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DIC and peat
A few days back Steve Pushak asked (paraphrasing here) how peat would
contribute to CO2 in an aquarium.
The consensus seemed to be that there was no direct relationship between
organic substrate materials like peat and the dissolved inorganic carbon
species. Additionally, CO2 contributed by bacterial breakdown of the
organic substrate would be small.
I agree with both of these points, but after some thought and a few
calculations I thought I'd mention another indirect relationship.
Organic substrate materials like peat, humus, organic garden soils, etc
contain organic acids that act as buffers. These act in conjunction with
the DIC buffer. The organic buffers are very complex and have highly
variable characteristics. Some peats will buffer water to pH near 4.
Others will have little effect. However, the end result of the
combined peat-DIC buffer could be to make more CO2 available.
Its difficult to guess just how important the effect might be. There are
lots of variables, so I ran some simulations to figure out how big the
effect might be. Without going into a lot of boring details, an organic
substrate (with all else constant) might increase available CO2 by a
ppm or so. The organic acids produce large declines in the pH and
alkalinity, but these are such that their net effect on CO2 concentrations
is small.
The extra CO2 is probably generated from bicarbonate in the tap water when
the tap water reacts with the (acidic) peat. The simulations assumed 15%
water change per week, but performed the change at a continuous rate, so
the resulting CO2 was available at a continuous rate. In a real tank with
water changes once a week, or every two weeks the CO2 might be generated
in much larger pulses but over a rather short time after the new water is
added.
Also, the effect isn't permanent. DIC in the tap water eventually
depletes and overrides the organic buffer.
I have a paludarium with a lot of peat in the substrate. I wouldn't say
that the peat has had any noticable effect, other than to turn the water
the color of weak tea.
Roger Miller