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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.