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Re: CO2 sufficiency

On Tue, 24 Mar 1998, Stephen Pushak wrote:

> Is there a way to determine by pH measurements or otherwise, that you
> have enough CO2 to exceed requirements thus maxing out the rate of
> photosynthesis? Humic acids preclude direct concentration estimates.
> Steve

A few months back I put a letter on the digest that described how to 
calculate the % of CO2 injected that was actually used by plants.  It's a 
rough calculation, using pH and alkalinity measurements at morning and 
evening.  I used it, and like the results.

Qualitatively, the results say that a large daily pH swing indicates that
a large part of the injected CO2 is used.  It may also mean that CO2
reaches low concentrations at times of peak demand.  A small pH swing
means that a small part of the CO2 injected is being used.  That could be
caused by CO2 escaping before the plants get to it (turbulent surface,
perhaps) or by CO2 being injected in excess of the plant's needs.  The
actual meaning would have to depend on the details of the system. 

It might also be possible to get a feeling for the sufficiency of the
supply by taking a sample of the water at a time of peak demand,
measuring its pH, then allowing it to set awhile - preferably with
agitation - and measuring it again.  If the pH increases between the two 
measurments, then CO2 was available in excess at the time of peak 
demand.  No way to tell how CO2 without an alkalinity test.  If pH stays 
the same between the two tests, or drops between them, then the CO2 
supply was low and possibly low enough to be limiting.

Why do you say "humic substances preclude direct concentration