Pastor, A., J. Ll. Riera, M. Peipoch, L. Cañas, M. Ribot, E. Gacia, E. Martí, and F. Sabater. 2014. Journal of Environmental Science and Technology 48(12):6612–9.
Understanding the variability of the natural abundance in nitrogen stable isotopes (expressed as δ15N) of primary uptake compartments (PUCs; e.g., epilithon or macrophytes) is important due to the multiple applications of stable isotopes in freshwater research and can give insights into environmental and anthropogenic factors controlling N dynamics in streams. While previous research has shown how δ15N of PUCs varies with δ15N of dissolved inorganic N (DIN) among streams, less is known about how δ15N of PUCs varies over time. Here, we examined monthly variation of δ15N of PUCs and of DIN species (nitrate and ammonium) over a year, and compared it among streams with contrasting human impacts and PUC types. Our results showed no evidence of isotopic seasonal patterns. Temporal variability in δ15N-PUCs increased with human impact, being the highest in the urban stream, probably influenced by the high variability of δ15N-DIN. Among compartments, in-stream PUCs characterized by fast turnover rates, such as filamentous algae, showed the highest temporal variability in δ15N values (from −3.6 to 23.2 ‰). Our study elucidates some of the environmental and biological controls of temporal variability of δ15N in streams, which should be taken into account when using stable isotopes as an ecological tool.