Nitrogen pollution promotes changes in the niche space of fish communities

Historically, anthropogenic fixed nitrogen has been purposely increased to benefit food production and global development. One consequence of this increase has been to raise concentrations of nitrogen in aquatic ecosystems. To evaluate whether nitrogen pollution promotes changes in the estimates of niche space of fish communities, we examined 16 sites along a Brazilian river basin highly impacted by anthropogenic activities, especially discharge of domestic and industrial sewage from a region with more than 5 million inhabitants. We analysed the carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios of fish species and both autochthonous (periphyton) and allochthonous (course and fine particulate organic matter) basal food resources. To estimate the magnitude of nitrogen pollution, we measured the nitrate and ammonium concentrations at each site. Sampling was conducted in the dry and wet seasons to evaluate the influence of seasonality. Nitrogen pollution generally increased estimates of niche space, and seasonality influenced only the niche estimates of fish communities from polluted sites. In addition, isotopic analyses of nitrogen polluted sites yielded unrealistic estimates of trophic positioning (detritivores at the top of the food web). We conclude that changes in niche space estimates reflect both alterations in baseline isotopic values and differential trophic behaviour among fishes. Our study suggests that under conditions of high pollution, other factors appear to influence isotopic estimates of niche, such as isotopically distinct sources that have not been sampled, and/or differences in δ¹⁵N turnover rates between fish tissue and basal resources, creating isotopic baselines that are challenging to interpret.