Using δ15N of periphyton and fish to evaluate spatial and seasonal variation of anthropogenic nitrogen inputs in a polluted Brazilian river basin

Abstract

Nitrogen stable isotope ratios (δ15N) of aquatic organisms have been used widely as indicators of the intensity of anthropogenic N loading into watersheds; nevertheless, this approach has rarely been applied in the Neotropics. Considering the potential utility of δ15N to assess aquatic pollution, the present study aims to use the δ15N of periphyton and its direct consumers (catfishes from the genus Hypostomus) to evaluate spatial and seasonal variation of anthropogenic nitrogen inputs. The study was carried out in 16 sites distributed along the Rio das Velhas basin, a Brazilian river basin highly impacted by anthropogenic activities, especially the discharge of domestic and industrial sewage from a region with more than five million human inhabitants. We correlated the δ15N of periphyton and Hypostomus with nitrate (NO3−) and ammonium (NH4+) concentrations in water from each site, during both the dry and wet seasons. The δ15N of periphyton and Hypostomus was correlated, and varied with NO3− and NH4+ concentrations. NO3− was considerable consistent with high δ15N values (especially in the dry season), reaching values above 30‰ in periphyton and 36‰ in Hypostomus. In contrast, NH4+ was correlated with a more depleted δ15N (especially in the wet season), reaching values of −1‰ in periphyton and 5‰ in Hypostomus. However, periphyton was more sensitive than Hypostomus to changes in NO3− and NH4+ concentrations between seasons. In addition, the Δ15N (enrichment of primary consumer relative to the primary producer) was higher in polluted sites than in unpolluted sites (especially in the wet season), suggesting that Hypostomus may be feeding on resources other than periphyton. We conclude that the δ15N of periphyton and Hypostomus reflects reactive nitrogen concentrations and suggests the occurrence of different nitrogen sources within this basin due to diverse anthropogenic activities. Unfortunately, the mixing and diversity δ15N sources mean the values of periphyton and Hypostomus are best used as an indicator of the intensity of anthropogenic nitrogen loading, but not as tracers of individual nitrogen sources.