Abstract
The aim of this work was to present a quantitative and qualitative analysis of a phytoplankton assemblage under the influence of Ceratophyllum demersum L. during a two-year field study. Literature data obtained under laboratory conditions indicate that C. demersum may cause a decrease in the blue-green algae biomass, which is considered to be evidence of the significance of allelopathy. The observations reported in the current paper indicated that there was no clear effect on cyanoprokaryota whereas decreases in the quantity of Cryptophyta, Dinophyta, Chlorophyta, and Chrysophyceae are noted.