Resumo. Abstract: Aim We evaluated changes in periphyton biomass and the composition of benthic diatom communities along a gradient of urbanization in 10 coastal streams located on the coastal plain of southernmost Brazil. Methods At each coastal stream, we obtained limnological variables and periphytic material from the stolon of the aquatic macrophyte Hydrocotyle ranunculoides for further analyses of chlorophyll a and diatoms. Results Total phosphorus was the only limnological variable selected by the statistical models, showing a positive relationship with periphyton biomass and a negative relationship with diatom species richness in these streams. Species composition (for both presence-absence and abundance data) was also explained by total phosphorus. Further, we observed a nested distribution of diatom species along the streams, in which poorer communities of streams with higher concentrations of phosphorous are subsets of richer communities from streams with lower concentrations of the nutrient. Conclusions Our study shows that water quality modifications caused by eutrophication are leading to the loss of species and changes in the structure of biological communities in ecotones such as coastal streams.