Editing Photic zone (section)

==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 />