CO2 concentration and the Prandlt layer
Dave Huebert brought up the subject of CO2 concentration and the
Prandtl layer in an offline discussion and I thought I'd bring it
to the APD for comments. A very valid point is that on the surface
of a submerged leaf, there is a thin layer of water which is not
in motion called the Prandtl boundary layer. This acts as a buffer
zone which prevents mixing or diffusion of CO2 to the leaf surface.
(the same boundary layer as in aerodynamics and fluid dynamics!)
The thickness of this layer can be reduced by inducing circulation
into the aquarium water; this concept has been mentioned before and
although we don't mention it often, it is fairly important to
maintain a good strong circulation underwater in the aquarium.
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
The question is then, how low a concentration of CO2 would be
sufficient to maintain optimal growth? Remember that we need to
provide above average growing conditions in an aquarium in
order for the plants to outcompete algae. Ideally, we need a
experimental study which attempts to quantify the results by
the average surrounding water velocity (which can be measured
accurately by pitot tube). We could presume adequate nutrient
quantities and 20-50% of light saturation which might be typical
of our HIGH lighting category of +4 watts/gal. (of course it
varies for plant type; I suppose we'll get vastly different
results studying H. poly than Anubias or Crypts)
I wonder if this might be the hidden factor in some of Dorothy
Reimer's low-tech tanks without CO2 injection?
Steve in Vancouver BC (Go Sonics Go!)