# RE:[APD] LUX and photosynthesis

```Meryl Kwan wrote:
> I noticed that the list of "compensation points" has floating
> plants like, highest.

I think you might mean Light Saturation Point, not compensation point;
floating & terrestrial plants have the highest LSP; that is to say they
can utilize intense light more effectively. The Light Compensation Point
(LCP) is the light level where a particular plant species is just able
to produce a net gain in carbon from photosynthesis. If a plant of algae
is illuminated below its LCP, it will slowly die.

> Here's my questions:
> ** What's the values for some algae?

I could not easily find the answer to this question. I think you want to
know the LCP & LSP for several species of algae.

> ** What's the values for BGA when is floats on the surface
> (as a colony)

I could not easily find the answer to this question. I think you want to
know the LCP & LSP for typical species of Cyanobacteria especially those
which grow on the water surface.

> ** How many lux does a very lightly planted 55 gallon with
> 110 watts of CF light have

If the 55 gallon tank has a height of 18" and a base area of 706 square
inches (.455 square meters) and we assume an LCP efficiency of 0.4 or
40% and if 110 watts of CF lighting produce 9350 lumens (85 lumens/watt
for CF lighting) then the average light intensity will be 9350 * 0.4 /
.455 = 8220 lux on the base of the aquarium with no plants. The actual
light intensity on the top leaves will approximate this while those
underneath will have much less. You can calculate the average leaf
illumination if you know something called the leaf-area index (LAI), the
ratio of total leaf surface area to ground surface area. The average
illumination at the leaves is the base illumination divided by the LAI.

A maximum LAI, that of coniferous trees is 47. A typical LAI is around
8, for terrestrial plants. For aquatic low-light plants such as
Cryptocoryne, I would guess an LAI of 2-4 depending upon maturity &
density. If we arbitrarily define a lightly planted tank as one with an
LAI of 1 then the average illumination on the leaves will be somewhat
less than 8000 lux. This is many types the LSP of most aquatic plants.

See <http://www-eosdis.ornl.gov/VEGETATION/lai_des.html> for the basis
of my guesses regarding LAI.

> ** How many lux in a 55 gallon with 110 watts of compact
> fluorescent light when it has a lot of plants?

The difference between a lightly planted tank & a heavily planted tank
is that the leaf-area indexes will be quite different. Bear in mind that
its not necessary to have all of the leaves of a plant receiving an
optimal light intensity in order for the plant to grow. If you want to
limit light intensity at the highest point of the leaves to below the
LCP for BGA, then I suspect that as long as the light intensity at some
of the leaf surfaces is above the LCP for the plant, it can still grow,
albeit slowly. The botanists might correct me on that point. I expect
that lower shaded leaves might die off however the plant should still be
able to circulate carbohydrate nutrients to other parts of the plant.

If the tank is densely planted and we guess an LAI of 4, then the
average leaf illumination is still around 2000 lux, still about double
that necessary to keep most plants alive & growing.

Another way to estimate the light intensity at the leaves is to examine
the angle from the horizontal that they are growing. If the leaves grow
at angle alpha from the horizontal, the light intensity is given by the
cosine of alpha. The cosine of 45 degrees is 0.71 so unshaded leaves
growing at 45 degrees to the horizontal might get about 8000 * 0.71 =
5660 lux.

Using the LAI adjustment & assuming that the Allgayer & Teton values
were computed based upon illumination adjusted for LAI, then 1 WPG of CF
with a moderately effective reflector seems about right. If Allgayer &
Teton based their data upon base-surface illumination levels, then we
probably need about 0.3 to 0.5 WPG.

No one with the Allgayer & Teton book has commented about how they
conducted their experiments yet. (hint-hint)

If we could find some data for LCP & LSP for aquatic plants & algae in
uEinsteins, that would also be useful. I'm sure we can find a way to
fudge the numbers back to lumens or vice versa.

It will be quite easy to gather data for calculating the lighting
efficiency coefficients of reflectors. These are fun experiments to do.
I'd sure like to know what kind of numbers we can get for really good
reflectors in aquariums with water in them. The 45 to 55 gallon
aquariums make good candidates for testing because they are so typical
and easy to light with 4' bulbs.

Steve P

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