Stable isotope variation in macroinvertebrates indicates anthropogenic disturbance along an urban stretch of the river Tiber (Rome, Italy)

Running waters in urbanized areas are large-scale systems of anthropogenic energy dissipation that receive effluents from point and diffuse sources, potentially inducing changes in organic matter decomposition and deposition and thus modifying river metabolism and the feeding patterns of inhabiting populations. Based on the hypothesis that anthropogenic disturbance provides important trophic constraints that influence the trophic niches of local communities, stable isotopes analysis was used to evaluate possible alterations in resource assimilation by aquatic species in response to diffuse and point sources of pollution from the city of Rome. For this purpose, the isotopic signature (15N and 13C) of detritus-based benthic communities and fish was determined upstream and downstream of two wastewater treatment plants (WWTPs) located before and after the urban stretch of the river Tiber. Community-wide metrics as the carbon range and convex hull area encompassing all taxa in a 15N and 13C bi-plot were used to measure the species' niche width and overlap. Differences were found between the upstream and downstream signatures, regarding the 15N and 13C of both detritivores and predators. The differences were found to be more pronounced at the southern WWTP, located downstream of the city. The lower 15N in macroinvertebrates at the WWTP-impacted sites reflected the lower 15N of suspended particulate organic matter and was associated with higher inorganic and organic loads. The decreasing range of 13C values in macroinvertebrates and fish indicated a narrowing of the niche width downstream of the treatment plants, particularly downstream of the urban area itself. The effects were stronger on detritivores than predators due to direct incorporation of the sewage-derived material that dominated the locally available food sources. These data suggest that isotopic signals coupled with community-wide metrics can be used as functional indicators of treated and untreated sewage impacts on aquatic communities even when the primary targets (species abundance and community structure) appear to be largely unaffected