Recently, I was talking in depth with a professor about gas exchange in terrestrial systems. I asked how important was air flow to exchange CO2 and of course he said "Extremely!".
I asked another question: how important would this issue be if the medium was changed to water? Something far denser?
"Far more important!"
We often assume that the tank water is homogenous in terms of CO2 concentration.
In longer tanks with sensitive equipment, we can measure some water column differences. However, there is a scale, one that is very important and presents perhaps the greatest resistance to diffusion: the boundary layer, sometimes referred to as the "Prandtl boundary layer".
Some basic references to show that in fact flow and CO2 are related:
Crossley et al, 2002
SpringerLink - Journal Article
Nutrient demand increases as CO2 increases. when there's low CO2, then there will be little nutrient demand and little growth. This makes sense.
What about when exposed to air and allowing high rates of gas exchange?
I mentioned intense pearling and growth after exposure to air during large water changes several years ago.
Effects of Air Contact on Growth, Inorganic Carbon Sources, and Nitrogen Uptake by an Amphibious Freshwater Macrophyte -- Madsen and Breinholt 107 (1): 149 -- PLANT PHYSIOLOGY
In Madsen's paper, we see yet another link between no net effect if the nutrients are high or low, until we remove the CO2 limitation, then the plants grow like mad(3x faster for this plant). This is common in every study done on aquatic plants.
There is a defining theme here.
But I digress...............
One part of the boundary layer issue is laminar versus turbulent flows around the leaf. With more current and energy, more turbulence occurs.
Adding to this, the CO2 that is being added is actually getting to where it needs to go, the plants! Also, O2 if low in the water can diffuse from the air at the surface into the water.
So current is a critical component for fish and plants.
More so than many of us often acknowledge.
I've often suggested "adding more CO2".
I sound like a broken record with that advice.
I get folks that have issues with KH and the measurement of CO2.
I get folks with issues with their fish and cannot add 30ppm because they gasp.
I get folks claiming that the plants are still not growing with 30ppm.
CO2 alone is not the whole picture, just like adding some Trace iron is not going to solve everyone's plant problems.
It's also the flow and current in the tank.
When we give advice, it is wiser to ask and suggest current and CO2, as well as good careful observations when adding more and getting to optimal amounts.
This is much different than doing "whatever works".
Low limiting levels of N, P, etc can reduce CO2 demand down as well, but then you do not have an optimal CO2 under all conditions, just a certain set of conditions.
By finding the optimal conditions for all plants and each parameter, then you can best optimize a method and reduce algae.
CO2 is critical (if additions are used).
Flow/current is critical by default if you use CO2.
However, flow is much harder to measure in aquariums than NO3 or CO2.
And the scales involved tend to be a mm or so thick, yet can reduce growth down many times.
CO2 mist, the idea that gas phase bubbles can help increase growth rates may be nothing more than this, the mist breaking up and helping increase diffusion rates by decreasing the resistance to diffusion of CO2(and expelling of O2).
Using N2 gas can answer this, and it can be measured and quantified.
I'm not sure if it's been done prior, but it would be relative simple.
Any difference between gas mist due to boundary layer impacts could be measured in isolation, then adding CO2 gas phase back, we could compare the effects of pure CO2 [gas] vs CO2 dissolved [aq].
This can be measured as O2 production via the plant's growth, a standard unit of measure in aquatic systems.
In any event, regardless of the theory there, the theory regarding the boundary layer is certainly a real one and there's a lot of evidence to suggest that flow rates are a very dominant factor for exchange of nutrients and especially gases like O2 and CO2.
Regards,
Tom Barr
I asked another question: how important would this issue be if the medium was changed to water? Something far denser?
"Far more important!"
We often assume that the tank water is homogenous in terms of CO2 concentration.
In longer tanks with sensitive equipment, we can measure some water column differences. However, there is a scale, one that is very important and presents perhaps the greatest resistance to diffusion: the boundary layer, sometimes referred to as the "Prandtl boundary layer".
Some basic references to show that in fact flow and CO2 are related:
Crossley et al, 2002
SpringerLink - Journal Article
Nutrient demand increases as CO2 increases. when there's low CO2, then there will be little nutrient demand and little growth. This makes sense.
What about when exposed to air and allowing high rates of gas exchange?
I mentioned intense pearling and growth after exposure to air during large water changes several years ago.
Effects of Air Contact on Growth, Inorganic Carbon Sources, and Nitrogen Uptake by an Amphibious Freshwater Macrophyte -- Madsen and Breinholt 107 (1): 149 -- PLANT PHYSIOLOGY
In Madsen's paper, we see yet another link between no net effect if the nutrients are high or low, until we remove the CO2 limitation, then the plants grow like mad(3x faster for this plant). This is common in every study done on aquatic plants.
There is a defining theme here.
But I digress...............
One part of the boundary layer issue is laminar versus turbulent flows around the leaf. With more current and energy, more turbulence occurs.
Adding to this, the CO2 that is being added is actually getting to where it needs to go, the plants! Also, O2 if low in the water can diffuse from the air at the surface into the water.
So current is a critical component for fish and plants.
More so than many of us often acknowledge.
I've often suggested "adding more CO2".
I sound like a broken record with that advice.
I get folks that have issues with KH and the measurement of CO2.
I get folks with issues with their fish and cannot add 30ppm because they gasp.
I get folks claiming that the plants are still not growing with 30ppm.
CO2 alone is not the whole picture, just like adding some Trace iron is not going to solve everyone's plant problems.
It's also the flow and current in the tank.
When we give advice, it is wiser to ask and suggest current and CO2, as well as good careful observations when adding more and getting to optimal amounts.
This is much different than doing "whatever works".
Low limiting levels of N, P, etc can reduce CO2 demand down as well, but then you do not have an optimal CO2 under all conditions, just a certain set of conditions.
By finding the optimal conditions for all plants and each parameter, then you can best optimize a method and reduce algae.
CO2 is critical (if additions are used).
Flow/current is critical by default if you use CO2.
However, flow is much harder to measure in aquariums than NO3 or CO2.
And the scales involved tend to be a mm or so thick, yet can reduce growth down many times.
CO2 mist, the idea that gas phase bubbles can help increase growth rates may be nothing more than this, the mist breaking up and helping increase diffusion rates by decreasing the resistance to diffusion of CO2(and expelling of O2).
Using N2 gas can answer this, and it can be measured and quantified.
I'm not sure if it's been done prior, but it would be relative simple.
Any difference between gas mist due to boundary layer impacts could be measured in isolation, then adding CO2 gas phase back, we could compare the effects of pure CO2 [gas] vs CO2 dissolved [aq].
This can be measured as O2 production via the plant's growth, a standard unit of measure in aquatic systems.
In any event, regardless of the theory there, the theory regarding the boundary layer is certainly a real one and there's a lot of evidence to suggest that flow rates are a very dominant factor for exchange of nutrients and especially gases like O2 and CO2.
Regards,
Tom Barr
