Although T. dealbata roots aqueous extract is capable of inhibiting both A. flos-aquae and M. aeruginosa in experimental conditions, we wanted to test whether it also has the potential to inhibit natural phytoplankton assemblages in lake water. As such, we collected the phytoplankton assemblages from eutrophic lake water and tested their response to T. dealbata roots aqueous extract. As expected, the chlorophyll a and TFP contents decreased significantly, which indicated that T. dealbata roots aqueous extract stress leads to a decrease in the phytoplankton biomass and an inhibition in the total dehydrogenase activity.