melting sea and land ice, and increased sunlight cause the surface layer of the ocean to increase in temperature. This layer sits on top of the large winter mixed layer that was previously created and forms a seasonal pycnocline above the main pycnocline, with the
387:
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
374:
Below the mixed layer, a stable density gradient (or pycnocline) separates the upper and lower water, hindering vertical transport. This separation has important biological effects on the ocean and the marine living organisms. However, vertical mixing across a pycnocline is a regular phenomenon in
533:
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,
313:
Pycnocline during stable stratification of deep water layers. The pycnocline is the transitory region between a surface layer of water (warmer and less dense) and deeper layer of water (colder and more dense). Mixing occurs across the pycnocline, driven primarily by waves and
424:, the surface density for all oceans follows surface temperature rather than surface salinity. At the highest latitudes over 50°, surface density follows salinity more than temperature for all oceans because temperature consistently sites near the freezing point.
439:
separating them. This phenomenon is reflected in density due to the strong dependence of density on ocean temperature; two permanent pycnoclines are associated with the permanent thermoclines, and the density equivalent to the thermostad is called the pycnostad.
408:
winter mixed layer becoming a lower density gradient called a pycnostad. As the seasons begin to change again, a net loss of heat from the surface layer and continued wind mixing wear away the seasonal pycnocline until the next summer.
486:. 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.
367:, and tides caused by the gravitational pull of celestial bodies. In addition, the physical properties in a pycnocline driven by density gradients also affect the flows and vertical profiles in the
843:
Capotondi, A., Alexander, M.A., Deser, C., and Miller, A. 2004. Low-frequency pycnocline variability in the
Northeast Pacific. American Meteorological Society. Vol. 35, pp. 1403-1420.
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Hales, B., Hebert, D., and Marra, J. 2009. Turbulent supply of nutrients to phytoplankton at the New
England shelf break front. Journal of Geophysical Research. Vol. 114, C05010,
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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
546:
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
388:
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
427:
In low and mid-latitudes, a permanent pycnocline exists at depths between 200-1000 m. In some large but geographically restricted subtropical regions such as the
482:
is controlled by the nutrient concentration, and the regeneration of nutrients in the sea is a very important part of the interaction between higher and lower
562:
drops below 0.25. The
Richardson number is a dimensionless value expressing the ratio of potential to kinetic energy. This ratio drops below 0.25 when the
573:
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
416:
While temperature and salinity both have an impact on density, one can have a greater effect than the other depending on latitudinal region. In the
404:
are cooler, and waves tend to be larger, which increases the depth of the mixed layer even down to the main thermocline/pycnocline in some cases.
371:. These changes can be connected to the transport of heat, salt, and nutrients through the ocean, and the pycnocline diffusion controls upwelling.
296:
632:
474:
fjord, Sweden. The top of the largest jellyfish is breaching the surface, while its tentacles are stirring up the thin pycnocline layer.
400:
While the general structure of a pycnocline explained above holds true, pycnoclines can change based on the season. In the winter,
718:
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502:. The term "microbial loop" was coined by Azam et al. (1983) to describe the role played by microbes in the marine ecosystem
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538:. There, decomposition by bacteria contributes to the formation of oxygen minimum layers in stable waters.
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Hill, A.E. 1998. Diel vertical migration in stratified tidal flows: Implications for plankton dispersal.
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Vertical Mixing and
Transports through a Stratified Shear Layer, Journal of Physical Oceanography (2001)
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663:
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At the end of phytoplankton bloom, when the algae enter a senescent stage, there is an accumulation of
698:
Mann and Lazier (2006). Dynamics of marine ecosystems. 3rd edition. Blackwell
Publishing. Chapter 3.
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463:
230:
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Anand
Gnanadesikan. 1999. A simple predictive model for the structure of the oceanic pycnocline.
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401:
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Density
Stratification, Turbulence, but How Much Mixing? Annual Review of Fluid Mechanics (2008)
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147:
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Lalli and Parson (1993). Biological oceanography: an introduction. Pergamon press. Chapter 4.
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Lalli and Parson (1993). Biological oceanography: an introduction. Pergamon press. Chapter 5.
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Lalli and Parson (1993). Biological oceanography: an introduction. Pergamon press. Chapter 2.
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and an increased release of dissolved metabolites. It is particularly at this time that the
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435:, two permanent thermoclines exist with a layer of lower vertical stratification called a
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732:(1997). Introduction to Physical Oceanography. 2nd edition, Prentice-Hall. Chapter 1
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Talley, Lynne D.; Pickard, George L.; Emery, William J.; Swift, James H. (2011).
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Turbulent Mixing in
Stratified Fluids, Annual Review of Fluid Mechanics (1991)
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is generated by the forces such as breaking waves, temperature and
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Layer where the density gradient is greatest within a body of water
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379:. Such mixing plays a key role in the transport of nutrients.
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where phytodetritus accumulates by sinking from the overlying
368:
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disturbing the pycnocline in the top water layer of
570:, resulting in a turbulence which leads to mixing.
762:Descriptive Physical Oceanography: An Introduction
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375:oceans, and occurs through shear-produced
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566:exceeds stratification. This can produce
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558:Pycnoclines become unstable when their
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639:based on difference in water
625:based on difference in water
611:based on difference in water
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498:is a trophic pathway in the
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764:(6th ed.). Elservier.
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821:Journal of Marine Research
664:Thin layers (oceanography)
500:marine microbial food web
823:, Vol 56, pp. 1069-1096.
402:sea surface temperatures
548:diel vertical migration
542:Diel vertical migration
689:283 (5410): 2077–2079.
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865:Physical oceanography
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412:Changes with Latitude
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790:10.1029/2008JC005011
261:Ocean stratification
577:model based on the
455:Biological function
363:differences, wind,
326:or layer where the
256:Lake stratification
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266:Aquatic ecosystems
771:978-0-7506-4552-2
560:Richardson number
383:Physical function
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231:Thermohaline
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599:temperature
591:Thermocline
516:zooplankton
396:Seasonality
390:thermocline
220:Thermocline
113:Hadopelagic
80:Mesopelagic
854:Categories
673:References
605:Chemocline
564:shear rate
437:thermostad
377:turbulence
320:pycnocline
196:Nutricline
185:Chemocline
163:Pycnocline
57:Epipelagic
654:Isopycnal
641:turbidity
633:Lutocline
619:Halocline
613:chemistry
554:Stability
207:Halocline
174:Isopycnal
648:See also
627:salinity
581:(OCGM).
531:bacteria
472:Gullmarn
445:subpolar
433:Atlantic
361:salinity
331:gradient
243:See also
128:Demersal
687:Science
431:in the
418:tropics
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335:
328:density
322:is the
139:Benthic
69:Aphotic
36:Pelagic
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520:nekton
504:carbon
466:and a
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340:ρ
338:∂
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