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Re: PO4, etc.

On Mon, 21 Feb 2000, Steve Dixon wrote:

> Let me restate the question a second time.  :-)  Assume the above facts.
> Why do higher order plants out compete the algae?  Dr dave (Heubert)
> discussed this issue a couple of years ago and expressed considerable
> puzzlement over the question.

If I remember right, Dr. Dave's puzzlement (good word) was over
competition in a PMDD regime, where the tank is dosed with everything but
PO4 and competition pulls the PO4 down to unmeasurable levels.

In this case, you're dosing with everything, keeping iron and all the
macronutrients at measureable levels and balancing the micronutrients to
match the macronutrients.  It seems to me that measurable levels of those
nutrients are high enough that competition is not a factor.

Rather than restate the question yet again (which might make this thread
sound a lot like attorney's questioning a difficult witness), I'll go
ahead and propose three possibilities:

1)  The algae population in your tank isn't well-adapted to use the PO4
under the conditions of light, water hardness and so on that your tank
provides.  This would probably be a temporary state of grace because new
algae varieties are inevitably introduced.  I think this is least likely
of my three options.

2)  The algae population is being kept in check by grazing.  This would
mean by microscopic grazers (per James Purchase's recent quote from Craig
Bingman) or more normal grazing by SAEs, etc.  In this case, if you dosed
too much PO4 you might get growth rates that exceed the appetite of your
grazers.  I don't have any way to assess the likelihood of this option,
but I really like it anyway; it has such an ecosystem "feel" to it.

3)  Algae growth is actually being controlled by iron (or perhaps,
manganese). The observed growth could occur in the transition from
phosphate-limited growth to iron-limited growth, or it could be a response
to "luxury" levels of phosphorus.  I think this is probably the most
likely of the options.  In order for iron to control algae growth
(particularly in Karen's case, where she doses to some fairly high iron
levels) I think it's necessary that the iron test kits overstate the
amount of available iron; the amount of available iron would have to be no
more than a small fraction of the measurable iron.

I'm kindof assuming (as we usually have in our discussions) a simple
either-or choice between different elements controlling growth rates.  In
fact it probably isn't that simple but we just don't have the information
to indicate how things really work.

> Steve Dixon in San Francisco where the weather is definitely sub moderately
> optimal

Roger Miller

In Albuquerque, under calm, bright blue, sunny skies with a 60 dF
temperature and about 10% humidity.