There is a need to search for additional indicators allowing for more accurate identification of both the trophic status of waters and its chemical and biological consequences. Work detailed here involved a preliminary analysis of the possibility of an isotopic index being used in association with values for trophic and saprobic indicators in describing a dam reservoir experiencing far-reaching eutrophication. Water samples for physicochemical analysis were collected from three sites along the axis of the dam reservoir in Rzeszów three times during the spring and summer of 2013. Results sustained classification of the Reservoir’s waters as hypertrophic, irrespective of the particular zone sampled. While phytoplankton blooms characterised by reference to numbers of organisms per unit volume of water were also similar throughout the Reservoir, diversification in terms of taxonomic composition was to be noted, given the occurrence of cyanobacteria among the dominant diatoms in the area close to the dam. This presence was accompanied by enrichment of the Reservoir’s suspended organic matter in carbon of the heavier 13C isotope. On this basis, the δ13C isotopic index can be regarded as a potentially useful indicator allowing for more accurate identification of both the level and the nature of the trophic degradation ongoing in bodies of water.