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PO4 and Algae

Roger wrote about a lack of algae growth under seemingly adequate macro and
micronutrient conditions:

"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 don't think # 1 is likely.  For one, I have had these conditions in my
tanks for several months at a time.  #2 is attractive, but doesn't seem
plausible to me either.  My algae crew is fat and well-fed and happy.  Their
algae work is just a friendly pastime between feedings of brine shrimp,
blood worms, mosquito larvae, dry food, etc.  And there are less of them now
than at times when algae was a problem for me.  In addition, this crew has
been easily overwhelmed by algae problems associated with nitrate limitation
on repeated occasions a few years ago.  # 3 is definitely worth thought and
consideration.  But it's not iron.  I'm a careful student of iron.  I have
had intensive outbreaks of algae with less (actually much less) iron than I
presently have in either of my tanks.   (A word of caution about reported
iron levels.  Even the best hobbyist kits (LaMotte and Hach, I have both)
struggle a bit to pull out the chelated iron we commonly use in our
micronutrient solutions.  I continue to believe the cheap kits are
essentially worthless and actually often yield "false positives" if you

I'm interested in the conversations I've seen about manganese.  It's a
nutrient I want to learn about.  While I believe I understand the essential
logic of the view that the higher plants must be pulling some nutrient or
other to a "limited" condition to out compete algae (Liebniz law), my
experience with nasty algae outbreaks only in nitrate limited conditions,
and superb stable growth conditions with seemingly fully adequate nutrition
levels with very little bothersome algae growth, has me wondering whether
we've got all the pieces on the chess table.  Why wouldn't algae grow well
under conditions of adequate general nutrition?  Very odd idea, isn't it?  I
suppose that's Roger's point! :-)

I like the way Karen characterized the nitrate/phosphate limitation issue in
her recent note.  I might go one step farther.  I'm not actually sure that
limiting, or nearly limiting, phosphate is the important issue.  I'm rather
more certain that NOT allowing nitrate to become the limiting factor is the
important point.  I think if we studied this carefully, we might make this
type of revision to Paul and Kevin's original idea.

And by the way,  I stopped by Tom Barr's house in Marin County, CA yesterday
and was very surprised to find 0.8 ppm phosphate in his large tank, and 1
ppm phosphate in his water supply.  Many of us in the Bay Area have observed
that Marin water seems to have some magic about it.  The aquatic gardeners
can often do quite well without the difficulties that perplex their brothers
and sisters just over the bridge in San Francisco.  FWIW, for those of you
who know Tom and his tanks, he is rarely troubled with algae problems.  

So let's keep noodling on this!

Steve Dixon