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Re: O2 - CO2 relationship

> Date: Fri, 14 Aug 1998 22:21:16 -0500
> From: George Slusarczuk <yurko at warwick_net>
> Subject: Re: Aquatic Plants Digest V3 #448
> Hello Frank,
> There is no inverse relationship between CO2 and O2 in an open aquarium
> system. At *equilibrium*, the concentration of each dissolved gas is
> independent of each other and depends only on the partial pressure of
> each gas in the atmosphere above.

Hmm.  There's little or no chemical or physical reason for CO2 and O2 to
vary inversely.  There are very good biological reasons for CO2 and O2 to
vary inversely.

Photosynthesis and respiration both enforce that variation, as each
consumes one (oxygen or carbon dioxide) and produces the other.

Plants cause diurnal swings, with O2 up during the day and CO2 down; at
night CO2 increases and O2 increases.  The size of the day-night swing is
determined by the rates of photosynthesis and plant respiration.

Respiratory activity by microbes (especially) and animals in the tank is a
different matter.  An increase in respiratory activity due to changes in
population size, food supply or nutrient competition will increase CO2
(the product of respiration) and decrease O2 (consumed during

In a well-circulated tank, you might see drops of several mg/l in the O2
content and even larger increases in the CO2 content caused by increases
in respiration rates. Unless you are making some long-term change in your
tank maintenance those changes will be short-term, but they may last for a
few days or even weeks.

That inverse relationship may be very strong in a substrate where the
circulation is reduced.

That said, turning CO2 off at night won't change the O2 content of the
water at night.  There might me a lag in the increase in O2 the next
morning, as CO2 concentrations will have to build up before photosynthesis
gets into full swing.

I hope I never venture into a realm of aquarium keeping where that becomes
important.  That could be part of my "optimum" definition.

Roger Miller