Seasonal Changes in the Silica Content of Natural Waters in Relation to the Phytoplankton.

Abstract

1. The method of Dienert and Wandenbulcke permits of the estimation of silicate in solution down to the limit of 0-04 mg. of silica per litre, which cannot be distinguished from zero. The phosphate normally present does not interfere with the reaction. 2. All fresh waters examined were found to contain more silicate than does the water of the English Channel. 3. The water of Plymouth Sound is richer in silicate than that of the sea. 4. At Station El in the English Channel silicate decreases during spring, from a winter maximum of 200-240 mg. per cubic metre, so that from April to June, 1925, the amount left, recorded as 40 mg. per cubic metre, was indistinguishable from zero. At this time the water was stripped of phosphate also. Silicate increased again till early July, decreased in August and again in November ; the times of decrease are suggested as being those of diatom increase. 5. In deep Atlantic water silicate is much more abundant than in the English Channel; values from 360 mg. per cubic metre at 1000 metres to 1200 mg. at 3000 metres were obtained. In winter at any rate the surface value in the Atlantic is similar to that off Plymouth, about 110 mg.per m3 in 1925.