Still several inportant ones forms you didn't mention
. ADP has a friend that it is usually associated. Think cellular respiration. Also, there is another real important one. REAL important.
Phosphorous is a part of DNA. So, if you have bacteria, if you have growth, if you have a cell, you are going to have phosphorous in situ. In growth, naturally, more phosphorous is concentrated.
The bottom line is that phosphorous is going to occur in these depositional sandbeds. It has two forms, particulate and dissolved. There is going to be a lot of both in a closes system. Particulate phosphates in depositional areas mainly come from adsorption to detritus or sediments or other particulate matter. Warm, slightly acidic waters are ideal for absorption to occur, but it does happen in marine chemistries/situations. When pH is high and temperature is stable, phosphorous adsorption is dependent on salinity. At natural seawater salinities, adsorption is considerable (a reason most marine aquarists don't see leaching from their sandbeds for several years at a time).
This is where life comes in and screws with what seems like a great inorganic control system. In the wild, the culprit are any number of phytoplankton and bacteria, but in our system mainly bacteria. Bacteria use orthophosphates and organophosphates for cellular incorporation, grabbing them from anything they are adsorbed too (orthophosphates) or decaying from (organophosphates).
Algae can use the alkaline phosphatase to steal phosphorous from just about anything (Calcium, cells, carbon, you name it). Alkaline phosphatase is perfectly adapted for our aquariums too, where we maintain (yep, you guessed it) very alkaline conditions.
Any feeding you perform and any detritus you make is going to be sinked/adsorbed by the sandbed until it is incorporated. A sink with no drain fills up and overflows.
What can happen (and would be interesting to examine in a closed marine system) is the formation of ferric phosphate. When bacteria or algae cells leach iron (another chemical of life, so to speak), and bacterially (or otherwise) mineralized orthophosphate is present, it can (and does) adsorb to iron and become insoluble. In the wild, when sediment conditions become anoxic, the iron is reduced, releasing the phosphates into the water (Spanky: this is where you say "I told you so")