Is ocean color affected by Rayleigh Scattering?

Question

We know that sky color is due primarily to Rayleigh scattering. Is Rayleigh scattering the dominant effect for ocean color too?

Answer 1

Rayleigh scattering is also present in water, but the main reason of the blue color of the sea is absortion, due to vibrational transitions.

See: http://www.dartmouth.edu/~etrnsfer/water.htm

Answer 2

It looks like Raleigh scattering also makes an indirect contribution to the sea color, as "the surface of the water reflects the color of the sky" (http://en.wikipedia.org/wiki/Color_of_water )

Answer 3

When white sunlight enters the large masses of water the water molecules absorb most of the higher wavelengths in the red, green and yellow parts of the spectrum by vibrational transitions; they also absorbs blue but emit it immediately (Rayleigh scattering) so the ocean water looks blue.

We can say that both Raman scattering (scattering by vibrational transitions) and Rayleigh scattering are combinedly responsible for the blue colour of the ocean, yet some of its other colors like green and red are due to plant life, algae, and other factors like rocks, minerals etc.

Because Raman scattering is a weak effect, it involves only 1 out of 10^9 photons so it requires huge masses of water to manifest properly: a glass of water, a river, or a pond water will appear colorless.

Answer 4

One might expect to see Rayleigh scattering in water of the order of 1% of light scattered per 5 m of illuminated seawater. That would presumably be perceived as a faint additive 'glow' over dark regions of sea floor. I can't recall seeing such a thing myself, and presume that is partly because of attenuation by murky coastal seawater of both incoming and scattered light; also it has to compete with reflection from the surface, from plankton and particulate matter and possibly the sea floor.

Why is the sea blue?

The chosen answer refers to a 1993 article 'Why is water blue?' by Braun and Smirnov which provides an absorption spectrum of pure water. Another spectrum of pure water is given in Pope and Fry (1997) and reproduced on Wikipedia, showing greatest transparency at 420 nm in blue-violet. (There may be confirming measurements in this chapter.) Absorption would seem to explain why a diver in tropical waters or high seas sees predominantly blue light when looking up.

Reputable sources like Woods Hole also suggest absorption, along with surface reflection, is a significant effect in ocean colour as observed from above, but doesn't mention Rayleigh scattering, and nor does Britannica, which cites its absorption spectrum to Jerlov.

Murky waters

However, I live in Britain, and my answer to "Why is the sea blue?" is "It isn't." I often see a grey sky, with a grey sea beneath it. Any colour at a quayside is plankton green. If you look at any photograph of a blue sea or lake, you see it mirror the features of the hills or sky above, including the colour. Shallow angle of incidence in photography may play a role in the way the all the colour you see is copied from the sky; perhaps also there is some effect of the surface on polarised Rayleigh scattering from the sky. So to see the effects of the measured colour of pure water, a subtractive effect, you would need something near the surface to actually reflect the filtered light to the observer. This could be coccoliths or floating or solid carbonates, which would have to dominate over other 'coloured dissolved organic matter'. But even where there is a chalk shelf, the sea in some cases is only blue by virtue of reflecting the sky.

A NOAA diagram of light penetration shows that coastal waters are darker and explain a more green or cyan absorption effect, presumably because they have more nutrients and life. Recent measurements in the Arctic, Geoffroy et al (2021), question some assumptions about a blue water column: "Analysis of absorption and backscattering spectra suggested that coloured dissolved organic matter absorption was the dominant contributor to diffuse attenuation in the blue-green wavelengths while water absorption dominated the red end of the visible spectrum. Particle backscattering signals were generally low across the entire visible spectrum. Horizontal spectral penetration distance (i.e., the range at which irradiance diminishes to 1/e of its initial value) reached ~ 8 m for blue light,  ~ 14 m for green light and ~ 6 m for the red light, indicating that the red light was attenuated stronger than light at other colours." but particularly the red bordering infra-red rather than orange.

Conclusion

I've not found why Rayleigh scattering is too insignificant to see in water, but the other answers to why seawater is seen as blue may depend on whereabouts in the world the waters are.