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On Thu, 30 Mar 2000, Steve Dixon wrote:
> Fortunately, I think Roger Miller and I have just about exhausted our
> knowledge of the topic we have been discussing. Just a couple of additional
> points and my head will be empty for sure! :-)
The installments are getting shorter. I imagine the list can only be
> Roger: "That fact [of Leibniz Law] is of rather trivial value because
> excess nutrients aren't usually a problem." And also, "Um, if the plants
> aren't growing well then probably nutrient excess isn't the problem." So if
> it is patently obvious that NOTHING IS MISSING (yesterday's note) and excess
> nutrients are usually not a problem, what on earth are we talking about? Are
> the conditions always "optimum" (uh-oh) and we either have algae or we
> don't! You're leading me by the nose in tautological circles. And it must
> be Groundhog's Day! :-)
Well, what I was talking about was that rapid algae growth means that all
of the nutrients necessary for algae growth (likewise, for plant growth)
are present in the water. Nothing is missing.
> Roger: "Some people have reported success with [limiting nutrients other
> than nitrogen], and other people have not. Where it does work the reduction
> in algae growth is thought to be a secondary effect of causing some other
> nutrient to become limiting, plus the tertiary effect of competition. It
> isn't a direct effect of adding nitrate." Of course it's the direct affect
> of adding nitrate. No other variables are changed. Nitrate is added and
> the algae declines.
The only mechanism I can think of that would account for this directly
would be toxicity. I don't think that nitrate is particularly toxic to
algae, so it leaves me looking for indirect mechanisms. The mechanism
that Paul and Kevin proposed for the PMDD regime was an indirect one: 1)
dosing with everything but phosphorus made phosphorus the limiting
nutrient (assuming adequate light, CO2, water movement and so on); 2)
competition for phosphorus forced its concentration to low values; 3)
plants were better than algae at competing for low phosphorus levels, so
the plants out-compete the algae. Cool. When it works.
> Roger: "With* the simplifications of Leibnitz' rule and *if* nutrients are
> growth-limiting, and *if* the growth rate of algae is high enough to become
> a nuisance then it's because the *limiting* nutrient -- not any excess
> nutrient-is too readily available." Sorry to be dense, but say what! The
> limiting nutrient is too readily available, but no nutrient is in excess!!!
> I don't have a clue what you're trying to say here.
The fact that some nutrient is growth-limiting doesn't mean the nutrient
is absent. The higher the concentration of the growth-limiting nutrient
(or the greater its availability) the faster the algae and plants will
grow. In this simplest assessment it's only the limiting nutrient that
matters. All other nutrients are available in sufficient supply so that
they don't limit growth.