With the adoption of the Water Framework Directive, monitoring of surface waters has undergone a reform process. It introduced a new approach to the assessment of surface waters based on biological elements. In the case of water reservoirs, the monitoring is based on phytoplankton, phytobenthos, and macrozoobenthos, as well as chlorophyll concentrations. The ecological evaluation is supported by physicalchemical and hydromorphological characteristics. The study was based on national monitoring data coming from 10 reservoirs in the lowland landscape. Our analysis revealed that the classification of reservoirs was determined both by physical-chemical and biological factors. It was found that good and better than good ecological potential was achieved by three reservoirs, while three more were classified as moderate and four as bad. Principal component analysis showed that the biological elements are strongly associated with the level of phosphorus in the water. The ecological potential of the reservoirs does not depend on the surface area or depth of the water body, nor it is related to water retention time.