Re: Aquatic Plants Digest V4 #410

[busko at stsci_edu (Ivo Busko)]

Wright Huntley <huntley1 at home_com> wrote:

> > 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 ? According
to the "pure H2O" curve, which is the one relevant for this purpose, a 
16" layer of pure water should absob 15-20 % of the ligth at the 600-700 
nm band, about 5% at the visible range and nothing at the blue. Before 
resorting to that article I did a through (I think) internet search on 
the topic and found nothing usable. Maybe the true oldtimers (I'm not 
*that* old in this list !) that participated in the
"blue-water-in-a-white-bucket" thread could enlighten us with more
accurate data ? :-)

-Ivo Busko
 Baltimore, MD

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