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

Any one that choses to make a stink about how N does not affect CO2, or how CO2 does not affect N in aquariums and how Tom Barr does not know everything ought to read this article:

IngentaConnect The interaction between elevated carbon dioxide and nitrogen nutr...

There is a mountain of evidence. It's there, it's been done and all they have to do is a little web search. You want to learn nutrient "balance"?
Learn CO2 first, then you can do that. Or, learn nutrients first, then light and then you have isolated CO2(or have you? Current plays a huge role).

Do not believe everything you think. We all make assumptions that are bad at some point. And do not be too hard on yourself, we are all learning here.


Regards,
Tom Barr

Hopson and Zimba(1998) also showed that swirling plants in a flask for 45 second knocked off about 90% of the attached algae species. More current => reduced habitat for many algae. However, some are well adapted to high current, BBA, and GSA in particular.

Response to flash flooding:
Blackwell Synergy - Biotropica, Volume 33 Issue 4 Page 566-572, December 2001 (Article Abstract)

Najas and other species of aquatic plants simply grow like mad to maintain populations as seasons change and floods occur, what about Crypts and Swords?

They form strong roots and rhizomes to avoid being swept away by the current.
Najas does not invest heavily into roots and rhizomes so needs to produce seeds and grow fast and is an annual plant.

Some Effects of Low-velocity Currents on the Metabolism of Aquatic Macrophytes -- WESTLAKE 18 (2): 187 -- Journal of Experimental Botany

This gives an idea of the current ranges.

SpringerLink - Journal Article

this one is good and specific:

Resistance to CO2 Fixation in the Submerged Aquatic Macrophyte Callitriche stagnalis Scop -- MADSEN 35 (3): 338 -- Journal of Experimental Botany

Interesting one O2 loss inside plant tissue:
Blackwell Synergy - Plant Cell Environ, Volume 12 Issue 3 Page 293-299, April 1989 (Article Abstract)

This concept is also present in algae as well.

http://www.aslo.org/lo/toc/vol_32/issue_6/1181.pdf

Blackwell Synergy - Freshwater Biol, Volume 29 Issue 1 Page 7-17, February 1993 (Article Abstract)

Blackwell Synergy - Freshwater Biol, Volume 51 Issue 7 Page 1331-1340, July 2006 (Article Abstract)

CAT.INIST

Interesting paper on secondary rising flows. which like the CO2 mist bubbles rising up, may play a large role:

http://web.mit.edu/nepf/www/pdf/1072.pdf

Point is, there is a mountain of evidence from many research groups around the world suggesting that low current can reduce uptake and increase the boundary layer for CO2 as well as other nutrients.

Dense aquatic plant beds/groups like we normally keep also greatly reduce flow rates, finding methods to reduce this without causing a torrent in the tank and by producing better flow patterns can really dramatically help aquarist to grow plants better, add less CO2 without sacrificing growth rates, improve fish health and vigor and reduce algae.

So........when I nag about adding more CO2, or you hear about someone that claims they are adding so much CO2 that their fish are gasoing, ask about current and flow patterns, not just ppms of CO2.

Things are complex in aquariums and the processes not always so simple as excess NO3, or add more CO2, however, when you do a controlled experiment and isolate things, you can find the causes, but other things like current also affect CO2 exchange, so that is only one part of the answer, it's not limiting nutrients really, it's limiting nutrients to limit CO2 , which is the real cause of many folk's problem, not high NO3 or PO4 etc.

Such evidence strongly supports this reason, whereas there is no support for the high NO3 theory causing algae or fish health issues.

Think about that.

Many cases are a simpler matter, they have decent current and flow and adding a bit more CO2 is all it takes, however, others might not. It might be the same reason, low CO2, that's causing the problem, however, the current between the tanks is not the same. So it's still CO2 at the root of the matter, just need to look at why the CO2 is not getting to the plants very well.



Regards,
Tom Barr

Kick @$$ reference material ! Really Good Stuff !!!

This is exactly why I come here. Thanks Again Tom.

Hi Tom, to me this is very logical. I see in my tanks that where circulation is poor or stagnant that is the first place algae starts to grow. All my tanks with your DIY internal reactor have a power head close by to blast the CO2 enriched water around the tank.

Well it is not very logical to many on the forums.

They think it's solely an issue of CO2 ppms. pH/KH.

And they think that CO2 is the only thing that influences fish respiration.

So they think that's the only part, yet in the same admit "that there are many things we do not yet understand about aquatic plants".

Yeee uhuh

Regards,
Tom Barr

Does this need for circulation apply equally to other nutrients, such as NO3-, PO4-, K+? Those are ions in the water, a little more simple than CO2, so do they become locally depleted with poor circulation as CO2 does? I think the answer is yes, but my reason is far from theorectical - when I pour my fertilizer solution into the tank I can "track" it for awhile by the refraction of light due to the slight difference in density of that solution vs. the tank water. The variation doesn't disappear very rapidly, indicating that the nutrients take at least many seconds to move around in the tank, and that's with at least moderately good circulation. If I'm not way off base it looks like it isn't just CO2 that requires good circulation of the water.

Good question.
The answer is: yes!


Yes, but they are needed at far less concentrations and mass.
Carbon is 45% of the plant, N? 1.5%.


You'd be correct.

Regards,
Tom Barr