Chieko Manabe* and Syuhei Ban*

*University of Shiga prefecture

Vertical distribution of large phytoplankton and its chytrid fungal infection were monthly determined in Lake Biwa from May to October 2011. Phytoplankton and chytrid fungi were stained with a Calcofluor white solution and counted with an epi-fluorescence microscopy. Staurastrum dorsidentiferum var. ornatum and Fragilaria crotonensis were dominated in large phytoplankton during the study period. Aulacoseira granulate, Asterionella formosa Hassall and Ceratium hirundinella also occurred but sporadically. All five phytoplankton species were infected by chytrid fungi, and S. dorsidentiferum and F. crotonensis were infected at 20-90% and 5-80%, reaching maximum in July and August, respectively. Proportion of both empty and infected cells in S. dorsidentiferum decreased below euphotoic zone with depth, while that of F. crotonensis did not necessarily show such pattern. The proportion of infected cells in S. dorsidentiferum was therefore positively correlated to the proportion of empty cells, while that in F. crotonensis was not necessarily shown such relationship. The number of chytrid sporophytes on S. dorsidentiferum was 1-27 per cell and showed no trend with depth. The relationship between the vertical distribution of the phytoplankton and infection by the chytrid fungi was discussed.