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Re: Cable flow rates

Tom Barr wrote:

> Claus of Tropica mentioned that they tested one tank for optimum flow rates
> in the substrate and came out with a figure of .49 liters/m^2/day. Not much.
> Osmosis ain't far behind that.

That's an interesting value.

Ole Pedersen and Kaj Sand-Jensen (Aquatic Botany 44(1993) p 385-406)
measured the rate of transport of water from the substrate to the water
column generated by several different submersed plant species.  In an
aquarium, water transported upward by the plants will cause an equal
amount of water to be drawn from the water column down into the
substrate -- that is, it will cause substrate circulation.  Karen
Randall pointed that out to this group some time ago.

Pedersen and San-Jensen's results cover the range from 7 to 143
microliters per gram plant dry weight per hour.  I'll use a mid-range
value of 70 microliters/gr/hr for a little calculation.

I think my 55 gallon tank (0.4 square meters floor area) contains
something like a kilogram of plant wet weight.  It's reasonable to
estimate that the plants are about 95% water (about 5% dry weight) so
the tank would contain about 50 grams of dry plant mass.  That dry
weight of plants would circulate about 3500 microliters of water per
hour, of 3.5 milliliters/hour.  Spread out of the 0.4 square meter tank
area that comes out to 8.75 milliliters/square meter/hour, or 0.21
liters/square meter/day.

Gee.  Without any further finagling, that's already at 43% of Claus'
optimum circulation rate.

Petersen and San-Jensen argued that the water transport rate should
increase with growth rate.  They ran their tests in cold water (12
degrees C, 54 degrees F), with CO2 at atmospheric equilibrium levels and
with lighting that was low (90 micromole/sq meter/sec) compared to what
we are often keep in our tanks (for comparison, I'd estimate from Ivo
Busko's data at Aquabotanic.com that my 55 gallon tank with 160 watts of
NO fluorescent lights is lit with 200 micromoles/sq. meter/sec).  I can
easily envision that the growth rates in my tank (and by extension,
probably in the typical CO2-augmented, well-lit plant tank) are
considerably higher than they were in the Pedersen and San-Jensen
experiments; if so then perhaps that 0.21 liters/square meter/day
estimate should be taken as a *minimum* value for a planted tank.

The possibility that plant-induced circulation rates can be quite a bit
higher than this estimate is further reinforced by values that Pedersen
and San-Jensen site in their literature review where (for instance) one
submersed, 20 cm Nomaphila scricta was found to "pump" 2.1 ml/hour.

The uncertainties in these estimates are so large that the results can't
be used for much.  Just look at how the results change if the plants are
97% water instead of 95% water!  Despite that, I think they are
sufficient to demonstrate that plants can provide substrate circulation
without any additional intervention by the aquarist.

Roger Miller