To quantify the relative contributions of local community assembly processes versus γ‐diversity to β‐diversity, and to assess how spatial scale and anthropogenic disturbance (i.e. nutrient enrichment) interact to dictate which driver dominates. Location: France and the United States. Time period: 1993 - 2011. Major taxa studied: Freshwater stream diatoms. Methods: β‐ diversity along a nutrient enrichment gradient was examined across multiple spatial scales. β‐ diversity was estimated using multi‐site Sørensen dissimilarity. We assessed the relative importance of specialists versus generalists using Friedley coefficient, and the contribution of local community assembly versus γ‐ diversity to β‐ diversity across spatial scales, with a null model. Finally, we estimated the response of β‐ diversity to environmental and spatial factors by testing the correlations between community, environmental and geographical distance matrices with partial Mantel tests. Results: β‐ diversity generally increased with spatial scale but the rate of increase depended on nutrient enrichment level. β‐diversity decreased significantly with increasing nutrient enrichment level due to the loss of specialist species. Local assembly was an important driver of β‐ diversity especially under low nutrient enrichment. Significant partial Mantel correlations were observed between diatom β‐ diversity and pure environmental distances under these conditions, highlighting the role of species sorting in local assembly processes. Conversely, in heavily enriched sites, only spatial distances were significantly correlated with β‐ diversity, which indicated a substantial role of dispersal processes. Main conclusions: Nutrient concentration mediated the expected increase in β‐diversity with spatial scales. Across spatial scales, β‐ diversity was more influenced by local assembly processes rather than by γ‐diversity. Nutrient enrichment was associated with an overall decline in diatom β‐ diversity and a shift in assembly processes from species sorting to dispersal, notably due to the elimination of some specialists and their subsequent replacement by generalists.