INFECTION OF THE DIATOM ASTERIONELLA BY A CHYTRID .2. EFFECTS OF LIGHT ON SURVIVAL AND EPIDEMIC DEVELOPMENT OF THE PARASITE

A model is formulated to investigate the ability of chytrid parasites to survive or become epidemic within populations of their algal hosts. The model is used for an analysis of the effects of light on the occurrence of Rhizophydium planktonicum Canter emend., a chytrid parasite of the freshwater diatom Asterionella formosa Hass., using the information on the growth parameters of host and parasite presented in the first part of this article (J. Plankton Res., 13, 103-117). According to the model, conditions for survival of the parasite are optimal when the host grows at saturating light conditions. Under limiting light conditions, Rhizophydium needs higher host densities in order to maintain itself. The parasite is not able to survive prolonged periods of severe light limitation of the host. Epidemic development, however, turned out to be facilitated by a moderate light limitation of the host. Both light saturation and severe light limitation hamper epidemic development, but in the first case, epidemic development is still possible at sufficiently high host densities.