Re: CO2 concentration and the Prandlt layer

>From: Stephen.Pushak at saudan_HAC.COM

<discussion of strong currents and boundary layer>

>With this one can reduce the required concentration of CO2 
>and still achieve optimal growth rates. IMO, it also makes the
>aquarium look nicer to have the plants moving and swaying. The
>problem is if you have a good strong powerhead, any long stem
>plants growing in the current end up growing sideways!
>It's also more difficult than you'd think to get a good current
>to reach all parts of the tank when it's overgrown as mine tends
>to become occasionally. Need more powerheads!
>Some may argue (correctly) that increasing the circulation inside
>the aquarium will increase the diffusion of dissolved CO2 to the
>atmosphere. That is true however the dramatic increase of
>available CO2 at the leaf surface should more than compensate
>for the new lower steady state concentration of CO2 (assuming
>constant input).

Although increasing the circulation in a tank will almost always increase the
CO2 loss, there are ways of increasing it in such a way as to keep the surface
interaction relatively low.

I use injector heads rising up from a manifold under my substrate to provide a
swirl flow in my tanks.  The water in the tank doesn't stratify, but it does
proceed around the tank and if you watch a piece of debris for a while, you'll
see it cover the entire area of the tank within a few minutes.

Obviously, the swirl in my tank isn't strong enough to produce a whirlpool, but
since it isn't that strong, it also isn't strong enough to significantly sway
my stem plants or valisneria.  The H. polysperma and H. difformis seem to be
doing a good job of growing as if there isn't much of a current, even though
they sway gently all the time.  The Alternathera grows straight up, and the
Valisneria (right next to one of the diffusers) spreads out normally too.  

I'm guessing here, but I think the current towards the top of the tank is
significantly weaker than the current around the bottom, and this may be the
reason why I don't seem to have much of a problem.

Anyways, this may help people who are trying to increase nutrient uptake
efficiency without hurting their CO2 retention.

David W. Webb
Enterprise Computing Provisioning
Texas Instruments Inc. Dallas, TX USA
(214) 575-3443 (voice)  MSGID:       DAWB
(214) 575-4853 (fax)    Internet:    dwebb at ti_com
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