Re: Phosphate Levels...

> From: tmarch at primenet_com (Todd March)
> Subject: Please Share Your Phosphate Levels...
> I'm wondering if members of the list would share with me (and all) their
> phosphate levels--for thriving tanks, with minimal algae growth and optimum
> plant growth, as well as those with problematic algae inflected tanks. This
> is a "target level" that most of the literature and even some of the finest
> information sources tend to ignore; perhaps because of the crucial and
> integral relationship between phosphates and all photosynthesizing life
> forms.

I haven't measured phosphates in my tanks recently but last time I
did I believe it was around .5 mg/l. The scientific literature I've
read indicates that the critical threshold for phosphate is very low.
In natural environments dissolved reactive phosphorus (DRP) in lake
water rarely exceeds 10 ug/L (.010 mg/L) DRP and concentrations approaching
20 ug/L may cause exclusion of macrophytes as a result of light
attenuation associated with stimulated algal growth. Now we may be
able to tolerate higher concentrations of green water in aquaria since
the depths are not as great as lakes but we get the idea that P should
be less than .05 ppm. (Barko, Gunnison & Carpenter, 1991,
Aquatic Botany 41 41-65, "Sediment interactions with submersed
macrophyte growth and community dynamics")

In sediment studies by Barko and Smart they reported DRP (dissolved 
reactive phosphorus) concentrations in the sediment interstitial 
water ranging from 40 to more than 9000 ug/L with an overall 
average of 1150 ug/L which would indicate that in mesotrophic 
(low aqueous nutrient levels) lakes macrophytes probably get 
almost 100% of P from the sediment. It may be a moot point 
since I suspect that P will never be limiting in an aquarium 
even with sterile, inert substrate if there are fish.

Paul Krombholz wrote (jun 27, substrate discussions):
"My master's degree research on several aquatic plant species indicated that
they had a critical tissue value for nitrogen of 1.5% (of dry weight) and a
critical tissue value for phosphorus of 0.15%.  Critical values are the
minimum tissue contents of nutrients that allow maximum growth given that
all other nutrients, light, CO2, etc are in good supply.  If tissue
contents fall below critical values, growth is limited by the scarce
nutrient.  In other words, if the tissue content of N falls below, 1.5% or
if the tissue content of P falls below 0.15%, then yields will be reduced.
The aquatic plants, like other plants, could accumulate higher amounts in
their tissues than the critical values.  These excess amounts over the
critical values are called luxury consumption.  For nitrogen, I got tissue
contents as high as 4.25%, and for phosphorus, I got tissue contents as
high as 0.7%.  The highest contents of nitrogen were around 2.8 times the
critical value, whereas the highest contents of phosphorus were around 4.7
times the critical value."

In discussions with George Booth he said that Dupla laterite comes
with rooting tablets to be used during the initial setup of the
substrate and he feels that these do contain phosphate.

Conclusion: a substrate containing soil/clay/laterite will help
sequester or hold phosphates in the substrate where they can be
used by plants. It is preferable that plants get their phosphates
from the substrate rather than from the aquarium water where it
will stimulate algae growth. Top soil or composted manure may
contain a significant amount of phosphates. The amount of phosphates
in soils varies tremendously. The aluminum oxides and iron oxides
of clay or laterite are very effective at binding phosphate ions;
soils often contain a large proportion of clay. Barko & Smart
"suggest that growth was governed by the availability in
sediments of P and Fe" together with N which may be supplied in
the water. (Barko & Smart, "Sediment-related mechanisms of growth
limitation in submersed macrophytes", 1986, Ecology 67(5))

Another interesting note, Diana Walstad reported successfully
growing plants in soil substrates with strong lighting (TAG vol 4-?).
with high measured concentrations of both nitrates and phosphates
(>5ppm P I recall) without serious algae problems! This would
suggest that plants have mechanisms for inhibiting algaes even
when there is sufficient nutrients and light for them to flourish.

More than you ever wanted to know about phosphorus! ;-)