Color of the Ocean
John Hacunda
an endless palette.
Is the color appearance a function of the light filtering based on
the water clarity? How do particles (plankton, sediment, etc.)
affect the color of the ocean?
Thanks,
John
R T Tyrrell
: The color of the ocean seems to change like
: an endless palette.
the main factors affecting the sea colour are:
1. amount and type of phytoplankton pigments in the water
2. suspended sediment
3. sky conditions
4. degraded pigments of land or sea origin
4. phytoplankton "shells" (diatom tests, coccoliths)
the effects of coccoliths on ocean colour can be seen at:
http://www.soc.soton.ac.uk/SUDO/tt/eh/
regards,
Toby Tyrrell
Southampton Oceanography Centre
University of Southampton
European Way
Southampton
SO14 3ZH
UK
email: t...@socnet.soc.soton.ac.uk
Todd Bowers
The color of water is a function of the absorbing (which
wavelengths, or colors, are removed so they can't reach your
eye) and scattering (redirected so it *can* enter your eye)
properties of the water and its constituents. The color you
see then is a result of incident visible light with a certain
characteristic spectrum interacting with the water in a way
that certain frequencies (wavelengths) are "manipulated"
through absorption and scattering events, giving rise to a
new characteristic spectrum.
First, pure water (including seawater) is a blue liquid. Not
because the sky is blue, but because of the way in which the
water molecules scatter and absorb photons.
It happens that the water molecules scatter bluer light more
strongly than red light. This phenomenon is called molecular
scattering, density fluctuation scattering, and, incorrectly,
Rayleigh scattering. (Rayleigh scattering is very similar,
but technically not the same process. Rayleigh scattering is
responsible for the blue color of the sky, though)
So, you may ask, why does water preferentially scatter blue
light more strongly? Well, now you're really getting into the
physics of the problem. It has to do with the frequency of
the incident radiation and the "natural" frequency of the
oscillating molecules. This natural frequency is in the
UV, and blue light is closer to this frequency than red.
You've gotta kind of imagine a spring-mass system. If you
jiggle it at too high/low a frequency, the weight at the end
will just jiggle around a bit too. If you time your arm
movement at the right frequency, its natural frequency for
that particular spring-mass system, you'll get the maximum
displacement of the weight with each oscillation. Blue
wavelengths are closer to this natural frequency so have a
larger magnitude of displacement (read larger magnitude of
scattering) than other colors.
With absorption, just the opposite happens, water absorbs
the reds more strongly than the blues. I will not go into
this any deeper, but I will say that it's also associated
with the interacting oscillations of the photons and water
molecules. The result of these two actions gives pure water
its blue color when illuminated with "whitish" light.
Now that's how the water molecules affect the color, to
this you've got to "add" the effects of impurities in the
water. Photosynthetic plankton absorb blue and red light,
which tends to give the water a greenish color (same reason
a leaf is green). Now, these critters tend to die sooner or
later and decompose into dissolved organic matter(DOM). Other
things, sea and land based, also contribute to DOM. This
stuff absorbs blue and green light the most giving it, and
the water that contains it, a yellowish color. Some species,
especially when in an excessively productive mode (a bloom)
can impart a reddish color to the water locally. This is
commonly called a red tide. This obviously is caused by how
the light is absorbed and scattered off their little
collective bodies, but exactly why they give the water a red
color when individually they are green is not fully
understood at this time. Other pigments in plankton have
different absorption characteristics, but generally give only
a small contribution to color. Sediments tend to give the water
the color that they reflect if they are suspended sufficiently
close to the surface so they can receive light and emit it
back through the surface to you. Muddy sediments of course
are dark, meaning that they absorb a lot at almost all visible
wavelengths giving the water a dark appearance. I'm not
really sure why the Red Sea is called that, but I *believe*
that it's due to the ferrous nature of the sediments.
Other non-organic particles can also contribute to the mix.
Also, it is true that the color of incident light does
have some effect on the color of the water. If I shine pure
red light on the surface of pure water, it may appear black
since the water absorbs red light so well. In reality,
natural skylight is never this pure, even when a low sun
creates a brilliant red sky, so the color of the natural
incident light has little effect on the color of the water
except when light levels are low (cloudy and night) which
makes the water appear darker.
So really, the color of the water you see depends on what's
in the water and how much of each component is present.
There's a lot more to be said here and some things I've
simplified, but I think this gives the general picture.
Yes, there is more, like inelastic scattering, fluorescence,
and bio-luminescence, but then I'd be here all day and this
weekend. These effects are generally small and bio-illum.
is kind of a special case, but they are there.
Each topic I've touched on has at least one entire book
written about it. Sorry I can't write more, but I've *got*
go get back to work!
Hope this helps,
Todd
--
Todd Bowers
PSI-MSAAP Bldg. 9121
Stennis Space Center, MS
tbo...@nrlssc.navy.mil
John Hacunda <jh...@aquanet.com> wrote in message
news:8mha3.3701$gK.8...@typ22b.nn.bcandid.com...
> The color of the ocean seems to change like
> an endless palette.
>
> Is the color appearance a function of the light filtering based on
> the water clarity? How do particles (plankton, sediment, etc.)
> affect the color of the ocean?
>
> Thanks,
> John
>
William F. Campbell
> Wow, that's a complex question.
> <snip good discussion>
> So really, the color of the water you see depends on what's
> in the water and how much of each component is present.
> There's a lot more to be said here and some things I've
> simplified, but I think this gives the general picture.
> Yes, there is more, like inelastic scattering, fluorescence,
> and bio-luminescence, but then I'd be here all day and this
> weekend. These effects are generally small and bio-illum.
> is kind of a special case, but they are there.
> Each topic I've touched on has at least one entire book
> written about it. Sorry I can't write more, but I've *got*
> go get back to work!
One more book you neglected to mention -- the color perception of the human eye.
> Hope this helps,
> Todd
>
> --
> Todd Bowers
> PSI-MSAAP Bldg. 9121
> Stennis Space Center, MS
> tbo...@nrlssc.navy.mil
Bill Campbell
bkay
in key west recently, and i think I heard this somewhere,
Jim Ivey
Temperature only effects the color of the water itself very slighty.
Temperature affects the frequency of of the O-H bonds and results in
slight changes in water absorption. The difference is not detectable by
the human eye but can only be detected by the most sensitive
instruments. Temperature does effect the constituents in the water both
directly and indirectlly. Therefore temperature can have an affect on
what is in the water and the color perceived. An example would be a
fall mixing of the water column due to colder (denser) water at the
surface resulting in more nutrients from deeper water and resulting in a
bloom of phytoplankton. Of course ,if the water freezes, that is a
whole different story and temperature will have big affect on color :-).
Jim