This is a real "devil in the details" type problem. Being a mesocosm, you probably had some sediments in the system which became anaerobic. Under these conditions organic phosphates can become mineralized and become soluble and available. Much of the total phosphorous in the system was probably associated with the sediments. Meanwhile, those anaerobic conditions reduce NO3 to N2 gas while silicates come to thermodynamic equilibrium with the sediments where the pH is lower that the bulk water (CO2 generation from the bacteria) and the solubility of silicate decreases and silica (sand) crystals can grow.

The kinetics are related to the biological ecology of the sediments and their growth rates following your big disturbance of making the mesocosm.

How did you measure the phosphate?

I should also have asked how were the elements supplied to the mesocosm and what else was present

Paul Milham I measured phosphate following strickland and parson manual using uv vis spectrophotometer..

Also, silicate, nitrate were supplied following F/2 media composition. Phytoplankton natural assemblages were studied under elevated temperatures treatments. I also measured dissolved oxygen, daily community respiration, chl-a, species abundance on day-3, day5, day-7, day-9, day-11...

Dallas Weaver Thanks for the possible reasons. But, Its a pelagic study and there was no sediments in the systems.

Look into the details of you measurement methods. Phosphorous can exist in many chemical forms including polymers and these can change. Unless your test preparations get all of it into the form (chemical species) your test measures, you can get apparent changes over time.

Was this a fresh water or salt water system at what pH and Ca content? I know the pH can get high with photosynthesis and various insoluble phosphate compounds are stable. Both Ca apatite and Fe phosphate will come out of solution, but as the system matures and doesn't grow as fast and has more predation on the algae, more CO2 comes in from the air and decreases the pH back to normal and the solubility of these micro particles increases.

It is a interesting , more information are need, The most important is the total phophorus content. It is necessary to know what source of P in the system.

I did experiment with the phosphorus ester, including G6P , ATP, polyphosphate.and formal them can be significantly increased within hours in the artifical sea water system. I have dig it out that the system contain nano molar level inorganic iron, which was from the chemical regents, as inpurity composition, e.g. NaCl, always contain small ppb level Fe, I further demonstrated that it is the inorganic iron oxide nanoparticle as enzyme to promote the inorganic P release,

In your case, if you know the Total P and inorg P , you may have good explaination.

You can find my work, both p analysis and the hypothesis on inorganic enzyme

I have share you a couple of my publications and hope you can find useful information ,

Dr. Weaver, would you provide some experiment results, as you mentioned, sediment release inorg P and also have TP Changes, I have a hypothesis about inorganic enzyme on hydrolysis of phosphorus esters. which may explain such results. It will be greatly if you provide such documents.

You can find my work as following

https://www.researchgate.net/publication/327892167_Inorganic_phosphatase_aqueous_inorganic_iron_oxide_nanoparticles?_sg=B8KjMeETWQeB7OIZPt3oTE1ndPC1YSXqrJzqr_2Po1hEJYNxCu7w3Tg5ku8WRmkzWrVxc-qtowtWVNFFWmZg8mocVcSN1blZbvXsWvCc.oLGLfuroeeecQPSnwzYFeMcH2fg7CqaIrYQ-J0PJ-ekFXKQ_GvdGw1Jxa2ktO2C0sPhoqgH7EVVnY7mzrCPKxQ

@Xiaolan: thank you very much. I used F/2 media to the seawater before starting the experiment to make sure that phytoplankton as enough nutrients to grow in short time and also i thought it will be useful for me to study isolate temperature stress.

@Xiaolan: I measured reactive phosphate following Strickland and Parsons 1972..

You need test total p first

the Fe concentration in the medium should be higher enough as I mentioned for inorganic enzyme, you may notice the temperature response of such enzyme

which type of sea water? You made artificial one or nature one?

Xiaolan Huang (黄 晓澜) Its the sea water collected from the same coast where I collected plankton inoculum. But, it was filtered and ozonized before to use for experiment.

It is more likely that your sea water contain quite high level of dissolved organic P. These org P can be hydrolysis rapidly with the iron oxide nano particles promotion. Check total dissolved P is essential for you understand your results.

The method you used is likely only inorganic P, check the acid concentration that you used . The high conc of acid also can cause organic P hydrolysis

see my papers.

Xiaolan,

I didn't know about Fe as a catalyst, thanks. In sediments that go anerobic where a lot of the PO4 mobilization occurs from organic P sources, iron is biologically reduced to Fe++ from Fe+++ with ferrous form being very soluble. Bacteria near the sediment surface go the other way using O2 or NO3 as an electron acceptor creating Fe+++ particles often as Fe(OH)3 nano sized flocks and jells. That process produces both ionic Fe and micro particulate that would be catalytic.

The creation of phosphate mineral (Ca, F, Phosphate based minerals) deposits in the ocean appear to be the mineralization of organic phosphate materials which diffuse upward in the sediments while Ca and F ions diffuse down creating a very low solubility compound of Ca, F, apatite type minerals. I have no publications on this but found the literature when looking into where phosphate rock came from using scholar.google.com for searching.

Note that the f/2 nutrient medium used contains Fe in the trace element mix (one of the big trace elements).