SummaryThe species composition of planktonic diatom crops in Loch Leven, Kinross, Scotland varied considerably over the years studied (1968–71) with centric forms predominating. Dense populations were often produced and rates of increase and decrease were high.Relations with dissolved silica did not appear always to be simple; instances occurred during which both diatoms and silica were increasing or decreasing. Moreover, certain rates of increase in dissolved and frustule silica exceeded the rates at which silica flows into the loch even when the inflows appeared to be the only available dissolved silica sourc