Relative abundance, rate of increase, and fungal infections of freshwater phytoplankton

Fungal infections of seven species of phytoplankton were examined in relation to the host species' exponential rate of net increase, and to their proportional contribution to the total phytoplankton biovolume. Infections were observed to increase at biovolume proportions of the host species of as low as 1%. In most algal species, infected cells were always found at higher proportions, with the exception of Stephanodiscus rotula and Fragilaria crotonensis. In these two species, high proportions of biovolume were reached without any evidence of infected cells. The increase in infected cells was usually associated with a growing host population, whereas peak and decreasing densities of infected cells were usually observed when host populations were declining. The results show that the fungal parasites can exist on their host population even if it comprises only a small fraction of the total phytoplankton biovolume, and that the parasites become evident while the host population is still increasing.