As the traditional taxonomic classification proved insufficiently sensitive, concept of functional groups, deffined by Reynolds et al. (2002) and revised by Padisák et al. (2009), was used in this thesis with the aim of better understanding and monitoring changes of phytoplankton in river ecosystems. Qualitative and quantitative analysis of potamoplankton communities and physico-chemical properties of the River Drava were conducted at one site, the right bank of the city Osijek, during the spring period, in March and April 2016. As analyzed section is belonging to the downstream course of large lowland rivers, phytoplankton community was well developed and had diverse origin. In addition to real planktonic vegetation, most numerous were benthic species due to turbulent flow. Results showed the importance of hydrological conditions in plankton biodiversity and abundance. With a total of 142 found taxa, the largest number of species were represented by Chrysophyta and Chlorophyta. Dominant functional groups were C, D, E, H1, MP and P characteristic for turbulent, turbid habitats rich in nutrients. More than 80% of the total biomass accounted for diatoms, typical fast-growing species of dominant group C (Cyclotella meneghiniana) and group D (Stephanodiscus hantzschii). Functional groups proved to be excellent indicator of fitoplankton found in River Drava during early spring period.