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PO4 uptake

Back in the '60's and early 70's, when I had access to research literature
on aquatic plants and nutrient uptake, I ran across a number of articles
that showed how incredibly efficient aquatic plants are in taking up
phosphorus.  In lakes it was established that the amount of phosphorus in
the water could be extremely low without algae and higher plants becoming
deficient.  In fact, what was important was turnover times.  This is the
time it takes for all the phosporus in the water to be taken up and
replaced by an equivalent amount that came from decomposition.  The
research showed that dissolved phosphate has a rather short lifetime in the
water.  Soon after it is released, it is taken up again.  Turnover times in
lakes ranged from around two hours to as little as fifteen minutes.  A
fifteen minute turnover time means that the average lifetime of a phosphate
ion in the water is only fifteen minutes!  The water is not a storehouse of
phosphate, rather it is a conveyer belt from decomposers to users.

I recall a paper that showed that an alga (I don't remember what species)
when phosphorus deficient, became covered with phosphatase enzymes which
enabled the alga to extract organically bound phosphorus from both
dissolved and particulate organic matter in contact with the alga.  I
suspect that this capability isn't limited to a few species of algae, but
is probably widespread in algae and higher aquatic plants.

All plants can also store excess phosphate in their tissues for later use.
They can store quite a lot.  A number of species of aquatic plants were
found to begin to have reduced growth due to phosphorus deficiency at
tissue contents of 0.15% P (wt of P/dry wt of plant) This is known as the
critical value.  Above this value, the plants' growth rate is not increased
by increasing the amount of P.  Below it, the plant is clearly deficient.
Tissue P contents could be as low as 0.1% and as high as 0.7%  Stored P in
the tissues above 0.15% represents so-called luxury consumption, and can be
utilized if the availability of P decreases in the environment.  A plant
that has 0.7% P can increase in size by approximately 4.5 times without any
further P uptake before it gets near the critical value, assuming, of
course, that other nutrients are not limiting.

Ecologists find that organisms competing for resources tend to partition
the resources so that each species does not compete head-to-head with
another species for eactly the same resources.  Different species of
grazing animals eat different kinds of grass so that they don't directly
compete.  Different species of fruit-eating pigeons specialize on different
sizes of fruits so that they don't directly compete.  However, phosphate
would seem to be a resource that can't be partitioned.  All plant species
need it, and, unless the plant is parasitic, it is going to have to get it
from the same source.  Thus, it follows that the competition for phosphorus
is severe, and that all the tricks possible for improving uptake would have
to be possessed by most or all of the competing species.

Thus, if you can detect any phosphorus in your tank water at all, the
plants are probably well supplied and their tissue contents are likely to
be well above the critical value.  If the phosphorus content of the water
drops after you add phosphate, it could be due to the plants increasing
their luxury consumption.  It does not mean that the plants were actually
deficient.  Also, there may be other ways that phosphorus is removed from
the water column, such as reacting with iron or calcium and precipitating

Paul Krombholz, in warming central Mississippi, expecting a nice day