Abstract
Nitrogen dynamics in lotic ecosystems have been intensively studied in the light of eutrophication caused by anthoropogenic input of nitrogen. Natural abundance of 15N (δ15N) is a promising tool to elucidate in-situ processes in lotic ecosystems because δ15N can be a conservative tracer. However, many processes in intact ecosystems can exert isotope effects in different ways resulting in complex behavior of δ15N, which often makes our understanding origins and processes of nitrogenous compounds semiquantitative. Especially it is not easy to discuss quantitatively the effect of anthoropogenic N input in an intact ecosystem with δ15N data because endmembers of natural and anthoropogenic N compounds with respect to δ15N are not easy to define. Here, we summarized the existing isotopic data of inorganic N (ammonium and nitrate), the variability of with δ15N data, processes causing such variabilities in intact lotic ecosystems, and explore the possibility to use δ15N data for the better understanding of N dynamics.
