Factors affecting growth and viability of natural diatom populations in the meso-eutrophic Římov Reservoir (Czech Republic)

Algal population size may vary substantially over the season in response to varying growth and loss rates strongly affected by environmental drivers. In our study, we measured growth and viability of Fragilaria crotonensis, a dominant diatom of the summer phytoplankton in the small, meso-eutrophic, dimictic freshwater Řimov Reservoir (Czech Republic). Over two consecutive seasons, Fragilaria growth and viability were assessed with PDMPO, a specific Si-deposition tracer in diatoms, and SYTOX Green, a membrane permeability probe, respectively. Using multiple linear regression with stepwise forward selection of environmental variables, diatom growth and viability were found to be driven by different factors. Fragilaria growth was positively affected by daily light exposure during the 24-h incubation with PDMPO and by mean river discharge into the reservoir over the 10 days before our measurements. Fragilaria viability, in contrast, declined markedly with decreased ambient silica concentration. This indicates that while diatom growth is tightly related to physical properties of the environment, cell viability reflects the availability of silica, which is essential for generating and maintaining diatom siliceous frustules. Importantly, there was a significant negative correlation between Fragilaria viability and growth, suggesting that seasonal peaks of diatom growth are coupled with low cell viability.