We can find diatoms where there is water, but it is especially rewarding to study them in the sea. In temperate waters nutrients accumulate during the winter. This results in a spring bloom of phytoplankton.
Sufficient light is available then and temperatures are higher.
The most important factor however is the availability of nutrients. The spring outburst of phytoplankton is grazed down by zooplankton in the summer and occurs again as a second maximum in the autumn. Due to the death and decay of the spring plankton, fresh nutrients become available in the autumn. Of course, this is a generality and the occurrence of primary and secondary plankton blooms depends on the exact geographical and climatic factors. For diatoms, silica is especially important and a limiting factor.