Seasonal changes in the cell size and density of the diatom Fragilaria crotonensis Kitton in Lake Biwa

Few studies have explored changes in the sizes of diatom cells which can be due to sexual reproduction in lakes, as observations of natural populations have been limited. We measured seasonal changes in the cell sizes and densities of the diatom Fragilaria crotonensis Kitton in Lake Biwa from February 2012 to January 2017 and examined possible causal variables on sexual reproduction and cell density of F. crotonensis . Cell size tended to decrease with increase in cell density in spring and summer to autumn, though the pattern was not so clear in 2012 and 2013. The highest cell density (1.6 × 10 4 cells L -1 ) was recorded in May 2014, with the average cell size being 73.5 μm; the lowest density (7.1 cells L -1 ) was recorded in December 2016 (average cell size 86.5 μm), reflecting biannual cell size reduction and restoration which can be due to sexual reproduction. We used time series analysis to delineate the seasonality of cell size changes of the target diatom and Convergent Cross-Mapping (CCM) to explore the effects of possible causal variables on sexual reproduction and cell density of F. crotonensis and to evaluate the influences of environmental factors. We found that water temperature may influence the cell density (vegetative reproduction) of F . crotonensi s, though we could not identify any possible causal variables for the algal sexual reproduction. In addition, time series decomposition analysis, together with the seasonal changes in cell size mentioned above, proved that the target diatom followed reduction in cell size twice a year.