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 (C) and nitrogen (N) 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: , , and . The results revealed statistically significant differences in the C and N values of plant organic matter between river and lake environments. Higher N values were observed in rivers, whereas higher C values were recorded in lakes. Spearman correlation analysis showed a negative relationship between C and N, as well as correlations between N and the concentrations of Ca and HCO. A positive correlation was also found between C and dissolved oxygen levels. These findings confirm the utility of C and, in particular, N as indicators of anthropogenic eutrophication, including potentially domestic sewage input and its impact on aquatic ecosystems.