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Re: Carbon dioxide concentration in water

Amy wrote:

> I included the last sentence of her quote just to complete her thought. What 
> baffles me is the statement that "most natural waters have about three times 
> more mg/l CO2 than air". I thought, and the recent APD discussion appears to 
> confirm, that the concentration of CO2 in air was about 350 ppm while the 
> concentration in sterile water is about 0.5 ppm. Even if the concentration 
> in natural water is significantly higher due to decomposition of organic 
> wastes, etc., I thought it got no higher than 3-5 ppm. Walstad's statement 
> of "three times more mg/l CO2 than air" implies a CO2 concentration in 
> natural water of over 1,000 ppm.

Amy, what Ms. Walstad said was that a liter of "most" water contains
about 3 times as much CO2 as a liter of air.  I've done that calculation
myself, and I think she's about right.  What appears to be confusing is
that a kilogram of "most" water contains only about 1/100th as much CO2
as a kilogram of air.  In this case there's a big difference between
comparing concentrations on a volume basis (mg/l) and comparing
concentrations on a weight (ppm) basis.

The actual significance of the comparison is lost on me.

And for Scott and all discussing the source of CO2 in the aquarium...

The concentration of CO2 in aquarium water is typically quite a bit
higher than the CO2 concentration that would be present in the water at
equilibrium with air.  Therefore CO2 diffuses *out* of the water, not
into it.  In the absence of added CO2, the CO2 in most aquariums
originates largely from respiration in the aquarium.  Fish food would be
the most prevalent source for the carbon, though some would also
originate from the biological decomposition of an organic substrate or
plant detritus.

If plants are using more CO2 than respiration provides in the tank then
the CO2 levels can drop below atmospheric equilibrium levels and CO2
will enter the tank water from the atmosphere.  That (in my experience)
requires a lot of light, can drive the pH > 9 and result in an
environment that is hostile for most plants. 

Roger Miller