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Re: CO2 rates

On Mon, 19 Jan 1998, Jeff Kropp wrote:

>> snip ...I try to keep the plant population in the 55 gallon tank about
>> constant.  To do that I trim about enough every week to fill a small salad
>> bowl.  I think that's about 0.2 lbs (91 grams) per week, moist weight. If
>> the plants are 6% carbon (85% water, carbon 40% of dry weight), then
>> they're fixing (and I'm removing) about 5.5 grams of carbon/week.  The
>> lights in that tank are on 12 hours/day, or 84 hours/week, so the carbon
>> fixation rate is about 65 milligrams/hour of light.  The corresponding CO2
>> fixation rate is 240 milligrams of CO2/hour snip...

> Perhaps I don't fully understand Roger's calculations; am I correct to
> think they imply plants respire a lot of co2 without converting all that
> carbon to plant mass? If so where does it go?

I think I see what might be confusing.  I estimated the total grams of
carbon removed per week (5.5), then adjusted that to an hourly rate (65
mg/hour) based on the number of hours per week that the light is on.
That assumes that the plants are fixing carbon only when the lights are
on.  Probably a good assumption.

Then (and this might be the source of confusion) I converted the carbon
fixation rate to the corresponding rate of CO2 fixation.  I did that so
the fixation rate could be compared to the CO2 input rate.  The weight
ratio of CO2 to C is 44:12 or 3.67, so the CO2 rate is 3.67 times higher
than the C rate. 65 mg carbon/hour * 3.67 mg CO2 per mg carbon gives
238.55 mg CO2 per hour, which I rounded to 240 mg CO2/hour.

I think the vast majority of carbon fixed by the plants remains in the
plants.  Small amounts apparently do "leak" out, but I assume that is
negligible.  Some of the fixed carbon might be lost to grazing - that's
especially true if your plants include some algae and you have the
algae-eating critters to control it. Plants also lose carbon in dying
leaves or necrotic spots when something (snails for instance) remove the
dying material.  Any carbon that gets "recycled" that way is missed by my

> I have tank conditions
> (steady co2 injected, dKH 6) resulting in a pH change of .4 for each light
> cycle but have very slow growth to show for it. Could this be symptomatic
> of a particular nutrient deficiency?

Don't know.  I'll give it some thought.

Roger Miller