Diatoms need silicate

Diatoms need silicate in order to build their skeleton. In winter the supply of silicate from the rivers, the dissolution of silicate from detritus in the sediment and the low uptake of silicate by diatoms due to the short days and long night's results in the highest silica concentration from the whole year. As soon as phytoplankton growth takes off in early spring the silica concentration quickly decreases to limiting values; subsequently grazing by filter feeders and zooplankton leads to a nearly complete disappearance of diatoms in the water column whiles all the inedible parts (mainly diatom-skeletons or frustules) sink to the sediment and add to the large buffer of particulate silicate. Silica is the first of the macro-nutrients to become limiting.