The Calvin cycle does not involve evolution of O2.
The light reactions: they produce the reduction power for making carbohydrate sugars. Without that, the Calvin cycle will not work.
P is used a great deal in ATP-ATD the main energy source for cells.
NADPH-NADP also uses P.
The base sugars that plants use for storage are rich in PO4.
The addition of PO4 and increases in pearling seems to be more a fuction of ATP/NADPH and simpler compounds, these are quickly made.
The cell would not fix more CO2 if it's PO4 deficient, it shuts down it's CO2 fixing(reduction) and slows things down, "idles" till there's enough PO4 to resume full production. So it's not lost or wasted, it never really gets taken in. The plant cell is finely tuned to changes and environmental inputs, it'll down regulate things if there's not enough/too much etc.
N enters as NO3 or NH4. The NO3 is reduced into NH at the cholorplast cell wall and is very quickly converted from NO2=>NH4 as NO2 is toxic to plant cells internally.
NH4 is then converted into Glutamine via glutamine synthase. There are two types of GS(glutamine synthase), one in the cholorplast and an external one that plays a role in NH4 from the environment rather than from reduction of NO3.
Most of these studies are on agricultural crops but should apply in most cases to aquatic plants as well at this biochemical level.
Finding hard empirical data on aquatic plants is difficult.
Here's a few diagrams from UC Davis and some general test questions.
http://www.acad.carleton.edu/curric...udy guides/nutrient uptake/12nutruptake2.html
And here's a good one for you for NH4 and NO3 assimlation and rates etc:
This is a good site for some of the comments on lakes and plants:
Note Mn and NO3.
Here's something that might bother you also about high CO2 and NO3
You read all that, and you'll have a good understanding about plant biochem or a headache
Seriously, there are good info in each one of these sites.
These will help understand and argue for various observations in aquatic plants not just with N or P, but also with K, CO2 etc.
While we often want to narrow things down to one nutrient etc, it's very often several.
Enjoy, now I have to prepare fer da genetic class I have to teach, Fruitfly fun
Maybe I'll post some of these over on the APD to stir some disccusion up there also.