The characteristic species of Hydrocharitetum morsus-ranae (Hydrocharis morsus-ranae and Stratiotes aloides) are used to determine the ecological condition of surface waters in many countries of the European Union. Aquatic and marshland plants play an important role in shaping the chemical composition of the littoral zone of eutrophic and mezotrophic water reservoirs. The aim of this study was to determine the influence of the extent of coverage of water surface by Hydrocharicetum morsus-ranae on the physicochemical and chemical properties of water inhabited by both plant species. Conducted field studies indicate that Hydrocharicetum morsus-ranae existed in reservoirs that are considered polluted due to the concentration of lead. The cumulation of mineral nutrients by plants inhabiting the studied reservoirs affected the chemical composition of the analyzed waters by limiting the influence of Hydrocharis morsus-ranae and Stratiotes aloides on the quality of these waters. The ecological range of occurrence of Hydrocharis morsus-ranae relative to most chemical water quality indicators is wider than that of Stratiotes aloides.