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Re: Light spectrum
Paul Westall writes:
<snip>> I have read that "most" aquarium =
> plants prefer a buld with color temp less than 5300-5400K. But, many =
> people on the list seem to have great success with higher temp, bluer, =
> bulbs. In the past I have used 5500K metal halide which seemed to work =
> well. Do you think that 6500K flourescent bulbs are too blue for most =
> freshwater plants? Or, how about a mix of Chroma50 and Chroma 75?
> Paul
I've been thinking about this lately. Here's my opinion:
Many of the "aquarium" bulbs being sold are aimed at Reef tanks. Marine
coraline algae are found in nature at very great depths. The deeper you go,
the bluer the available spectrum of light. This is because water filters out
light by absorbing its energy. Light at the red end of the visible spectrum
gets absorbed first. So by the time you are down three or four hundred feet,
all the algae can use is blue. For some reason the specific chlorophylls in
this coralline algae is "focused" (okay, not scientifically accurate, but a
reasonably understandable explanation). on this available blue color. So the
aquarium trade has come up with bulbs that have lots of "blue" phosphors,
rendering colors "temperatures" as high as 10,000*K.
So then hobbyists in the freshwater end of the hobby start seeing all these
high temp, spendy bulbs. They strat thinking "Gee. There must be something
great about these bulbs." So they pop the big ticket, take them home, and get
great results. And so the myth grows.
But in reality, these high-temp bulbs are also higher in total light output
than the cheap bulbs, so there is more light available. Period.
The freshwater plants we grow in our tanks are commonly found at depths of ten
feet or less. Ten feet is not enough depth to lose the available red light.
Anyone who has ever picked up a ring or coin at the bottom of the deep end, or
touched the drain just to prove they could, can verify this. So the plants in
our tanks will use all the light available. Sure, some of it is green light
and gets reflected back to some degree. Dark green plants like Anubius will
absorb more and reflect less green than a light green plant like Wisteria.
There has been some discussion on this list as to whether plants have
alternative pigments that can capture the green part of the spectrum for
photosynthetic use. The final conclusion was, yesh, maybe some plants, but
probably not others.
But light at both ends of the spectrum will be utilized. A lot of folks try
to use light at the blue end by buying bulbs at or above 7500*K. That's cool,
because they work good. Other folks buy cool whites, which run around 3500*K,
and dispite the fact that we rant at them that tese bulbs really stink for
growing plants, they get good results, too (see: Bumblebee aerodynamics).
Others still try to get an output temp of 6500-7000*K, because this most
closely resembles natural daylight. They get good growth, too, and they like
the way this light displays their tanks.
In the end, the meaningful issue is 'Photosynthetically Available Radiation'.
IMHO this is really hard to determine, because if coralline algae has
chlorophyl that seeks a specific end of the spectrum, then how do we know, or
even how can we assume that all the freshwater species we grow are going to
have chlorophyl that responds exactly the same to a given frequency of light.
So instead of concentrating on a bulb's 'tepmerature', I am working on getting
as much light as I can in my first plant tank, which I will get running in the
next few weeks. I'm going to pack 56 watts of GE cool white flourescents over
a ten gallon tank. If the spectrum range of the bulbs isn't perfect, I think
I will make up for it by giving bunches of it.
Bob Dixon