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Absorption and Retention of O2 and CO2

Discussion in 'CO2 Enrichment' started by aquabillpers, Sep 9, 2008.

  1. aquabillpers

    aquabillpers Lifetime Charter Member
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    There has been some discussion about the rates of absorption of oxygen and CO2, and about the rate of outgassing of the latter. To help me understand this, I have several questions.

    1. Which is more readily absorbed in water, O2 or CO2?

    If a container of water that contains no gasses is exposed to air, one would expect that the levels of O2 and CO2 would eventually rise to the level at which they existed in the atmosphere. Which gas would reach that level first?

    2. When water is agitated, dissolved gasses tend to be outgassed into the atmosphere. At the same time, the agitated water will absorb additional gasses from the atmosphere.

    Will CO2 be outgassed at a more rapid rate than O2?

    Is the rate of absorption of O2 and CO2 the same as the rate of outgassing of those two gasses?

    3. In a non CO2 injected aquarium, is it ever possible for the CO2 level to be less than that in the local atmosphere?

    Thank you!

    Biill
     
  2. SpeedEuphoria

    SpeedEuphoria Prolific Poster

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    Hey Bill, I just figured I would post these links over here also:

    starting on page 7 has good info:
    http://www.ieagreen.org.uk/oceanrep.pdf

    also this one has good info
    http://www.tos.org/oceanography/issues/issue_archive/issue_pdfs/14_4/14_4_feely_et_al.pdf

    The CO2 uptake rate depends on the PH, temp and how much CO2 is in the water/atmosphere to begin with.

    As for your question 3, I would deff say yes the CO2 levels will likely be less then atmospheric. If you have lots of plants and no surface agitation it will be much lower then atmospheric most of the time, at least during the day when plants are using up the CO2. I would say that also with great surface agitation and a high PH/temp if will not be as likely to bottom out and levels should stay closer to atmospheric. This is just my opinion as there are many variables/scenarios.

    Now talking about El natural or some king of organic substrate then that is a big difference compared to regular substrate.

    It would give Tom something to test with his nice CO2 meter, just testing same tank/plants/fish and changing the surface agitation.
     
  3. Tom Barr

    Tom Barr Founder
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    CO2, which is pretty soluble, O2 is rather insoluble.

    It'd be hard to get all the gas out. I'm not sure which might be first, nor that it matter that much. Most important is the rate of use/output and new inputs.

    I think you are painting yourself into a corner here a bit.
    When they discuss degassing, they mean it relative to the air ambient condition.
    So say 100% with air is 7ppm for O2 say at 85F.
    If you have 5ppm if the water, the air above will act as source for O2, if the plants have been growing well all day, and the O2 is not 10ppm, the water, not the air above, acts a source.

    So it depends on what the differences are between the air and water which way this goes.

    If they are both equal, agitation will not matter at all.
    You have a larger amount of CO2 added in enriched systems, so the amount of CO2 degas will be more than the O2, which we do not enrich, just whatever extra the plants add. Generally not more than 150% say 2-5ppm of O2.

    CO2 can be 20-30ppm difference.
    Even within the aquariums, you can find regions of lower O2 and CO2.


    I'm not sure.
    I'd guess they are similar.
    The diurnal time graphs suggest so from field site, but precise measurements and looking specifically at that question, I really do not know.

    Certainly, it is most of the day/light cycle BTW. Why might that be?
    It's likely a good thing.

    Regards,
    Tom Barr
     
  4. VaughnH

    VaughnH Lifetime Charter Member
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    A simple thought experiment should answer this. Assume that the rate of absorption of CO2, for example is greater than the rate of outgassing. That means that the amount of dissolved CO2 in the water would rise to a new equilibrium value. Then it would remain constant. But, to remain constant, the rates of absorption and outgassing would have to be equal, which is contrary to our assumption that the rate of absorption is greater. If the amount of dissolved CO2 in the water never did reach an equilibrium value, the concentration would increase without limit, which is physically impossible.

    In reverse, assume the rate of outgassing is greater than the rate of absorption. That has to mean that the amount in the water would drop until an equilibrium is reached. And, at that equilibrium point, the rates would be equal, again contrary to the assumption that the rate of outgassing is greater than the rate of absorption. And, if an equilibrium is not reached, the water soon has zero CO2 in it, but that, too, isn't possible, since the rate of absorption is known not to be zero, so some must always be absorbed.

    My conclusion is that there is always an equilibrium concentration of CO2 in water for any given condition of the water and the air.

    The nice thing about this type of experiment is that it doesn't require buying any equipment or supplies, although a beer does seem to make the experiment go easier.
     
  5. Carissa

    Carissa Guru Class Expert

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    Yeah, I should have had my glass of wine before I read that. Thanks for reminding me.
     
  6. Tom Barr

    Tom Barr Founder
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    Well, any time you talk about gas transfer and % saturation, take a sip:p
    Solubility makes some difference, not a lot in most of the cases in aquariums really.

    I have a nice O2 meter and have play with the CO2 meter, I suppose I could test and answer it if I still had the CO2 meter.

    The O2 I can.
    I'd have to bubble out all the other gases using N2 gas, then measure how long it takes for O2 to reach 100%. I'm too lazy:)
    Your question, you answer it:)


    Regards,
    Tom Barr
     
  7. aquabillpers

    aquabillpers Lifetime Charter Member
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    Thanks for your answers! Below is what I've learned, along with a few more questions.


    OK, CO2 is absorbed much more readily than O2.

    Therefore, in a container of water that had all CO2 and O2 removed from it, and then was exposed to the atmosphere, CO2 would reach ambient before the O2 did.

    Right?


    Is the rate of absorption of O2 and CO2 the same as the rate of outgassing of those two gasses? Assume that the amount of each gas dissolved in the water is, say, twice that of ambient. After agitation, in the water will both return to ambient at about the same time?

    Some say that it is, because the plants consume it faster than it dissolves in the water from the atmosphere. Is the rate of plant utilization of CO2 really greater than the rate of CO2 absorption from the atmosphere?

    Then Vaughn said: My conclusion is that there is always an equilibrium concentration of CO2 in water for any given condition of the water and the air.[/quote]

    Then the plants do not consume CO2 faster than it is absorbed?

    Also, in Walstad-type tanks the concentration of CO2 is higher than ambient because of decomposition. That is why excessive surface agitation is to be avoided in such tanks.

    Thanks again.

    Bill
     
  8. VaughnH

    VaughnH Lifetime Charter Member
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    Plants will consume CO2 at a rate determined by how fast they are growing, which is determined by the availability of nutrients and light. So, CO2 can be consumed faster that it is absorbed, but only for awhile, and a short while. Then, unless there is very good water circulation in the tank, no more CO2 will be available for the plants to consume. Adding growing plants prevents an equilibrium from ever being reached, other than a dynamic one, with a gradient of CO2 concentration in the water.

    In a Walstad-type tank the same reasoning would apply. Unless the light and other nutrient availabilities are low enough to restrict the plant growth rate, there may never be more CO2 in the water than ambient. Again, with very good water circulation, the amount available to the plants can be greater.

    (One more sip of wine - Oh heck, one more glass!)
     
  9. Tom Barr

    Tom Barr Founder
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    When it comes to which one would be absorbed faster, O2 is less soluble, however, there is a lot more concentration of O2 in the air above. CO2 is far more soluble, but there's a lot less of it.

    Fick's 1st and second law of diffusion addresses this:

    Fick's law of diffusion - Wikipedia, the free encyclopedia

    The concentration gradient matters in other words, not just solubility.
    The coefficient of diffusion will be the same for both gases however.
    I'm not sure because I do not know how much the solubility and the concentrational gradient are influenced by each other with respect to time.

    Regards,
    Tom Barr
     
  10. aquabillpers

    aquabillpers Lifetime Charter Member
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    Please read Ms. Walstad's "Ecology of the Planted Aquarium", particularly pages 59 and 60 and 100 in edition 1. You are close to becoming a NPT enthusiast; this might the straw that does it.

    Page 100 explains why surface agitation should be avoided in NPT's, since it drives off the over-ambient CO2.

    Bill
     
  11. Carissa

    Carissa Guru Class Expert

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    Ok, here's a somewhat related question.

    When dosing EI, the point is to always avoid a 0 level of any nutrient, thus limiting the plants.

    Does this apply to co2? One would think that it wouldn't, since it seems that people who inject co2 need to be adding 20 - 30 ppm, not just keeping it non-zero. Right? But why does the plant care if there are 3ppm or 30ppm, as long as it's non-zero?

    Or ....does 25ppm on a drop checker simply usually mean non-zero near the plants?

    If that's the case, it would seem that a non-co2 injected tank would need to be near or at 0ppm of co2 most of the time, for carbon to be the limiting factor, right? How can the plants be limited if co2 at the plant is not non-zero?

    So if this is the case, circulation must be a good thing in non-co2 tanks with no significant source of carbon. i.e. a source of carbon that replenishes itself FASTER than atmospheric equilibrium could ever be accomplished by excellent circulation.

    Forget the glass, pass me the bottle.
     
  12. VaughnH

    VaughnH Lifetime Charter Member
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    Carissa, I read that twice, and finally realized I must have drank the whole bottle! Now, I'm not sure I understand any of this.
     
  13. jeremy v

    jeremy v Guru Class Expert

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    Carissa,

    The plants can be limited by a nutrient without the nutrient being zero. Zero is just the point at which the plant ceases growth completely, because it can’t continue without more of that nutrient.

    Tom posted something a few months ago that I think has value in this discussion.

    http://www.barrreport.com/general-plant-topics/3120-good-bookmark-worthy-link-plant-nuttrients-ratios.html

    It seems that just keeping the nutrients above zero is not the whole point, but only part of it. Zero is just the point at which the plants completely stop growing. Anything above zero just means that the plant CAN grow (or at least maintain itself), not that it will be growing ideally, efficiently, and at maximum speed.

    Plants will continue to grow faster the more of any nutrient they have all the way up until that particular nutrient reaches the point of “luxury uptake”.

    It would also make sense to me that (if light levels remain the same) the higher the nutrient and CO2 levels in a tank, the less important circulation is, because a single unit of water has more nutrients in it, so that unit of water can sit on a leaf for a longer period of time before being depleted. That would add credence to your statement of circulation possibly being more important in a non-CO2 tank than is now accepted, since the levels of CO2 are low to begin with.

    I think of plants and nutrient levels being like humans and the air we breathe. Lower nutrient concentrations in the water are just like trying to breathe while on the top of a mountain where the air is thinner. You can still grow, survive, and live, but your total energy levels are lessened and it takes much more effort to do anything. Now take that same breath at sea level where the oxygen levels in the atmosphere are much higher. You can live and do everything you want to do much more easily.

    Along the same train of thought, I think of high light CO2 injected tanks as being like trying to live on the summit of Everest. It is possible, but unless you are a Sherpa that has adapted to that atmosphere, you will most likely need an oxygen tank to do so. If your oxygen tank runs out things go bad quickly, just like they do in a high light tank when the CO2 runs out, haha.

    Have a good one, Jeremy
     
  14. jeremy v

    jeremy v Guru Class Expert

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    Aquabillpers,

    I think surface agitation is a tricky thing that has more factors than just whether or not you are injecting CO2. Diana Walstad is using a complete system to get the plants to grow how she does. One of the main parts of that system is the fertile soil and the fish wastes which remain in the tank and combine with the dense soil to add a lot of bacterial action to the tank. That increased bacterial action would enrich the tank with higher than equilibrium levels of CO2 if the water surface isn't disturbed enough to cause it to become lost to the air.

    One thing I have found, is that if you try to combine an EI style tank (that doses nutrients in the water column instead of having a rich dense substrate) you run into problems very quickly without having enough circulation in a non-CO2 tank. I think it is because with water column fertilization (and a pretty neutral substrate) there isn't much bacterial action adding CO2 to the tank water. In that situation the plants quickly strip the water of CO2 and then they can no longer grow (assuming they can't use carbonates for a carbon source instead) since the only additional replacement CO2 of any magnitude (from the air at the air-water interface) is almost nonexistent because of the low circulation levels.

    Have a good one, Jeremy
     
  15. Tom Barr

    Tom Barr Founder
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    You got it Jermey,

    But folks will not think that much about it in general forums, public plant forums etc, they will get lost, the wind bags that want to yak will critique anything you say and never get the big picture.

    I think while some might understand this and care about it:

    The Relative Nutrient Requirements of Plants

    Most will not really read it or try and understand the point.
    I would not have years ago myself.
    Today I do. Things change, so do folks.
    Some however will never care. That's fine, I might not ever care much about knitting....but some are very passionate about it. Maybe I'll change later on, but I doubt it:eek:

    But it does matter enormously when you start discussing all the things that influence aquatic plant growth.

    Since growth is a wide range of parameters that allows plants to "grow", zero and non limiting nutrients(say EI with high light/CO2 etc), allows folks to run the entire range of possible growth rates.

    Everything as far as a method is defined by that rate of growth.

    Light, CO2 and nutrients, dosing routines, location of the nutrients, plant species etc.

    Trying to make something simple and reducing it all down simply because one clown blow hard happened upon a method that worked for them does not imply it will work for everyone.

    Many wind bags spend a lot of time trying to suggest why their method is the best and all the others are bad, excessive, bad for shrimp, fish, wasteful, does not add enough, the wrong ratio of nutrients, spiritual, poetry, natural etc...........

    You can shine a cow paddy many different ways:cool:

    Rather than that, I've focused on the range of plant growth parameters and explore them in the context of a planted aquarium and with fish, shrimp etc.
    Knowing the range allows us to weigh the trade offs, know the risks and design something specific for our goal.

    What method allows for good very slow easy to care for planted tanks?
    What method can rule out nutrient related issues so I can focus on CO2?
    What methods can I use to limit the rates of growth with PO4?
    What method is bets for my Dutch gardening tank?
    How might light change the method, the rates of growth, CO2 demand or uptake of N, P, Fe etc as they go from low to high?

    The answers cannot possibly be the same in all cases.
    Just make sure to think about parameters as they go from zero to very high and how they might influence the others.

    Also, realize that many simply are never going to think about it this way and will fall into their own routines that work for them. Many will never realize why the other methods work nor master them. Generally, that's not a goal for them, they just want a planted tank that works for them personally.

    Nothing more.



    Regards,
    Tom Barr
     
  16. Tom Barr

    Tom Barr Founder
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    Still, if you consider low surface movement and all that bacterial action...............and you have low rates of plant growth(but still some), where does all the CO2 come from?

    E.g. The bacteria are aerobic, so they much burn the reduced carbon to produce this CO2, and if there's not much O2 and it's being used up fast, how's this good for fish?

    A balance must be struck with good stable O2 levels coming in and stable CO2 levels going out.

    Otherwise very low O2/high CO2 situation occurs and then lower O2 levels reduces the CO2 from fish and also bacteria.

    4C (reduced carbon energy source, say glucose) + 2O2 => 2CO2 + energy for the bacteria/critter/plant etc.

    Plants and algae will use the CO2 and give off O2, but only so much.





    Regards,
    Tom Barr
     
  17. Carissa

    Carissa Guru Class Expert

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    Ok this is interesting. So I would like to know - what are the levels for the particular nutrients, above which they become non-limiting?

    Do plants adapt to other nutrient levels like they adapt to co2 levels (such as, in their ability to utilize nutrients when they are present in varying concentrations)?

    I didn't think about this in my above post, the fact that plants need an enzyme to use co2 and they manufacture more or less of this in response to varying levels of co2.
     
  18. jeremy v

    jeremy v Guru Class Expert

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    Carissa,

    This is how I have been thinking. I have heard Tom and others say that most aquatic plants are around 7-1-8 N-P-K by mass. Since that can translate directly to ppm ratios of nutrients in water, I assume that to mean that if I dose Nitrogen, Phosphate, and Potassium in those ppm ratios (7:1:8), that if all the plants in the tank sucked up what they needed, that all the nutrients in the tank would all theoretically run out at the same time. This is assuming that fish wastes aren't adding any nutrients to the tank. To account for that, I usually just add a little extra Potassium to the dosing. That is just my way of trying to minimize wasting of nutrients or creating any drastically imbalanced nutrient levels.

    I would imagine that the levels of nutrients required before the plant gets into the zone of "Luxury Uptake" would be different for each plant. Maybe not though. I would think the best way to test it would be to just maintain that 7:1:8 nutrient ratio (so you know that all nutrients are in equal excess), and then increase your ferts a little each week (or every two weeks) until the plant growth stops increasing, or at least until you stop seeing any benefits. If you increased them one at a time or at different rates you would most likely run into less accurate results, because one individual nutrient uptake interacts with and affects other nutrient uptakes as well.

    I am starting to lose my fear of over fertilizing my tanks anyways, and the more I think about it the more I am starting to think there might actually be benefits other than increased plant growth by dosing higher levels of nutrients in a tank.

    I am thinking that if I dose my tank once a week (after a water change) and I dose 15ppm of nitrate, and my plants use say 10ppm of nitrate over that same period, that means that between the high and low of the week I would have a 66% drop in nitrate levels in the tank. If I dosed to 30ppm instead, that same 10ppm uptake from the plants would only cause a 33% total drop in nitrate throughout the week.

    I don't see that as being a waste of nutrients, I see that as being a more stable system, because the nutrient levels in the tank are more consistent, and increased stability usually always equals less algae issues. I also use the water change water to water plants, so the nutrients never really get wasted anyways.

    That would also create a smaller swing in nutrient levels for the plants to adjust to as well, so they could then become more efficient at taking in those nutrients, because they are dealing with a narrower range, so they can have more energy left over to put towards growth.

    I think that plants do actually adjust to optimize their uptake at different nutrient levels. I have seen Java Fern leaves of mine suddenly die in large numbers within a few days (in a very unique manner unlike regular dying leaves), and a new boost of fresh leaves come up very soon after when all that changed was that my ferts were increased. Light and CO2 remained the same. The new leaves quickly replaced the old, and they even looked a little different.

    Plant leaves and stem segment lengths change under different light levels, and plants change their leaf density, etc. under different CO2 levels. CO2 is just a nutrient, so why would the same not happen for the other nutrients as well, possibly just in more subtle (less visible) ways?

    I would think that if plants benefit from stable CO2 levels when the lights are on, then they would also benefit from all the other nutrients being as stable as possible as well.

    One thing I didn't figure out from the "Relative Nutrient Levels in Plants" related to the "Luxury Uptake" zone is this. It seemed to indicate that the plants will continue to uptake more nutrients even after the point of "Luxury Uptake" is reached, but they will just not continue to increase growth levels in response to that. Does that mean that the plant is just storing those nutrients away for a possible rainy day? If so, that would mean that having higher levels of nutrients all the time would also buffer your plants better in case of a time of lean or missing nutrients as well.

    Interesting stuff.

    These are just some thoughts I have. If anyone has any thoughts or criticisms of any of this I would like to hear them. I just want to get better at growing plants, I don't care about being right or not. I also hope that Aquabillpers isn't feeling like his thread has been hijacked, because that was not my intention.

    Have a good one, Jeremy
     
  19. Carissa

    Carissa Guru Class Expert

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    That all makes sense to me. I've been dosing that way as far as thinking about ratios - I never dose one thing at a time, I dose the same ratios and use nitrates as a guide as to when I need to be fertilizing since that's what I have a test kit for (keeping nitrates at or above 10ppm generally).

    I thought that Luxury Uptake was storage (i.e. the plant can't use it, but is storing it). In my understanding, this is why you can take some new plants and put them in a tank and they seem to do fine for a week or two, then start dying off.
     
  20. Tom Barr

    Tom Barr Founder
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    Yes, that's why the goal is to keep them in a stable range of ppm's for say NO3, CO2, K+, they will adapt and modulate to get the most out of their habitat.

    What choice do they really have?

    Main thing is to keep the environment stable.

    Here's an inducible K+ uptake enzyme(which cost the plant a lot of nutrients, energy to make and operate) example:

    [​IMG]

    [​IMG]

    There are ranges where we start to see reduced growth, but not regions where we see dramatic changes and stunting etc.

    Plants, like us, have some reserves, both for Carbon and nutrients like N and P.
    They can borrow when limited from other areas, like enzymes and chop them up and rearrange things to go after the most limiting growth nutrient, CO2 or K+, or whatever.

    This switching back and forth, has a price.
    It takes time and energy.
    And when that occurs, algae can creep in when it see the niche.

    So.........
    Also, at higher levels of CO2, say 30ppm vs 3 ppm, the enzymes run really well, concentration drives uptake(this is testable), and without more CO2, the uptake and supply is easier for the plant to assimilate.

    You can do this with DIY CO2 yeast and measure the amount of CO2 produced with various concentrations of sugar/yeast etc vs time. Simply use a test tube inverted in water to measure the productions of Co2 gas.
    Get a ruler and measure the differences.

    Simple.

    You can measure the effect of concentration, temp, pH, etc on the CO2 production.

    Likewise, using an O2 meter, you can do the same with a plant(O2 is a good measure of the by product of growth in aquatic systems).
    This is a non destructive method to measure growth of plants in whole aquariums.
    You can compare the differences this way.

    Regards,
    Tom Barr
     
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