As a monitoring tool for ecological status assessment of freshwater bodies, phytoplankton assemblage index (Q index) was so far successfully applied in the evaluation of different types of lakes. Currently, its wider application possibilities are being investigated. The aim of this study was to evaluate ecological status of the floodplain lake, Lake Sakadaš, part of the complex river-floodplain system in the Middle Danube (Croatia), by assessment of phytoplankton functional groups and applying the Q index. Phytoplankton research was conducted in the period July 2011-October 2012, in conditions of more or less usual flooding pattern. Results were compared with the historical data of phytoplankton dynamics within the period July 1972-September 1973, when the lake was under the strong anthropogenic pressure, which resulted in rapid eutrophication and deterioration of its ecological status. The data set of phytoplankton functional groups and the main group arrangements in the multivariate nMDS analysis showed variation related to the past and recent conditions. The frequent appearance of heavy bloom of only one species or assemblages characteristic for polluted waters was the most distinctive feature of phytoplankton dynamics in the past. In such an impacted environment, values of the Q index were low and indicated bad to tolerable ecological status. Recent data suggest that water quality improvement and near-natural hydrological condition support algal assemblages characteristic for naturally eutrophic lakes and the values of the Q index varied between medium to good ecological status. Altogether, results of this study showed that Q index can very precisely reflect anthropogenic pressure what was recognized as its major advantage in comparison with other indices used for ecological status assessment.