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Light attenuation by plants
The discussion on light penetration and effects of shading are fascinating.
I am grateful to Wright and Charley for 'illuminating' this obscure subject.
George says
>>A taller tank DOES need proportionally higher light at the surface if low
foreground plants are too receive enough light.
Clearly, the light changes as it is interrupted by tall plants in our
planted aquaria. But how? Also, do all plants need the same quality (and
intensity) light. Is it possible that plants that like to grow in unshaded
conditions need a different quality light than those that are used to
growing under the canopy of the forest or under other aquatic plants. For
example Glossostigma and Lilaeopsis need more than my chainswords and
crypts. Nevertheless, it would be nice to know what light gets through the
tall and floating plants in our tanks.
The spectral distribution of light changes according to what it passes
through. The Adley book (Dynamic Aquarium) reproduces the figure taken from
Gates (1980) which I attempt to show as a series of ascii drawing below.
[this was my therapy this morning :) ]. Gates' figure highlights the
differences between full sun, cloudy light and light under a tree canopy. I
would love for the experts to comment. With my naive interpretation it seems
that the former two may be appropriate for marine systems and for plants
that like to grow in full sun, while many freshwater plants may be more used
to the latter.
Several conclusions I draw from this figure are:
-an artificial light that simulates the sun may not be the most appropriate
for our aquaria. While some bulbs try to match the red and blue absorption
regions of plants, others try to appeal to our visual appeal. Others may
follow different principles: According to Doug's recent response from ADA,
Amano's fluorescent lights are high in green (From my discussion with him, I
think this is intended to simulate an overcast sky). Dennerle markets their
TROCAL bulb that is intentionally low in blue light (to discourage algae).
-there are changes in the light spectrum outside the visible range which
are probably important to plants (especially in the far red region).
- there may be similar changes in the quality of light caused by tall
aquatic plants shading others below.
Hopefully these figure will shed some light (and hopefully stimulate further
discussion). I am largely in the dark on this subject. :-)
Taken from Gates (1980)_Biophysical Ecology_ :
B G Y R
|...............|.....|.....|...|.........|...........|
0.3 0.4 0.5 0.6 0.7 1.0 10.0
Wave length (um)
| -ultraviolet- | -- visible ---|----infrared ---- |
_
| / | || | \
| / | | |
| / _ | | | \
| direct ---> ___/ || |
| Sunlight / || | |
| / /\ \
| / / \ | || |
| / / \ \
| / - Cloud light \ -
| / --> \ \
| / \ -
|../..................................................|
| -ultraviolet- | -- visible ---|----infrared ---- |
_
| / | || |
| / | |
| / _ |_| \
| direct ---> ___/ || |
| Sunlight / || ||
| /
| / | || |
| / \
| / skylight (shade)
| / -------_____ \
| / -
|../..................................................|
| -ultraviolet- | -- visible ---|----infrared ---- |
_
| / | || |
| / | |
| / _ |_| \
| direct ---> ___/ || |
| Sunlight / || ||
| /
| / ____
| / || | | \
| / filtered ----> | |
| / thru veget.______ | | \-
| / __________------ |
|../....................................\.............|
From the last drawing, you can see that the spectral distribution of light
changes dramatically as it passes through green foliage. The slope of the
irradiance curve is reasonably linear and is approximately 2 over the
visible spectrum. The blue is reduced more than the long waves, and there is
a significant amount of far red light (.7-.9 um) remaining.
Neil
Visit the Aquatic Gardeners Association home page
at <http://blake.oit.unc.edu/~fish/aga/>