Spatial heterogeneity of diatom silicification and growth in a eutrophic reservoir

Summary Diatoms are one of the most important phytoplankton groups. As they grow, diatoms use silicon to produce a siliceous frustule, which protects the cell. During April–November 2011, at 1–3 week intervals, we estimated diatom silicification rates at two distinct sites along the longitudinal profile of the canyon-shaped eutrophic Řimov Reservoir (Czech Republic): (i) a nutrient-depleted lacustrine zone near the dam and (ii) a nutrient-rich transition zone upstream near the river inflow. Diatom silicification was estimated using the 24-h in situ incubation of natural phytoplankton assemblage with a fluorophore 2-(4-pyridyl)-5-{[4-(2-dimethylaminoethyl-aminocarbamoyl)methoxy]phenyl}oxazole (PDMPO) which fluorescently stains the newly synthesised diatom frustules. Diatoms contributed an average c. 40% to the total phytoplankton biovolume in the lacustrine zone, but only c. 20% in the transition zone where a cyanobacterial bloom developed during summer. Asterionella formosa and Fragilaria crotonensis were the most abundant diatom species, while Aulacoseira italica, Nitzschia acicularis, Synedra acus and Stephanodiscus sp. were less important. Silicification rates of all diatom species were significantly higher in the transition zone than at the dam. The intensity of PDMPO fluorescence per diatom cell was tightly related to the growth rates of three diatom species calculated from changes in cell counts during the incubation. The PDMPO technique can thus be used as a proxy for diatom growth. Growth rates of the two most abundant diatom species were positively correlated with daily light exposure but not with nutrient concentrations.