FUNGAL CONTROL OF POPULATION CHANGES OF THE PLANKTONIC DIATOM <i>ASTERIONELLA FORMOSA</i> IN A SHALLOW EUTROPHIC LAKE¹

Abstract

ABSTRACT Parasitic fungi infected an Asterionella formosa Hass. population for 10 months of the year, during which time the population density of the alga was over 100 cells‐L −1 . High injection rates of more than 20% of the cells were observed between February and July except on a few occasions such as late April and late June. Mortality of infected diatom cells was temperature dependent in dialysis tube culture experiments; the loss rate of field populations due to fungal injection also was affected by temperature. Loss rates of diatom cells were negligibly small m February even though infection percentages were more than 20%, but increased toward summer with an increase m temperature. High infection percentages and correspondingly high loss rates (e.g. 0.3 d −1 ) occurred periodically during the summer. The results suggest that this type of host‐parasite interaction is an important control of algal population size in natural waters.