Concentration of CO2 in nature

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rasbora

Aquarium Advice Regular
Joined
May 5, 2011
Messages
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When injecting CO2, usually we have a target of 15-30 ppm of CO2 to make aquatic plants grow. I am curious about the concentration of CO2 in natural environment, like river, lake, etc. I think they should be close to the concentration of CO2 in air (which is very low, I think). If the concentration is low, how could plants grow ? Or the plant's growth in nature is very slow ?
 
Maybe rivers arent as diversely planted and thick with all the plants people usually stick in aquariums?
 
My guess would be that in nature there are many more sources of carbon available, such as increased fish waste due to larger populations, as well as animal waste (ducks, etc etc), and dying and rotting leaves and other organic matter like dead organisms that we normally clean out of our tanks.
 
The amount of CO2 in the atmosphere is around 350 ppm or so. If still water is allowed to fully equilibrate, you'll get a concentration of 2-7 ppm (depending on a number of factors, especially temp). Something else must be at work here.

I believe the general consensus is that the majority of CO2 comes from two sources: groundwater and decomposition. Ground water will carry in CO2 from topsoil and from reactions between acidic water and carbonate based rocks, such as limestone. The biggest contributor is probably decomposition of organic matter on the bottom.

I found this nifty little chart on the University of Wisconson - Madison website:

Time of Day, ppm

6 am, 23 ppm
9 am, 16 ppm
12 pm, 9 ppm
3 pm, 6 ppm
6 pm, 3 ppm

So CO2 levels do get up to around recommended levels and are slowly depleted during the day. It's also interesting to note that while the overall levels are somewhat lower, the sheer mass of water means that while their may only be 23 ppm available at any one spot, there is a huge amount of CO2 available to the plants.

I may also be worth noting that natural ecosystems have no sense of aesthetics. Algae can run rampant, which is something that we generally strive to eliminate in our tanks, further backing the need for supplemented CO2.
 
Thanks for all replies. Now I know that there are a lot of CO2 source, there is a high concentration of CO2 in water, there is a huge stock of CO2 in river, lake, etc.
I tried to google this topic but did not receive a good information. Perhaps I need better keyword for googling.
 
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