Re: PMDD's and nitrate, etc.

> From: Elizabeth Worobel <eworobe at cc_UManitoba.CA>
> Subject: Re: Of fish loads and nitrogen
> In effect, the turnover rate in alga is much faster 
> than in higher plants. This means that over time, even though at any
> given instant the alga take up a greater proportion of available P, all 
> the P in the tank is slowly absorbed by the higher plants.

	This is also consistent with the tendency for algae to grow
on plants that are doing poorly or actually dying.  It looks as if
they may tend to lose their phosphorous, and the algae grow where
it is available.
	Thank you Dave.

> From: Stephen.Pushak at saudan_HAC.COM
> Subject: Re: PMDD's and nitrate
> that's true. The total dosage in that period of time would have
> been about 40 ml standard PMDD dosage. I don't think the nitrate
> all came from the PMDD but I wasn't sure how much would accumulate
> in that short period of time.

	Even if the plants used _none_ of the added nitrate, the PMDD
contribution was only a few ppm in that time.

> The problem is in the startup period because Fe
> will not become available until the redox in the Fe layer becomes
> low enough and the bacterial cultures become established.

	I'm not convinced about this.  Terrestrial plants seem to
get iron from loose soil in small pots.  I suspect they can get
iron from soils that are not anoxic. 

> 1) The Ca & Mg ions will displace NO3 which may occupy CEC sites
> This will liberate nutrients to the water column.

	Nitrate is an _anion_.  It will not be held on _cation_
exchange sites!  Nitrate is difficult to hold anywhere; it is
_very_ mobile.

> 2) It will mediate the pH and encourage bacterial activity which
> will stimulate decay and liberate even more NO3

	Could you please explain this?  "mediate?"

> *** I really wish I had a redox meter!! ***

	As someone already pointed out, redox potentials apply to
equilibrium situations.  They are typically used to see which
phases are stable in possible corrosion situations (i.e. very
long-term stuff, with simple component mixtures).  They will
be of very limited use in a non-equilibrium situation, such as
we have in a soil substrate.  Also, the reading you get from your
"redox meter" will depend on exactly which electrode reactions it 
uses.  I think you are really interested in oxygen concentrations,
and these may indeed give you some insight into what is happening
in your substrate, but please don't look at a "redox potential" as
a magic number that describes everything.

Paul Sears        Ottawa, Canada

Finger ap626 at freenet_carleton.ca for PGP public key.