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substrate circulation rates


George was talking about circulation rates induced by heating coils.  I
wondered about the circulation rates that would apply *without*
convection, and estimated for one of my tanks that the water in the
substrate might turn over about every 6 weeks or so.  Details follow.

Karen Randall once commented that plants by themselves would induce
circulation in the substrate.  Her idea was based on some research
summarized in an article by Ole Pedersen at the Tropica web site.  
Briefly, the idea is that aquatic plants actively transport water from
their roots to their shoots and into the water column.  Water pumped out
of the substrate is replaced by water moving down through the substrate
from the water column.  Thus there is a continuous circulation of water
through the substrate.

I went looking for the original research by Pedersen and Kaj Sand-Jensen
so that I could get a sense of how much water the plants might move.  The
article (complete reference at the end of this letter) makes for
interesting reading.

The authors measured flow rates induced by several species of cold water
plants.  From the range of values they report, a rough mid-range estimate
of the flow rate would be about 0.1 cc of water per gram plant dry weight
per hour.  This varies from species to species and (inferring from their
discussion) may also vary with the plant growth rate and a few other

It's a little tough to apply that to an aquarium.  I have to admit that I
have never ripped all the plants out of a tank, reduced them to ash and
determined their dry weight.  Just the same, it seems like it might be
estimated.  I'll use my 55 gallon tank as an example.  I'll guess that it
contains about 3 lbs of plants.  If the plants are 95% water (ball park
guess) then the tank contains 0.15 lbs or about 68 grams of plant dry

68 grams of plant dry weight then could induce a flow of 6.8 cc of water
per hour.  The substrate has an area of 3774 square cm, of which about 25%
(944 square cm) would be open for water movement.  The flow rate through
the substrate would be 6.8 cc/hour divided by 944 square cm -- a
remarkably small 0.0072 cm per hour, or 0.17 cm per day.  The substrate is
about 7.5 cm thick, so it would take about 44 days, or about 6 weeks, for
the plants to turn over the whole volume of water in the substrate.

In reality the substrate would contain some stagnant areas and other areas
where the flow is concentrated.  Also, the downward flow rate at any depth
in the substrate would depend on how much root mass there is below that
depth, so there would be more turnover near the top of the substrate then
at the bottom.

At those slow rates the diffusion rates and stirring work done by
burrowing critters are probably significant factors.

Roger Miller

The reference is:

Perdersen, Ole and Kaj Sand-Jensen, 1993.  Water transport in submerged
macrophytes.  Aquatic Botany, v 44, pp 385-406.

No time to reach a library?  If you have a fax, a credit card and internet
access to Uncover or some similar library service then you can have the
article faxed to you.