Spatio-temporal study of phytoplankton cell viability in a eutrophic reservoir using SYTOX Green nucleic acid stain

Despite the global importance of phytoplankton primary production, the ecological role of cell death as an important loss process in phytoplankton is poorly understood. To assess the significance of cell death in phytoplankton, we studied cell viability of dominant species in the canyon-shaped eutrophic A similar to imov Reservoir (Czech Republic) at weekly and biweekly intervals from April to October 2011. Surface samples were taken from the lacustrine zone (near the dam, low nutrient level) and transition zone (near the river inflow, high nutrient level) of the reservoir. Moreover, samples from euphotic depth (1% of surface irradiance) were taken from the lacustrine zone. We used the membrane-impermeant nucleic acid dye SYTOX Green to examine seasonal and spatial differences in phytoplankton cell viability. Three species (diatoms Asterionella formosa, Fragilaria crotonensis, and cyanobacterium Aphanizomenon flos-aquae) were studied in detail. There was no difference in Asterionella cell viability among sampling sites. In the lacustrine zone, Fragilaria and Aphanizomenon exhibited lower viability than in the transition zone. In addition, Aphanizomenon viability was significantly lower at the euphotic depth. Nutrient levels were revealed as a factor influencing Fragilaria viability, while light availability was more important for Aphanizomenon. Our results evidenced that the importance of cell death, in particular phytoplankton taxa, varies both spatially and temporally. Moreover, our study indicates that coexisting taxa may differ in their capacity to cope with different environmental stressors.