Observations on a diatom bloom in Loch Leven, Scotland

A study was made on a freshwater planktonic population of centric diatoms, mainly Stephanodiscus rotula (Kütz.) Hendey, in Loch Leven, Kinross, Scotland. The observations were analyzed in relation to changes in water temperature and chemistry, parasitism by a chytridiaceous fungus, grazing by a recently described protozoan and competition from another alga (Synechococcux sp.). Each of these factors is thought to have affected the recorded rise and fall in diatom population density, in particular, silica limitation, fungal parasitism and protozoan grazing. The protozoan involved (Asterocaelum algophilum Canter) forms digestion cysts, the predominantly diatomaceous contents of which suggested very selective feeding but laboratory experiments with dual cultures did not confirm this. They indicated that both the size and quality of algae affected their suitability as food. Feeding activities of Asterocaelum seemed to be more or less confined to a solid surface; grazing rates in cultures shaken to maintain the animals and algae in suspension were low relative to those observed in unshaken cultures. This suggests that much of the grazing occurred on diatoms that had sunk onto the sediments.