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Abstract

An epidemic caused by a Zygorhizidium species infecting the single-celled planktonic centric diatom Stephanodiscus alpinus was analyzed for parasite distribution within the host population, final parasite size relative to host cell size, and size changes of infected and uninfected S. alpinus cells. Infections in the lake occurred at random within the whole host population. There was no evidence for aggregated or even distribution of the parasite individuals, indicating that the infections occur independently of each other. In enclosures in which light was enhanced compared to the lake, there tended to be an even parasite distribution within the host population, irrespective of whether plant nutrients were added. This suggests that infected host cells were negatively selected by the parasite zoospores under these conditions. Final parasite sporangium size and host cell size were positively correlated. Thus, parasite fecundity was limited by host cell size. Infected S. alpinus cells tended to be larger than uninfected cells, and the mean size of host cells within the population decreased during the epidemic. This might be due to selective infection of larger host cells or to the peculiar mode of cell division in diatoms.