Stable isotope-based statistical tools as ecological indicator of pollution sources in Mediterranean transitional water ecosystems

tN-stable isotope analysis of macroalgae has become a popular method for the monitoring of nitrogenpollution in aquatic ecosystems. Basing on changes in their ı15N, macroalgae have been successfullyused as biological traps to intercept nitrogen inputs. As different nitrogen sources differ in their isotopicsignature, this technique provides useful information on the origin of pollutants and their extension inthe water body. However, isotopic fractionation potentially resulting from microbial nitrogen processing,and indirect isotopic variations due to effects of physicochemical conditions on algal nutrient uptake andmetabolism, may affect anthropogenic N isotopic values during transportation and assimilation. This inturn can affect the observed isotopic signature in the algal tissue, inducing isotopic variations not relatedto the origin of assimilated nitrogen, representing a “background noise” in isotope-based water pollutionstudies.In this study, we focused on three neighbouring coastal lakes (Caprolace, Fogliano and Sabaudia lakes)located south of Rome (Italy). Lakes were characterized by differences in terms of anthropogenic pressure(i.e. urbanization, cultivated crops, livestock grazing) and potential “background noise” levels (i.e. nutri-ent concentration, pH, microbial concentration). Our aim was to assess nitrogen isotopic variations infragments of Ulva lactuca specimens after 48 h of submersion to identify and locate the origins of nitro-gen pollutants affecting each lake. ı15N were obtained for replicated specimens of U. lactuca spatiallydistributed to cover the entire surface of each lake, previously collected from a benchmark, unpollutedsite. In order to reduce the environmental background noise on isotopic observations, a Bayesian hierar-chical model relating isotopic variation to environmental covariates and random spatial effects was usedto describe and understand the distribution of isotopic signals in each lake.Our procedure (i) allowed to remove background noise and confounding effects from the observedisotopic signals; (ii) allowed to detect “hidden” pollution sources that would not be detected when notaccounting for the confounding effect of environmental background noise; (iii) produced maps of thethree lakes providing a clear representation of the isotopic signal variation even where background noisewas high. Maps were useful to locate nitrogen pollution sources, identify the origin of the dissolvednitrogen and quantify the extent of pollutants, showing localized organic pollution impacting Sabaudiaand Fogliano, but not Caprolace. This method provided a clear characterization of both intra- and inter-lake anthropogenic pressure gradients, representing a powerful approach to the ecological indicationand nitrogen pollution management in complex systems, as transitional waterbodies are.