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.

从历史上看，人为固定氮被有意增加以有利于粮食生产和全球发展。这种增加的一个后果是提高了水生生态系统中的氮浓度。为了评估氮污染是否会促进鱼类群落生态位空间估计值的变化，我们检查了巴西河流流域中受人为活动影响很大的 16 个地点，特别是人口超过 500 万的地区排放的生活和工业污水。我们分析了碳 (δ ¹³ C) 和氮 (δ ¹⁵N) 鱼类和本地（附生生物）和异地（粗粒和细颗粒有机物）基础食物资源的同位素比率。为了估计氮污染的严重程度，我们测量了每个地点的硝酸盐和铵浓度。在旱季和雨季进行抽样以评估季节性的影响。氮污染通常会增加对生态位空间的估计，而季节性仅影响来自污染地点的鱼类群落的生态位估计。此外，对氮污染地点的同位素分析产生了对营养定位（食物网顶部的食腐动物）的不切实际的估计。我们得出结论，生态位空间估计的变化反映了基线同位素值的变化和鱼类之间不同的营养行为。鱼类组织和基础资源之间的¹⁵ N 周转率，创建了难以解释的同位素基线。