Context. Anthropogenic activities producing nitrogen have affected freshwater ecosystems. The delta N-15 isotope is a key indicator of these impacts. Macroinvertebrates are vulnerable to pollution. Studying the macroinvertebrate functional feeding groups (FFGs) helps us understand the structure, function and balance of fluvial ecosystems, and how pollution alters ecosystem dynamics. The Maipo River Basin is one of the most polluted in Chile. Aims. This study examines how pollution affects macroinvertebrate family diversity, FFGs, delta N-15 content and trophic position. Methods. Using a Surber net, five replicates were taken from two polluted and two non-polluted sites to analyse diversity at the family and FFGs levels. Differences in delta N-15 content and FFGs trophic position were assessed using a permutation Student's t-test. Key results. Gathering-collectors inhabiting polluted sites showed less richness of taxa than they did in the non-polluted sites. delta N-15 contents were higher in polluted sites, although gathering-collectors had a lower trophic position. Conclusion. Our results indicated that greater nitrogen input creates conditions that may affect the diet of studied organisms, potentially explaining the observed trophic position changes.