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{{Short description|Uppermost layer of a sea water column that is exposed to sunlight}}
{{aquatic layer topics}}

The '''photic zone''' (or '''euphotic zone''', '''epipelagic zone''', or '''sunlight zone''') is the uppermost layer of a [[body of water]] that receives [[sunlight]], allowing [[phytoplankton]] to perform [[photosynthesis]]. It undergoes a series of physical, chemical, and biological processes that supply [[nutrient]]s into the upper [[water column]]. The photic zone is home to the majority of [[Aquatic ecosystem|aquatic life]] due to the activity ([[Marine primary production|primary production]]) of the phytoplankton. The thicknesses of the photic and euphotic zones vary with the intensity of sunlight as a function of season and latitude and with the degree of water turbidity. The bottommost, or aphotic, zone is the region of perpetual darkness that lies beneath the photic zone and includes most of the ocean waters.<ref>{{Cite web |title=Photic zone {{!}} Marine Life, Photosynthesis & Light {{!}} Britannica |url=https://www.britannica.com/science/photic-zone |access-date=2023-11-27 |website=www.britannica.com |language=en}}</ref>

== Photosynthesis in photic zone ==
In the photic zone, the photosynthesis rate exceeds the respiration rate.  This is due to the abundant [[solar energy]] which is used as an energy source for photosynthesis by [[primary producers]] such as phytoplankton. These [[phytoplankton]] grow extremely quickly because of sunlight's heavy influence, enabling it to be produced at a fast rate. In fact, ninety five percent of photosynthesis in the ocean occurs in the photic zone. Therefore, if we go deeper, beyond the photic zone, such as into the [[compensation point]], there is little to no phytoplankton, because of insufficient sunlight.<ref>{{cite book |doi=10.1016/B978-0-12-370518-1.X5000-0 |title=Evolution of Primary Producers in the Sea |date=2007 |isbn=978-0-12-370518-1 |editor1-first=Paul G. |editor1-last=Falkowski |editor2-first=Andrew H. |editor2-last=Knoll }}{{pn|date=June 2025}}</ref> The zone which extends from the base of the euphotic zone to the aphotic zone is sometimes called the dysphotic zone.<ref>[http://www.britannica.com/EBchecked/topic/457662/photic-zone Photic zone] ''Encyclopædia Britannica Online''. 14 August 2009.</ref>

== Life in the photic zone ==
[[File:Pelagiczone.svg|thumb|upright=0.74|right| Layers of the pelagic zone]]

[[File:Light penetration zones in the water column.png|thumb|upright=1.1|left| Zones of the water column as defined by the amount of light penetration. The mesopelagic is sometimes referred to as the ''dysphotic zone''.]]

Ninety percent of [[marine life]] lives in the photic zone, which is approximately two hundred meters deep. This includes phytoplankton (plants), including [[dinoflagellate]]s, [[diatom]]s, [[cyanobacteria]], [[coccolithophore]]s, and [[cryptomonad]]s. It also includes [[zooplankton]], the consumers in the photic zone. There are [[Carnivore|carnivorous]] meat eaters and [[herbivorous]] plant eaters. Next, [[copepod]]s are the small [[crustacean]]s distributed everywhere in the photic zone. Finally, there are [[nekton]] (animals that can propel themselves, like fish, squids, and crabs), which are the largest and the most obvious animals in the photic zone, but their quantity is the smallest among all the groups.<ref>{{Cite web|url=https://sciencing.com/trophic-levels-coral-reefs-5523723.html|title=Trophic Levels of Coral Reefs|website=Sciencing|date=21 May 2018 |access-date=2019-11-22}}</ref> Phytoplankton are microscopic plants living suspended in the water column that have little or no means of motility. They are primary producers that use solar energy as a food source.{{citation needed|date=January 2024}}

"Detritivores and scavengers are rare in the photic zone. Microbial decomposition of dead organisms begins here and continues once the bodies sink to the aphotic zone where they form the most important source of nutrients for deep sea organisms."<ref>{{cite book |doi=10.1016/B978-044452739-4/50006-1 |chapter=Open Ocean |title=Ecology of Insular Southeast Asia |date=2006 |last1=Göltenboth |first1=Friedhelm |last2=Schoppe |first2=Sabine |last3=Widmann |first3=Peter |pages=71–84 |isbn=978-0-444-52739-4 }}</ref> The depth of the photic zone depends on the transparency of the water. If the water is very clear, the photic zone can become very deep. If it is very murky, it can be only fifty feet (fifteen meters) deep.{{cn|date=August 2025}}

Animals within the photic zone use the cycle of light and dark as an important environmental signal, migration is directly linked to this fact, fishes use the concept of dusk and dawn when its time to migrate, the photic zone resembles this concept providing a sense of time. These animals can be herrings and sardines and other fishes that consistently live within the photic zone.<ref>{{cite book |doi=10.1016/B978-0-12-409548-9.10778-X |chapter=Fish Migration, Vertical |title=Encyclopedia of Ocean Sciences |date=2019 |last1=Neilson |first1=John D. |last2=Ian Perry |first2=R. |last3=Crawford |first3=Lisa M. |pages=217–223 |isbn=978-0-12-813082-7 }}</ref>
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== Nutrient uptake in the photic zone ==
Due to biological uptake, the photic zone has relatively low levels of nutrient concentrations. As a result, phytoplankton doesn't receive enough nutrients when there is high water-column stability.<ref>{{cite book | doi=10.1007/0-387-30843-1_325 | chapter=Photic zone | title=Beaches and Coastal Geology | series=Encyclopedia of Earth Sciences Series | date=1982 | last1=Sheppard | first1=Charles R. C. | page=636 | isbn=978-0-87933-213-6 }}</ref> The [[spatial distribution]] of organisms can be controlled by a number of factors. Physical factors include: temperature, hydrostatic pressure, turbulent mixing such as the upward [[Turbulence|turbulent flux]] of inorganic nitrogen across the nutricline.<ref>{{cite journal |last1=Longhurst |first1=Alan R. |last2=Glen Harrison |first2=W. |title=Vertical nitrogen flux from the oceanic photic zone by diel migrant zooplankton and nekton |journal=Deep Sea Research Part A. Oceanographic Research Papers |date=June 1988 |volume=35 |issue=6 |pages=881–889 |doi=10.1016/0198-0149(88)90065-9 |bibcode=1988DSRA...35..881L }}</ref> Chemical factors include oxygen and trace elements.  Biological factors include grazing and migrations.<ref>{{cite journal |last1=Gundersen |first1=K. |last2=Mountain |first2=C. W. |last3=Taylor |first3=Diane |last4=Ohye |first4=R. |last5=Shen |first5=J. |title=Some Chemical and Microbiological Observations in the Pacific Ocean off the Hawaiian Islands |journal=Limnology and Oceanography |date=July 1972 |volume=17 |issue=4 |pages=524–532 |doi=10.4319/lo.1972.17.4.0524 |bibcode=1972LimOc..17..524G |doi-access=free }}</ref> Upwelling carries nutrients from the deep waters into the photic zone, strengthening phytoplankton growth. The remixing and upwelling eventually bring nutrient-rich wastes back into the photic zone. The [[Ekman transport]] additionally brings more nutrients to the photic zone. Nutrient pulse frequency affects the phytoplankton competition. Photosynthesis produces more of it. Being the first link in the food chain, what happens to phytoplankton creates a rippling effect for other species. Besides phytoplankton, many other animals also live in this zone and utilize these nutrients. The majority of ocean life occurs in the photic zone, the smallest ocean zone by water volume. The photic zone, although small, has a large impact on those who reside in it.

== Photic zone depth ==
[[File:Light Penetration Spectrum in Water 01.png|thumb|Depth of light penetration]]
The depth is, by definition, where radiation is degraded down to 1% of its surface strength.<ref>{{cite journal |last1=Lee |first1=ZhongPing |last2=Weidemann |first2=Alan |last3=Kindle |first3=John |last4=Arnone |first4=Robert |last5=Carder |first5=Kendall L. |last6=Davis |first6=Curtiss |title=Euphotic zone depth: Its derivation and implication to ocean-color remote sensing |journal=Journal of Geophysical Research: Oceans |date=March 2007 |volume=112 |issue=C3 |doi=10.1029/2006JC003802 |bibcode=2007JGRC..112.3009L |doi-access=free }}</ref> Accordingly, its thickness depends on the extent of light [[attenuation]] in the water column. As incoming light at the surface can vary widely, this says little about the net growth of  phytoplankton. Typical euphotic depths vary from only a few centimetres in highly [[turbidity|turbid]] [[eutrophic]] lakes, to around 200 meters in the open [[ocean]]. It also varies with seasonal changes in turbidity, which can be strongly driven by [[phytoplankton]] concentrations, such that the depth of the photic zone often decreases as [[primary production]] increases. Moreover, the [[respiration rate]] is actually greater than the photosynthesis rate. The reason why phytoplankton production is so important is because it plays a prominent role when interwoven with other [[food web]]s.

==Light attenuation==
[[File:Linear visible spectrum.svg|thumb|upright=2| {{center|Phytoplankton growth is affected by the colour spectrum of light,<br />and in the process called [[photosynthesis]] absorb light<br />in the blue and red range through [[photosynthetic pigment]]s}}]]
[[File:NOAA Deep Light diagram3.jpg|thumb|upright=1.1|Comparison of the depths which different colors of light penetrate open ocean waters and the murkier coastal waters. Water absorbs the warmer long wavelengths colours, like reds and oranges, and scatter the cooler short wavelength colours.<ref>[https://oceanexplorer.noaa.gov/explorations/04deepscope/background/deeplight/media/diagram3.html Ocean Explorer] NOAA. Updated: 26 August 2010.</ref>]]

Most of the solar energy reaching the Earth is in the range of visible light, with wavelengths between about 400–700&nbsp;nm. Each colour of visible light has a unique wavelength, and together they make up white light. The shortest wavelengths are on the violet and ultraviolet end of the spectrum, while the longest wavelengths are at the red and infrared end. In between, the colours of the visible spectrum comprise the familiar “ROYGBIV”; red, orange, yellow, green, blue, indigo, and violet.<ref name=Webb2019>Webb, Paul (2019) [https://rwu.pressbooks.pub/webboceanography/chapter/6-5-light/ ''Introduction to Oceanography''], chapter 6.5 Light, Rebus Community, Roger Williams University, open textbook. [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>

Water is very effective at absorbing incoming light, so the amount of light penetrating the ocean declines rapidly (is attenuated) with depth. At one metre depth only 45% of the solar energy that falls on the ocean surface remains. At 10 metres depth only 16% of the light is still present, and only 1% of the original light is left at 100 metres. No light penetrates beyond 1000 metres.<ref name=Webb2019 />

In addition to overall attenuation, the oceans absorb the different wavelengths of light at different rates. The wavelengths at the extreme ends of the visible spectrum are attenuated faster than those wavelengths in the middle. Longer wavelengths are absorbed first; red is absorbed in the upper 10 metres, orange by about 40 metres, and yellow disappears before 100 metres. Shorter wavelengths penetrate further, with blue and green light reaching the deepest depths.<ref name=Webb2019 />
[[File:Cycling of marine phytoplankton.png|thumb|upright=1.5|Cycling of marine phytoplankton]]
This is why things appear blue underwater. How colours are perceived by the eye depends on the wavelengths of light that are received by the eye. An object appears red to the eye because it reflects red light and absorbs other colours. So the only colour reaching the eye is red. Blue is the only colour of light available at depth underwater, so it is the only colour that can be reflected back to the eye, and everything has a blue tinge under water. A red object at depth will not appear red to us because there is no red light available to reflect off of the object. Objects in water will only appear as their real colours near the surface where all wavelengths of light are still available, or if the other wavelengths of light are provided artificially, such as by illuminating the object with a dive light.<ref name=Webb2019 />

Water in the [[open ocean]] appears clear and blue because it contains much less [[particulate matter]], such as phytoplankton or other suspended particles, and the clearer the water, the deeper the light penetration. Blue light penetrates deeply and is scattered by the water molecules, while all other colours are absorbed; thus the water appears blue. On the other hand, [[Neritic zone|coastal water]] often appears greenish. Coastal water contains much more suspended [[silt]] and [[algae]] and [[Marine microorganism|microscopic organisms]] than the open ocean. Many of these organisms, such as phytoplankton, absorb light in the blue and red range through their photosynthetic pigments, leaving green as the dominant wavelength of reflected light. Therefore the higher the phytoplankton concentration in water, the greener it appears. Small silt particles may also absorb blue light, further shifting the colour of water away from blue when there are high concentrations of suspended particles.<ref name=Webb2019 />

The ocean can be divided into depth layers depending on the amount of light penetration, as discussed in [[pelagic zone]]. The upper 200 metres is referred to as the photic or euphotic zone. This represents the region where enough light can penetrate to support photosynthesis, and it corresponds to the epipelagic zone. From 200 to 1000 metres lies the dysphotic zone, or the twilight zone (corresponding with the mesopelagic zone). There is still some light at these depths, but not enough to support photosynthesis. Below 1000 metres is the aphotic (or midnight) zone, where no light penetrates. This region includes the majority of the ocean volume, which exists in complete darkness.<ref name=Webb2019 />

== Paleoclimatology ==
[[File:Triceratium morlandii var. morlandii.jpg|thumb| {{center|Intricate silicate (glass) shell, 32-40 million years old, of a [[diatom]] microfossil}}]]
{{further|diatoms|microfossils}}

[[Phytoplankton]] are [[unicellular]] [[Marine microorganisms|microorganisms]] which form the base of the [[ocean food chain]]s. They are dominated by [[diatom]]s, which grow silicate shells called [[frustule]]s. When diatoms die their shells can settle on the [[seafloor]] and become [[microfossil]]s. Over time, these microfossils become buried as [[opal]] deposits in the [[marine sediment]]. [[Paleoclimatology]] is the study of past climates. [[Proxy (climate)|Proxy data]] is used in order to relate elements collected in modern-day sedimentary samples to climatic and oceanic conditions in the past. [[Paleoclimate proxies]] refer to preserved or fossilized physical markers which serve as substitutes for direct meteorological or ocean measurements.<ref>{{Cite web|title=What Are "Proxy" Data? {{!}} National Centers for Environmental Information (NCEI) formerly known as National Climatic Data Center (NCDC)|url=https://www.ncdc.noaa.gov/news/what-are-proxy-data|access-date=2020-10-20|website=www.ncdc.noaa.gov}}</ref> An example of proxies is the use of [[diatom]] [[marine isotope stage|isotope records]] of [[δ13C]], [[δ18O]], [[Isotopes of silicon|δ30Si]] (δ13C<sub>diatom</sub>, δ18O<sub>diatom</sub>, and δ30Si<sub>diatom</sub>). In 2015, Swann and Snelling used these isotope records to document historic changes in the photic zone conditions of the north-west [[Pacific Ocean]], including nutrient supply and the efficiency of the soft-tissue [[biological pump]], from the modern day back to [[Marine Isotope Stage 5#Marine Isotope Stage (MIS) 5e|marine isotope stage 5e]], which coincides with the [[Eemian|last interglacial period]]. Peaks in opal productivity in the marine isotope stage are associated with the breakdown of the regional [[Halocline|halocline stratification]] and increased nutrient supply to the photic zone.<ref name=Swann2015>{{cite journal |last1=Swann |first1=G. E. A. |last2=Snelling |first2=A. M. |title=Photic zone changes in the north-west Pacific Ocean from MIS 4–5e |journal=Climate of the Past |date=6 January 2015 |volume=11 |issue=1 |pages=15–25 |doi=10.5194/cp-11-15-2015 |bibcode=2015CliPa..11...15S |doi-access=free }} [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/3.0/ Creative Commons Attribution 3.0 International License].</ref>

<gallery mode=packed style=float:left heights=220px>
File:Ocean changes from the last interglacial to the modern day A.png
File:Ocean changes from the last interglacial to the modern day B.png
</gallery>
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The initial development of the halocline and [[Stratification (water)|stratified water column]] has been attributed to the onset of major [[Würm glaciation|Northern Hemisphere glaciation]] at 2.73 Ma, which increased the flux of freshwater to the region, via increased monsoonal rainfall and/or glacial meltwater, and [[sea surface temperature]]s.<ref>{{cite journal |last1=Sigman |first1=Daniel M. |last2=Jaccard |first2=Samuel L. |last3=Haug |first3=Gerald H. |title=Polar ocean stratification in a cold climate |journal=Nature |date=March 2004 |volume=428 |issue=6978 |pages=59–63 |doi=10.1038/nature02357 |pmid=14999278 |bibcode=2004Natur.428...59S |url=https://gfzpublic.gfz-potsdam.de/pubman/item/item_230448 }}</ref><ref name=Haug2005>{{cite journal |last1=Haug |first1=Gerald H. |last2=Ganopolski |first2=Andrey |last3=Sigman |first3=Daniel M. |last4=Rosell-Mele |first4=Antoni |last5=Swann |first5=George E. A. |last6=Tiedemann |first6=Ralf |last7=Jaccard |first7=Samuel L. |last8=Bollmann |first8=Jörg |last9=Maslin |first9=Mark A. |last10=Leng |first10=Melanie J. |last11=Eglinton |first11=Geoffrey |title=North Pacific seasonality and the glaciation of North America 2.7 million years ago |journal=Nature |date=February 2005 |volume=433 |issue=7028 |pages=821–825 |doi=10.1038/nature03332 |pmid=15729332 |bibcode=2005Natur.433..821H }}</ref><ref>{{cite journal |last1=Swann |first1=George E. A. |last2=Maslin |first2=Mark A. |last3=Leng |first3=Melanie J. |last4=Sloane |first4=Hilary J. |last5=Haug |first5=Gerald H. |title=Diatom δ 18 O evidence for the development of the modern halocline system in the subarctic northwest Pacific at the onset of major Northern Hemisphere glaciation |journal=Paleoceanography |date=March 2006 |volume=21 |issue=1 |doi=10.1029/2005pa001147 }}</ref><ref name=Nie2008>{{cite journal |last1=Nie |first1=Junsheng |last2=King |first2=John |last3=Liu |first3=Zhengyu |last4=Clemens |first4=Steve |last5=Prell |first5=Warren |last6=Fang |first6=Xiaomin |title=Surface-water freshening: A cause for the onset of North Pacific stratification from 2.75 Ma onward? |journal=Global and Planetary Change |date=November 2008 |volume=64 |issue=1–2 |pages=49–52 |doi=10.1016/j.gloplacha.2008.08.003 |bibcode=2008GPC....64...49N }}</ref> The decrease of [[abyssal]] water upwelling associated with this may have contributed to the establishment of globally cooler conditions and the expansion of glaciers across the Northern Hemisphere from 2.73 Ma.<ref name=Haug2005 /> While the halocline appears to have prevailed through the [[late Pliocene]] and [[Gelasian|early Quaternary]] [[glacial cycle|glacial–interglacial cycles]],<ref>{{cite journal |last1=Swann |first1=George E.A. |title=Salinity changes in the North West Pacific Ocean during the late Pliocene/early Quaternary from 2.73 Ma to 2.52 Ma |journal=Earth and Planetary Science Letters |date=August 2010 |volume=297 |issue=1–2 |pages=332–338 |doi=10.1016/j.epsl.2010.06.035 |bibcode=2010E&PSL.297..332S |url=http://eprints.nottingham.ac.uk/2015/ }}</ref> other studies have shown that the stratification boundary may have broken down in the [[late Quaternary]] at glacial terminations and during the early part of interglacials.<ref>{{cite journal |last1=Sarnthein |first1=M. |last2=Gebhardt |first2=H. |last3=Kiefer |first3=T. |last4=Kucera |first4=M. |last5=Cook |first5=M. |last6=Erlenkeuser |first6=H. |title=Mid Holocene origin of the sea-surface salinity low in the subarctic North Pacific |journal=Quaternary Science Reviews |date=November 2004 |volume=23 |issue=20–22 |pages=2089–2099 |doi=10.1016/j.quascirev.2004.08.008 |bibcode=2004QSRv...23.2089S }}</ref><ref>{{cite journal |last1=Jaccard |first1=S.L. |last2=Galbraith |first2=E.D. |last3=Sigman |first3=D.M. |last4=Haug |first4=G.H. |title=A pervasive link between Antarctic ice core and subarctic Pacific sediment records over the past 800kyrs |journal=Quaternary Science Reviews |date=January 2010 |volume=29 |issue=1–2 |pages=206–212 |doi=10.1016/j.quascirev.2009.10.007 |bibcode=2010QSRv...29..206J }}</ref><ref>{{cite journal |last1=Galbraith |first1=Eric D. |last2=Kienast |first2=Markus |last3=Jaccard |first3=Samuel L. |last4=Pedersen |first4=Thomas F. |last5=Brunelle |first5=Brigitte G. |last6=Sigman |first6=Daniel M. |last7=Kiefer |first7=Thorsten |title=Consistent relationship between global climate and surface nitrate utilization in the western subarctic Pacific throughout the last 500 ka |journal=Paleoceanography |date=June 2008 |volume=23 |issue=2 |doi=10.1029/2007pa001518 |bibcode=2008PalOc..23.2212G }}</ref><ref>{{cite journal |last1=Brunelle |first1=Brigitte G. |last2=Sigman |first2=Daniel M. |last3=Jaccard |first3=Samuel L. |last4=Keigwin |first4=Lloyd D. |last5=Plessen |first5=Birgit |last6=Schettler |first6=Georg |last7=Cook |first7=Mea S. |last8=Haug |first8=Gerald H. |title=Glacial/interglacial changes in nutrient supply and stratification in the western subarctic North Pacific since the penultimate glacial maximum |journal=Quaternary Science Reviews |date=September 2010 |volume=29 |issue=19–20 |pages=2579–2590 |doi=10.1016/j.quascirev.2010.03.010 |bibcode=2010QSRv...29.2579B }}</ref><ref>{{cite journal |last1=Kohfeld |first1=Karen E. |last2=Chase |first2=Zanna |title=Controls on deglacial changes in biogenic fluxes in the North Pacific Ocean |journal=Quaternary Science Reviews |date=November 2011 |volume=30 |issue=23–24 |pages=3350–3363 |doi=10.1016/j.quascirev.2011.08.007 |bibcode=2011QSRv...30.3350K }}</ref><ref name=Swann2015 />

== Phytoplankton ==
[[File:Phytopla.jpg|thumb|upright|Drawn image of phytoplankton]]An increase in the amount of phytoplankton also creates an increase in zooplankton, the zooplankton feeds on the phytoplankton as they are at the bottom of the food chain.<ref name = "Accumulation" />

Phytoplankton are restricted to the photic zone only, as their growth is completely dependent upon photosynthesis. This results in phytoplankton only occupying the uppermost 50–100&nbsp;m of the water column. Phytoplankton growth within the photic zone can also be influenced by terrestrial factors, like the weathering of crustal rocks or nutrients from the respiration of plants and animals on land that are carried to the ocean via runoff or riverine input.<ref name = "Accumulation">{{Cite web |title=Accumulation |url=https://www.dnr.louisiana.gov/assets/TAD/education/BGBB/3/accumulation.html#:~:text=Planktonic%20aquatic%20organisms%20such%20as,is%20totally%20dependent%20upon%20photosynthesis. |access-date=2023-12-01 |website=www.dnr.louisiana.gov}}</ref>

[[File:Dimethyl sulfide structure.svg|thumb|Dimethyl sulfide structure]]
Dimethylsulfide loss within the photic zone is controlled by microbial uptake and photochemical degradation.


==See also==
* [[Aquatic photosynthesis]]
* [[Electromagnetic absorption by water]]
* [[Epipelagic fish]]
* [[Mesophotic coral reef]]

==References==
{{Reflist}}

{{aquatic ecosystem topics|state=expanded}}
{{physical oceanography|expanded=none}}

[[Category:Aquatic ecology]]
[[Category:Oceanographical terminology]]