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Re: Higher CO2 in natural waters
- To: <Aquatic-Plants at actwin_com>
- Subject: Re: Higher CO2 in natural waters
- From: Thomas Barr <tcbiii at earthlink_net>
- Date: Thu, 20 Dec 2001 17:45:21 -0800
- In-Reply-To: <200112192048.fBJKm5Q26017 at actwin_com>
- User-Agent: Microsoft-Outlook-Express-Macintosh-Edition/5.02.2022
> On page 93, she says, "The low productivity of submerged plants is not
> because there is less CO2 in water than in air. (On average, most natural
> waters have about three times more mg/l CO2 than air [8,9]). It is because
> CO2 diffuses so slowly in water (i.e., 10,000 times slower than air)."
Others have addressed the units issue for but I'll add to this:
Natural water have extreme variations in many parameters. Head waters are
extremely high in CO2 in many cases. Lakes and ponds are quite different
than streams. Soft water versus hard water etc. Relating these systems to
ours, needs to be considered carefully. Soil/peat based tanks are much more
in line with her point and to natural processes.
HCO3->CO2 ** slow reaction
CO2 uptake due to photosynthesis **fast reaction
I have not seen this in any planted tank book so far. It is relevant in this
discussion also. Many aquatic plants can and do use HCO3 as a carbon source
like algae. Many don't. But the speed at which these reactions happens is
slower than for CO2. The same slower reaction speed be said for Excel and
other similar products that do not use CO2.
Light is often a limiting factor in headwaters, a little further down the
canopy opens up and you get good plant growth. Lakes/ponds are different and
have issues such geology/substrate types, inflows/outflows, wind, depth,
shape and a few more issue to consider. The lumping together of these
systems is dangerous.
> 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".
Again, _depends_ on the type of water. A shallow lake vs a deep lake(say
Lake Victoria vs Tanganyika) will have quite different level of CO2/O2.
A fast flowing brook will have a different CO2 level than and slow moving
river./They should be considered separately.
Springs/ground waters are loaded with CO2.
This is because of microbial respiration as water seeps into the ground.
A few lakes in Africa are CO2 seltzer lakes from geologic seeps. A few are
high pH soda lakes(pH of 12). Mono lake has a pH of 10.1.
> Am I not understanding something or is Walstad wrong?
See Paul's explanation and no, she is not wrong. Microbial respiration
creates a large amount of CO2 in waters and she is pointing to this issue I
believe. Breakdown/decomposition of organic matter uses up O2 and produces
CO2. This generally means adding flow to drive off the CO2 and bring in the
O2. Plants use the CO2 too fast though and don't produce higher O2 levels as
a result. When there's enough CO2 then the plants make a lot of O2. Folks
seem to think that everything is at equilibrium in planted tanks and this is
not the case. O2 and CO2 are in excess and being given off. O2 will
sometimes dip below that 100% make then it's not a bad idea for surface
exchange. CO2 is almost always over 100% ambient in every tank unless
plants/algae remove it.
In surface waters CO2 gets used up fast if light is available
generally(turbid/muddy waters are light limited, as are most headwaters). In
ground water CO2 builds up and has a large area reservoir for production of
microbes and O2(from the air, O2 has the same diffusion problems as CO2)
from above so this can produce large amounts of CO2 which is picked up along
the way. The areas near springs or down stream of them have great plant
growth. Lakes with their production coming from the substrate can also have
relatively good plant growth also. Since it is locked up in the ground the
CO2 builds up. Also, when rain falls it picks up CO2 as it falls. When the
ground water(acidic) hits geologic strata is can react with it if it's say
karst(limestone- a good buffer in this case) or remain unaffected (say
granitic strata). A place such as the Sierra's that have granitic water
sheds and a low TDS/alkalinity/buffering. Someplace like Mammoth Cave KY
will have high KH valves from dissolving limestone. Guess which one taste
better? Consider the ground in the Sierra's(all rock and a few pine trees,
very low organic content) and the ground in KY (all nice soil and dense
woodland with good organic content-think microbes).
A number of larger lake will have plants growing only where there is organic
build up along the littoral shores. The rest of the lake is bare as a bone
with only some slight algal growth. How localized the CO2 production and
content are would be interesting to find out.
Some spirulina for thought.