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Light and CO2

Discussion in 'General Plant Topics' started by Chiya, Feb 18, 2009.

  1. Chiya

    Chiya Prolific Poster

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    Hi all,

    Been thinking about this for a long time.

    Sunlight is approx 2000mols. It's proven that we can begin growing plants 30-50mols onwards.

    However, CO2 concentration in water that we aim to achieve is approx 30ppm, or 0.03%, which is the same concentration as that of CO2 in the air that we breathe.

    Considering the huge difference in PAR between sunlight and the artificial lights that we have on our tanks, is the 30ppm CO2 concentration overkill?

    Given the evolution of planted tanks, it seems that 30ppm is the benchmark, especially if we use pressurized CO2. I know I'm sure to run into algae problems if I lower the CO2 content.. Why is that so?

    If plants in nature grow with a PAR reading of 2000mols & 30ppm CO2, why do we need 30ppm of CO2 when our lights only attain 200mols?

    Lastly, do plants in nature grow faster than plants in our tanks? Or do we always "lose" because 2000mol aquarium lights aren't available yet?

    Comments?

    Regards,
    Ryan
     
  2. Tom Barr

    Tom Barr Founder
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    You assumed that the concentration in air and the water are the same.
    We do not have 380ppm of CO2 in water at equilibrium.


    The required air concentration to bubble and maintain a 30ppm in water will be about 2000-2200 ppm in air. 25ppm will require 1800ppm in air and so forth.

    Ask yourself why this is.
    Otherwise would we even need CO2 gas enrichment, couldn't we simply bubble air into the tank at 380ppm and have more than enough?

    Why isn't the O2 level, which is 21% in air, 210,000ppm in water?
    Units, volume versus
    The partial pressure of CO2 and and the K solubility constant:

    Carbon Dioxide - Carbonic Acid Equilibrium

    Paul's reply might also help:
    Re: CO2 concentrations in water

    CO2 Concentration

    Regards,
    Tom Barr
     
  3. Chiya

    Chiya Prolific Poster

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    Hi Tom,

    Thanks for the great reads.
    Didn't know plants can get their carbon from carbonate ions too.

    Got the CO2 concept but I'm still lagging behind the light idea. :eek:

    Plants in nature are receiving sunlight and CO2. Whereas plants in our tanks receive artificial light and CO2.

    Assuming we up our aquarium lights to the maximum possible, with no limiting nutrient / CO2, are we able to achieve the same growth rates as plants do in nature?

    I'm wondering about the maximum potential growth rate for a particular plant when exposed sunlight VS aquarium lights.

    The sun produces a PAR reading of approx 2000mols, the most I've ever seen from a reef tank (MH bulb) is 800mols. So does this >2x PAR reading make any difference? Or does the 800mols meet the maximum requirement for the plant?

    Comments?

    Regards,
    Ryan

    P.S. It'll be so much easier if the sun is overkill. But then it takes the fun out of the question. :D
     
  4. ccLansman

    ccLansman Guru Class Expert

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    Just checking... were did you get 2000mol number from? It seems like the light from the sun would be a lot less, also would vary a lot due to dist from earth, clear day vs hazy day ect...
     
  5. Tom Barr

    Tom Barr Founder
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    That's generally a max value for midday in the sub and tropical regions.
    So light will be about this or less(might be a tad more in some spots, generally 2000 is an accepted full noon time sun, don't worry, we bring light meters along just in case)

    Regards,
    Tom Barr
     
  6. Tom Barr

    Tom Barr Founder
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    Plants have at least 3 very different methods to do that also.
    So do algae.


    Plants in natural systems tend to be limited by CO2, not light.
    Most aquatic weeds, those that are very competitive with respect to CO2, have light compensation points of 600 micromols of light. So they are low light plants.
    Plants that are less competitive for CO2, cannot use bicarb etc, will likely have even less light demands.

    Algae are not CO2 limited ever really. They tend to be more light limited.

    the other more basic question is do you want a faster rate of growth than you get with EI+high CO2 and moderate to high light?

    Do you want more/faster rates of growth than say 300-400micromols?
    I don't.

    Makes taking care of it, much harder and far more likely to have some go wrong and go wrong much faster. Same for the Reef tank also. You do not need those values to do well.

    It's a waste of energy, light cost etc.

    Some anti EI Folks cry like babies about wasting water, or adding N and P from water changes into the their drain...........then never comment about electric and light curiously or the fact that the hobby is a luxury item to begin with and we could stop doing altogether if we "really cared" about the ecology and environment. Funny how hypocritical some folks are in their agenda and cognitive dissonance.

    It kills me sometimes, mostly from laughter. Then they come back for more and never get it. It's almost cruel.
    But you know...........they influence other folks and make them second guest their good judgment, so I cannot forgive these folks for not thinking through things better even when they have had it spoon fed to them.

    So I'm a bad guy:p

    Light, that's where it all starts.

    Regards,
    Tom Barr
     
  7. aquabillpers

    aquabillpers Lifetime Charter Member
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    I have learned a lot about light in the last few months, and the more I learn, it seems the more confused I get.

    For example, Tom said that "most aquatic weeds" have "light compensation point" of 600 micromols. (LCP is the amount of light energy that an aquatic plant needs to neither grow nor decline, balancing photosynthesis [output] with CO2 [input].) He further says that they would be low-light plants.

    But other sources, including (I think) Tom himself, define low light light as 30 to 40 micromols and high light as 80 to 150 micromols. 600 micromols would be off the chart. Hence my confusion.

    The difference is so great that I must be missing something. What is it?

    Bill
     
  8. Tom Barr

    Tom Barr Founder
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    To clarify, the maximal amount of light weeds might use is 600micromols(addign more light further will not increase growth rates no matter how much nutrients/CO2 are added)), the LCP is about 10-20micmols for most of those plants. Hydrilla is 12 micromol for LCP.

    So lower limits are about 20-30 for perhaps most plants, and upper is about 400-600.

    Regards,
    Tom Barr
     
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