Concentrations of amorphous particulate silica (biogenic silica) in the uppermost layer of the inner Oslofjord, Norway, varied between 2 and 324 µ g Si·liter −1 over a 1‐year period in 1977–1978. Most of the silica was present in frustules of living or dead diatoms but there was always a certain amount (up to about 50 µ g Si·liter −1 ) in a form that could not be recognized under the microscope. Silica was produced mainly during three large blooms of Skeletonema costatum in February–March, May, and June. At the peak of the June bloom, there was up to 20 times more Si present as diatom silica than as dissolved orthosilicic acid. Uptake experiments in June indicated depletion times of dissolved Si as short as 3 h. Although the S. costatum cells were less heavily silicified in May and June than in February–March, they showed no morphological signs of a drastic Si shortage.
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