Inorganic phosphate in natural waters was
usually orthophosphate in its original form.
However, it is water soluble, so I suspect all of the lab tests are actually measuring the PO4(-3) anion, so it really doesn't matter what it is called. (Can any experts confirm if that is what is actually measured in the lab test?) That common anion is the way phosphorus is available to the algae and other photosynthesizing plants in the pond. (There can also be P2O7(-4) anions, but I believe they are uncommon in most locations.)
The PO4 anion can bind with the suspended clay minerals in the pond.
(The commentary above is pushing my old chemistry knowledge.)
I have no idea if clay "bound" phosphorus can be released back into the water column as the pond chemistry cycles through natural changes. I suspect clay bound phosphorus that remains suspended in the water column MAY be accessible to some autotrophs.
I also have no idea if a big rain event brought turbid water (that had a high content of clay minerals) into the pond if a phosphorus reading a week after that event would be measurably lower than the reading prior to the rain event? I suspect it might if the chemistry of your pond water does allow for electrically charged clay particles to settle out of suspense.All of my speculation/questions in red need to be addressed by pond experts, because they are well above my pay grade. (My background is more when the phosphorus is still in the soils.)
Phosphorus in Groundwater (USGS)
