Phosphates form insoluble salts with bivalent metals and precipitate while autoclaving. 

Autoclaving solutions in glass water bottles can lead to increases in silicon concentration and under some circumstances, crystals may form. Also composition/conc. of dissolved gases may change after autoclaving a solution.

Hi is there some reading I can follow up on this?

whats even more weird is that the PO4 concentration jumped up a day or 2 later

You might be having some of your inorganics precipitate out during the autoclave cycle.  What specific type of teflon bottle are you using? Not all of teflon bottles can be autoclaved without potentially sorbing some of the material.

In addition to the chemistry going on in the solution due to precipitation, complexation and dissolution contamination in the autoclave could also create a problem.

Do you close the bottles during auto claving or do you keep them open? If they are open, there might also be some contamination from the autoclave coming in. This can be condense water dripping down from the lid or also aerosols produced during boiling of the water. And you will find phosphates in low concentrations pretty much everywhere. 

THanks everyone.

# Niko Finke

They were capped but lossened by half a stop

Much of the inorganic carbon is driven off as the CO2 is lost (due to heating) and the carbonate equilibrium shifts to compensate. After autoclaving you need to add sterile bicarbonate solution and/or bubble with sterile air to re-establish equilibirum (for a large volume > 1 L, this can take hours to days of bubbling). The iron will also precipitate so autoclaving can help to get a low-Fe medium without going the Aquil route (though you cannot be assured of exact Fe concentration>).