Phytoplankton infections by fungal parasites in the upper, mixed
layer of a mesotrophic northern temperate lake
were analysed according to the following parameters: host and parasite
species,
host population density and
prevalence of infection, resting spore formation by the parasite, and
the lowest host density at which parasites
appeared. The phytoplankton taxa recorded included the Cyanobacteria,
Dinomastigota, Chrysophyceae, Bacillariophyceae, Chlorophyceae, Cryptophyceae
and Haptophyceae, but infection was never found in the last
two classes. The parasites belonged almost exclusively to the monocentric
Chytridiomycetes. Fungal epidemics
occurred at all times of the year. Parasites appeared at population densities
as low as about 1 cell ml−1 in some host
species, with infection prevalence sometimes exceeding 80%. The proportion
of the total phytoplankton
biovolume infected by fungi was usually much <1%, but occasionally
reached 10%. Parasitism proved to be
highly species-specific, with one parasite species usually infecting only
one host species. In the case of
Zygorhizidium planktonicum, which infected both
Asterionella formosa and Synedra acus, there is evidence
that two
species-specific formae speciales, each infecting only one
of these two host species, are present in the lake.