AN EXPERIMENTAL TEST OF HOST POPULATION-SIZE CONTROL BY FUNGAL PARASITISM IN THE PLANKTONIC DIATOM ASTERIONELLA-FORMOSA USING MESOCOSMS IN A NATURAL LAKE

Population control by fungi parasitic on diatoms was experimentally evaluated with the freshwater diatom species Asterionella formosa. For this purpose experiments were performed using mesocosms in the eutrophic shallow Lake Suwa (4 m average depth) during summer when the parasitic fungi were active. The lake is characterized by frequent vertical mixing due to wind and convection currents. Based upon previous kinetic studies, a working hypothesis was tested, that the diatom population biomass can decrease as a result of increased loss rates by fungal infection, relative to the growth rate of the alga in summer. It was possible to acclerate the growth rate, and thereby exceed the parasitic loss rate, by decreasing the mixing depth to 1 m (where the euphotic zone is about 2 m) using mesocosms. The diatom population captured in the mesocosms increased, whereas that in the lake consistently decreased to a yearly minimum density. These results correspond well with estimates predicted by models from previous studies, in which changes in diatom population biomass were determined mainly by the balance between specific growth rate and parasitic loss rate. Further, the importance of fungal control of the diatom population has been confirmed experimentally, as was suggested from previous modelling approaches. The existence of a threshold population density for parasitic fungal attack of the host diatom is also discussed.