The amount of phosphoric acid in sea-water off Plymouth was at a maximum of 0.06 mg. per litre of P2O5 at the end of December, 1915, after which it fell irregularly to a minimum of less than 0.01 mg., which extended from the last week of April to the latter part of May; it then increased again and in January, 1917, reached the same value as the average for the first part of the month in the previous year.

This seasonal variation is probably to be attributed to the removal of the phosphates from solution, at first by the fixed algæ, and later in the spring by the diatoms and for a short time by Phcœcyslis. There is also present in sea-water taken near Plymouth another soluble compound of phosphorus which can be converted into phosphoric acid by oxidising agents.