Impact of the fungal parasite Rhizophydium couchii (Chytridiomycota) on the population dynamics of the freshwater alga Closterium aciculare (Chlorophyta)

Plankton samples from a eutrophic-hypertrophic reservoir from Argentina (Paso de las Piedras Reservoir) were found to contain individuals of Closterium aciculare parasitized by a chytridial fungus. The parasite on this particular host and the impact on its population during the period of its greater abundance in relation to abiotic variables are described. The life cycle of the fungus was observed with light and scanning electron microscopy. The chytrid was identified as Rhizophydium couchii Sparrow. The prevalence of infection reached a maximum of 33% coinciding with a dramatic fall in the density of the host population and there was a significant negative correlation between the density of C. aciculare and the prevalence of infection. The major change in abiotic variables was at the beginning of the infection, namely a fall in water temperature and solar radiation. This parameter shows a positive correlation with the density of C. aciculare and a negative correlation with the prevalence of infection. The parasitism by Rh. couchii played an important role in the dynamics of the host population. The decline in the population of Closterium aciculare might be due to unfavorable environmental conditions but, on the base of the high percentage of parasitism reached, probably it was accelerated by the chytrid.