I agree, not much has been done specifically in planted aquariums.
We have done growth studies at various depths and micromols on a few aquatic weeds.
They have entire text on aquatic photosynthesis(I'm looking at a couple right now). It's well studied, but................this is an aquarium, not a terrestrial or natural aquatic system. It's it's own unique system.
Same with each of the 400 or so species commonly kept in aquariums, very few have ever been studied horticulturally and in any real depth, of those, mostly noxious weeds which few aquarist want to keep or are allow too.
However, many folks(myself and quite a few these days) have tested and compared various brands of lighting, asked several questions that make more applied sense to the horticultural question/s. We get the light curves and then compare those to the goals aquarist have.
These are what matters to aquarists.
How does light affect growth through time and space, or on this sized shape of aquarium? What is the best evenness of light spread and what types of light do that? How do I change the intensity easily to control growth rates and algae, and thus=> CO2 demand which influences=> nutrient demand? What light should I buy for my tank?
Some folks want to make a model/calculator to plug a various brand/bulb types and then it would give them a PAR curve for their tank based on the data from a PAR meter, distance plots, in water, ballast type, bulb type, reflector type etc.
Takes measuring a lot of brands etc, but can be done, bit by bit, slowly.
Then new folks can predict the light they need for a given goal.
We know what a micmol range is required for nice slow decent growth for most species already based on nice scapes that have been measured usign a wide range of species(maybe 200 or so thus far)
Aquatic plants live in a varied environment, more so than other terrestrial systems typically. This is due to the water and turbidity, tannins etc, depths and water level changes throughout the year/seasons.
They can and do adapt fairly rapidly to changes to the light using Chl a/b ratios, accessory pigments that catch other wave lengths and funnel the energy into the reaction centers for use, most are low light "shade" plants.
Chambers and Lambert's text is good in how plants adapt to various light intensity and quality.
While we can make arguments against using PAR, it is what most use for comparative purposes when growing similar plants and we want to see the effect of light intensity from the same source. Plants adapt and do it well, there's not much difference in aquariums here. Some have suggested using PUR, but again, there's a practical end to this issue, PAR is better than say Watt/gal, and PAR meters are lots cheaper than say a PUR system ........particularluy if it can go underwater.
An oft cited study in this hobby done comparing Cool whites to fancy plant bulbs showed no significance difference. Many aquarist also report the same, however, the aesthetics of cool white are hideous.
"Critical experiments show that maximum growth of most plants under cool white fluorescent lights will be equivalent to or better than that obtained under the blue-red phosphors. Work by V. A. Helson, Canada Department of Agriculture, Ottawa; and J. W. Bartok and R. A. Ashley, University of Connecticut, indicate that there is no advantage to the use of blue-red fluorescent tubes except for aesthetic purposes. Some of the names of these special tubes are Grolux, Plant-Gro, Plant Light, Vita Light and Optima. The higher cost of these fluorescent tubes may be justified on experimental or aesthetic grounds but is hardly warranted on the basis of plant growth."
I read the orignal paper a decade or so ago, basic stuff, but cannot not find the reference since. If there's no difference really between the growth rates(RGR's), then this really does not matter much as long as the PAR is 400-700nm really. Most FL's and MH's fall into that grouping.
Aquarist are not really that interested in every last photon or doing large scale monoculture, they have a wide range of goals. Some only care about color and not anything to do with RGR's.
N is not always plentiful(some limit it and forget to dose KNO3 etc and not all plants have the same N demands) and no aquarist I've met will do this for each plant and for each species. A Li COR 6400 will do this as well without destructive sampling but not for submersed plant growth.
RGR's also can do this if you are interested in light per unit of biomass yield.
Aquarist tend to want to know what amount of light do I need to have a certain aesthetic goal.
Ther Apogee PAR meter is a useful meter for this comparative purpose and at 200-300$, and tested side by side to the LiCOR 193 intengrating sensor was only off by less than 5% down to 80cm depth. Not bad.
Horticulture adds an aesthetics factor and is dependent of that person's goal/s.
A good question is what is in the min amoutn of light in Micromols (LCP) and what is the max light where no more growth is produced for a set of aquatic plants??
Here's a good paper that also includes CO2 compensation points as well:
http://www.plantphysiol.org/cgi/reprint/58/6/761
Read that and the conversions for mM of CO2 to ppm is 44
So at 20-30ppm or so(0.5mM to .8mM), we have non limiting CO2 for any light intensity. At least for these 3 very agressive aquatic weeds.
Aquarist typically use 20-30ppm of CO2.
Hydrilla has a LCP of about 12 micromols, not much.
Plants also can take the resources from a higher light region and extend into even more shaded regions in hopes of gaining nutrients and CO2 less than the LCP, Crypts, and anything with rhizomes, tubers, buds, stolons can do this.
Still, a PAR meter is pretty useful and a practical tool to compare light between aquariums, lighting types and brands, space and time changes, for aquarist.
It'd be nice to do all sorts of test, but unless you have the time, and $, some trade offs have to be made.
At 200-300$, a PAR meter shared amongst aquarist clubs works very nicely.
ADA's lights put out much less intensity than I and anyone had thought.
This explained a great deal as far as why they had little issues, when we thought the light was almost 2x as much using the watt/gal rule.
Less light= less CO2 demand= less nutrient demand= easier horticulture, but slower more managed growth rates.
Most aquarist seem to like that goal when they use CO2, but many think more light is better, it's not, it's just more work.
Check out that paper.
If you want more, I can suggest a ton of references.
If you find that one from the Canandians on cool whites, I'd be interested.
Regards,
Tom Barr
