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Re: Optimum current in planted tank + H corymbosa

     Frank I. Reiter" <FIR at istar_ca> asks in the 8/7/97 Aquatic Plant 
     Digest what is an optimum current for plants?
     If your plant leaves are moving slightly this is an indication of 
     turbulent flow which is what you want. Turbulent flow greatly 
     increases the mixing of the fluid (water) at the boundary layer of the 
     leaf which improves the transfer of water containing CO2 to the leaf 
     surface where it can be absorbed. If you have CO2 injection, high flow 
     rates will also increase the surface interaction of the water and 
     result in a lower CO2 concentration at dynamic equilibrium. Again, 
     this gas interchange is greatly increased when there are turbulent 
     flow regimes at the water surface which are characterized by small 
     disturbances in the water surface e.g. ripples, eddies, any visible 
     disturbance. If you have bottled compressed CO2 and you are running 
     trickle filters, box filters or the like, the additional CO2 you need 
     to inject may not be worth worrying about. If you are using yeast CO2, 
     you probably want to take every precaution to minimize surface 
     disturbances in your water.
     As an aside, I have a Hygrophila corymbosa (or stricta) which sits in 
     the current from my powerhead which I use to inject CO2. I use a half 
     inch tube about 6-8" long to improve the CO2 dissolution rate and I 
     have to aim it so that I don't have surface disturbance and so that it 
     doesn't dig a hole in the substrate. The Hygro continues to grow and 
     has sent down a number of fairly thick root fibers into the substrate. 
     The _stem_ of the plant however has rotted away for about 6 inches 
     from the gravel up. I don't know if this is related to the water flow. 
     Paul K and I have been speculating whether this is related to the 
     fertility of the substrate and caused by decomposition byproducts. I 
     wondered if this was not a natural phenomenon of the plant. Many 
     species of Hygrophila seem to loose small twigs which float on the 
     surface and might be a mechanism for propagation and dispersal.
     I'm curious about other folks observations of H corymbosa and stem 
     melting of Hygrophila species in general.
     Steve P in Vancouver BC