EFFECTS OF PHOSPHORUS LIMITATION ON THE EPIDEMIOLOGY OF A CHYTRID PHYTOPLANKTON PARASITE

1. The effect of phosphorus limitation of the diatom Asterionella formosa Hass. on growth, survival and epidemic development of its fungal parasite Rhizophydium planktonicum Canter emend. was estimated, using measurements of production and infectivity of the zoospores of the chytrid grown on host cultures with different phosphorus-limited growth rates. 2. Phosphorus-limited host cells were less susceptible to infection with zoospores of the parasite than non-limited host cells. 3. The sporangia on phosphorus-limited algae produced substantially less zoospores, but the development time of these sporangia was only slightly reduced. 4. As a result of these effects, Rhizophydium will reach lower growth rates at a given host density, and survival of the parasite will require higher host densities when Asterionella is phosphorus-limited. 5. The zoospore production remained high enough to enable the parasite to grow faster than the alga at sufficiently high host densities, both at limiting and non-limiting phosphorus levels. 6. In spite of the reduced growth rate of the parasite, phosphorus limitation of Asterionella was found to facilitate the development of a Rhizophydium epidemic. This was a consequence of the reduced algal growth rate at phosphorus limitation, which makes the host population more easily outgrown by the parasite. 7. Phosphorus limitation of the host could reduce the threshold host density required for the development of an epidemic by a factor of 2.5.