The in situ light microenvironment of corals


Article Title:
The in situ light microenvironment of corals

Daniel Wangpraseurt, Lubos Polerecky, Anthony W.D. Larkum, Peter J. Ralph, Daniel A. Nielsen, Mathieu Pernice, Michael Kühl

Limnology and Oceanography, Volume 59, Issue 3, May 2014, Pages 917–926

DOI: 10.4319/lo.2014.59.3.0917

PDF Link.


I don’t understand the choice of words “Scalar irradiance”, but the meaning in my words, is the total irradiance from every angle.  Sunlight from “up”, light from reflections off the sand, off the coral’s own skeleton, et al.

The typical light measurement you’ll see on a hobby website using a PAR or lux meter is only one aspect of “scalar irradiance”.

(Measurements of light in air don’t seem to have so many complications or aspects.)

Light is strongly scattered at the water–tissue interface and within the coral tissue, where photon trapping and redistribution leads to significant enhancement in the local scalar irradiance compared with the incident downwelling irradiance (Ku ̈hl et al. 1995; Wangpraseurt et al. 2012a).

Additionally, reflective, fluorescent, or both host pigments are synthesized by many corals, which further alters the intensity and spectral quality of light due to, for example, intense scattering and red-shifted emission (Salih et al. 2000).

Finally, photons that pass through the tissue are backscattered by the aragonite skeleton, further enhancing tissue scalar irradiance and thus photon availability for zooxanthellae photosynthesis (Enriquez et al. 2005; Marcelino et al. 2013).

Spectral scalar irradiance at the upper surfaces of faviid corals (E0) differed markedly from the incident downwelling irradiance (Ed; Fig. 3). Depending on the wavelength in the PAR region, the E0 : Ed ratio varied between 0.8 and 2.4, with the most pronounced enhance- ment at wavelengths 500–640 nm and . 680 nm (Fig. 3a–c).

[Ed:  500-640 nm is green of plant and human-color-vision “fame”, and 680nm is far-red like the light emitted from chlorophyll.]

it will be useful to compare differences between coenosarc and polyp tissue because they differ in total light exposure and spectral quality (Figs. 3, 4; Wangpraseurt et al. 2012a) and can exhibit different patterns of photoacclimation (Ralph et al. 2002).

There’s much more in there too!

They don’t go into this angle, but I think either all of, or a significant portion of, this “scalar light” other than the incident downwelling is actually “waste light”from the coral “blowing off” excess photons during photo-saturation.

Makes sense as green light isn’t well-absorbed and red light is low-function in the photosynthesis cycle.


“Light beyond 700nm has insufficient quantum yeild to drive photosynthesis.”

680nm may as well be 700nm as far as this goes….”mission accomplished” in modern parlance.

The photons have had their “kick” removed in the red cases, or they’ve been made ultra-reflective in the green cases.

Seemingly contradictory to the above  (and also mentioned at that link) is the Emerson effect, where chlorophyll can use 680nm + 700nm light to significantly boost photosynthesis.

Very interesting!

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