Stable Carbon and Nitrogen Isotope Signatures in Three Pondweed Species-A Case Study of Rivers and Lakes in Northern Poland.

Aquatic plants, as sedentary lifestyle organisms that accumulate chemical substances from their surroundings, can serve as valuable indicators of long-term anthropogenic pressure. In Poland, water monitoring is limited both spatially and temporally, which hampers a comprehensive assessment of water quality. Since the implementation of the Water Framework Directive (WFD), biotic elements, including macrophytes, have played an increasingly important role in water monitoring. Moreover, running waters, due to their dynamic nature, are susceptible to episodic pollution inputs that may be difficult to detect during isolated, point-in-time sampling campaigns. The analysis of stable carbon (δ13C) and nitrogen (δ15N) isotope signatures in macrophytes enables the identification of elemental sources, including potential pollutants. Research conducted between 2008 and 2011 encompassed 38 sites along 15 rivers and 108 sites across 21 lakes in northern Poland. This study focused on the isotope signatures of three pondweed species: Stuckenia pectinata, Potamogeton perfoliatus, and Potamogeton crispus. The results revealed statistically significant differences in the δ13C and δ15N values of plant organic matter between river and lake environments. Higher δ15N values were observed in rivers, whereas higher δ13C values were recorded in lakes. Spearman correlation analysis showed a negative relationship between δ13C and δ15N, as well as correlations between δ15N and the concentrations of Ca2+ and HCO3-. A positive correlation was also found between δ13C and dissolved oxygen levels. These findings confirm the utility of δ13C and, in particular, δ15N as indicators of anthropogenic eutrophication, including potentially domestic sewage input and its impact on aquatic ecosystems.