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Re: Aquatic Plants Digest V4 #410

Wright Huntley <huntley1 at home_com> wrote:

> > Date: Fri, 14 Jul 2000 09:24:51 -0400 (EDT)
> > From: busko at stsci_edu (Ivo Busko)
> > Subject: Ligthing
> > 
> > Wright Huntley <huntley1 at home_com> wrote:
> > 
> > <snip>
> > > There is *no* effective attenuation of the photosynthetically active
> > > spectrum in clear tanks that are less than a few *meters* deep. If you
> > <snip>
> > 
> > Not quite so. I converted the curves published at
> > http://www.aquabotanic.com/paper2-6.html from their standard 1m depth to a
> > "typical" aquarium depth of 16". For pure water, absorption at the blue end
> > (400-500 nm) is effectively zero. But for all other cases: pure water at the
> > red end (600-700 nm), and water with a reasonable amount of organics,
> > absorption is in the range 20-50%. Significant IMO. Of course, this should
> > affect mostly low lying plants. Stem plants and large swords for instance,
> > should see less of the effect.
> 
> That was truly turbid water, when compared to our crystal-clear planted
> aquaria. Try it yourself (which I did some years ago). You would be
> disgusted at the cloudiness of a tank with those parameters.
> 
> With our usual filtering, our tanks would compare favorably with the *old*
> Lake Tahoe for clarity. The pure water curve *is* more red-absorptive than I
> expected. That, of course is the photosynthetically more active part of the
> spectrum, too.
> 
> Nevertheless, I contend the *difference* in substrate-level available light
> between an 18"-deep tank and a 24"-deep tank is essentially insignificant.
> 
> Hey Ivo! I think you have done a wonderful job of resurrecting the old
> "blue-water-in-a-white-bucket" thread! <VBG>
> 
> Wright

Sorry if I misunderstood, but you mean the article is wrong ? So someone
must post an erratum ! In particular the "pure H2O" curve, which of course is 
the most sensible one for our purposes. According to that curve the absorption 
at 16" of pure water is more than 20% at the 600-700 nm band. What would be
the correct value then ?

Before resorting to that paper I did a through (I think) internet search for 
data on absorption of ligth by water and found nothing usable. Maybe real
oldtimers (I'm not *that* old in this list !) that participated in the
"blue-water-in-a-white-bucket" thread can enligthen me with good data ? :-)

- Ivo Busko
  Baltimore, MD