From shore to the bottom: detecting anthropogenic nitrogen inputs on Lake Bracciano through δ15N of epilithon and Characeae.

Nitrogen inputs represent a notable threat for aquatic ecosystem structure and functioning, promoting cultural eutrophication and algal blooms. Detecting external N inputs is particularly complicated due to the combined effects of dilution and assimilation processes. Moreover, in monitoring aimed at managing eutrophication, it is crucial to determine the sources of pollution. Due to its ability to reflect the ‘organic’ or ‘inorganic’ origin of the inputs, the nitrogen isotopic signature of algal primary producers (δ15N) is currently recognized as a flexible tool in the environmental monitoring of nitrogen pollution. In this study, we tested the ability of δ15N signatures of Characeae species to intercept the nitrogen inputs spreading from the coastline to depth in a lacustrine ecosystem. The results of this study highlighted the usefulness of Characeae for detecting sources of N inputs. We observed a rapid decrease in the nitrogen isotopic values along the bathymetry, from 0 to 15 meters depth, in the sampling sites characterized by steep slopes (North and East), while, in the sampling sites with a steep slope (South and West) we observed a propagation effect form the shoreline (0 m) to the first 5 meters. Moreover, we found a decrease in Characeae occurrence compared with previous studies. This reduction is likely related to the effects of the recent drought and water level reductionthat affected the investigated lake.