Ecological Indicators 28 (2013) 107–114 Contents lists available at SciVerse ScienceDirect Ecological Indicators jo ur nal homep age: www.elsevier.com/locate/ecolind Stable isotope variation in macroinvertebrates indicates anthropogenic disturbance along an urban stretch of the river Tiber (Rome, Italy) Antonella di Lascio a , Loreto Rossi a,∗ , Pasquale Carlino a , Edoardo Calizza a , David Rossi b , Maria Letizia Costantini a a Department of Environmental Biology, Sapienza University of Rome, via dei Sardi 70, 00185 Rome, Italy b CNR-IRSA, Water Research Institute Research Via Salaria Km 29.30 Monterotondo, 00015 Rome, Italy a r t i c l e i n f o Keywords: Stable isotopes Point and diffuse pollution sources Trophic niche Wastewater treatment plant a b s t r a c t 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 decom- position 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 ( 15 N and  13 C) 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  15 N and  13 C bi-plot were used to measure the species’ niche width and overlap. Differences were found between the upstream and downstream signatures, regarding the  15 N and  13 C of both detritivores and predators. The differences were found to be more pronounced at the southern WWTP, located downstream of the city. The lower  15 N in macroinvertebrates at the WWTP-impacted sites reflected the lower  15 N of suspended particulate organic matter and was associated with higher inorganic and organic loads. The decreasing range of  13 C 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 func- tional 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. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction Urbanization is increasing rapidly due to the growth of the world’s population and the movement of people from rural areas to cities. This rapid increase represents a threat to the ecological integrity of streams draining waters from urban basins, with neg- ative effects on aquatic communities due to increased flood flows, greater erosion of the stream banks and increased loads of pollu- tants (Grimm et al., 2008). Urban areas introduce into water bodies a series of substances including untreated sewage, organic and inor- ganic fertilizers from suburban agricultural areas, and wastewater ∗ Corresponding author. Tel.: +39 064940800. E-mail address: loreto.rossi@uniroma1.it (L. Rossi). plant outputs. Several studies of urban rivers have found increased oxygen demand and conductivity, huge quantities of suspended solids, and high ammonium, hydrocarbon and metal concentra- tions which are attributed to wastewater treatment plant discharge and diffuse source runoff (Latimer and Quinn, 1998; Lenat and Crawford, 1994; Porcella and Sorensen, 1980). Such inputs, mainly anthropic in origin, can have a series of secondary effects on inver- tebrate consumers by inducing changes in plant litter breakdown, organic deposition rates and food source availability, which modify river metabolism and the feeding patterns of inhabiting popula- tions (Cooper et al., 2007; De Lange et al., 2004; Young et al., 2008). Despite the acquired notions about the impact of nutrient loads from wastewater treatment plants (WWTPs) and diffuse urban river inputs (McCann et al., 1998; Vadeboncoeur et al., 2005; 1470-160X/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ecolind.2012.04.006