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CO2 in the San Marcos River
The San Marcos River in central Texas is a spring-fed, crystal clear
river with LUSH aquatic vegetation. It is literally carpeted shoreline
to shoreline with dense vegetation and it makes most planted tanks (at
least mine) look pretty shabby. It has the natural look the rest of us
are striving to achieve. Some of the plants in the river are familar
to aquarists, including Vallisneria americana, Ceratophyllum demersum,
Ludwigia sp., Hydrocotyle sp., Hygrophila polysperma and also
Potomogeton, Utricularia, Hydrilla. and Elodea. Yes, some of these
plants have been introduced. It is also home to a large population of
Texas Cichlids, which is the only cichlid native to the U.S. There are
some really big bruisers in there!
I go there often, as it is about 30 minutes from my house, and I enjoy
canoeing and diving/snorkeling there. This time, I brought along my
test kits to measure pH, CO2, and general hardness. Because of the
lush plant growth, I have always suspected it was naturally high in
CO2. I measured pH at 7.4, general hardness at 16 DH, and CO2 at 20
ppm. This is well above the typical values of 0-5 ppm CO2 found in
surface waters and unfertilized aquaria, but is within the target
levels recommended for planted tanks. The deep limestone deposits in
the area is what makes the springs so hard.
The soil is a sandy silty clay with a lot of organic matter in it, and
it is definitely anaerobic. Every step liberates many large bubbles,
although I could not sense any H2S; it must be CH4 and/or CO2.
Algae is present, but it is hardly a nuisance. Perhaps the strong
water currents help keep it in check. There were a few snails grazing
on the epiphytic algae, but you have to hunt to find one.
The river is mostly shaded by huge cypress, water oak, and pecan trees
that line the bank, but there are a few open, sunny areas. These
spots are dominated by Potomogeton and Vallisneria, while the shaded
areas have a greater diversity of plants. The Hygrophila carpets the
bottom and does not get very tall--instead, it creeps along the
bottom, growing red leaves at the crown, with few leaves along the
stem. Perhaps this is why Hygrophila and other stem plants grow roots
at their nodes--the current presses the plant down against the
sediment, and the nodal roots burrow into the substrate.
The springs keep the water temperature at about 70 F year round, so
plant growth is fairly constant. The water is very refreshing,
especially when the air temperature is 100 F.
Hope everyone finds this interesting to read.