The concentration of phosphates in sea water varies from 0.000 to about 2.00 microgram atoms (µg. atoms) phosphate-phosphorus per liter at 20° C. Occasionally a maximum of 3.00 µg. atoms may be found in some deep arms of the sea.
Robinson, Rex J., and Thomas G. Thompson. 1948. "The determination of phosphates in sea water." Journal of Marine Research 7, (1). https://elischolar.library.yale.edu/journal_of_marine_research/647