A growing body of mostly observational research has examined how β-diversity and its turnover and nestedness components respond to environmental gradients across taxonomic, phylogenetic, and functional dimensions. To our knowledge, this is the first manipulative investigation to assess how two major environmental influences—nutrient enrichment and herbivory—control β-diversity and its components across dimensions in phototrophs. We used algal data from field and lab experiments, manipulating herbivory and/or the number of added nutrients (NAN), which ranged from zero to three (N, P, and Fe) or four (N, P, Fe, and Mn). We compared control/nutrient treatment vs. control communities and non-grazed vs. grazed communities in terms of taxonomic, phylogenetic, and functional diversity. Taxonomic and phylogenetic β-diversity (β_Sor) was partitioned into turnover (β_Sim) and nestedness (β_Nes) components. We proposed a novel partitioning approach for functional β-diversity, outperforming the conventional approach, which calculates β_Sor, β_Sim, and β_Nes. Instead, we used Bray-Curtis distances derived from the number of taxa within morpho-functional groups and calculated overall functional β-diversity (β_BC) and its balance (β_Bal) and gradient (β_Gra) components. We developed three hypotheses predicting that i) β_Nes and β_Gra would rise at higher NAN because of increased taxonomic richness, and phylogenetic and functional diversity (hypothesis 1); ii) grazing would either reduce (hypothesis 2a) or elevate β_Nes and β_Gra (hypothesis 2b) depending on the balance between extinction of grazer-sensitive taxa vs. establishment of grazer-resistant taxa; and iii) the relative importance of β_Nes and β_Gra would depend on NAN and dimension. Our results supported hypotheses 1, 2b, and 3. Across dimensions, enrichment with multiple nutrients elevated biodiversity, the nestedness and gradient components of β-diversity, and often, the overall β-diversity. Herbivory contributed to this increase by promoting grazer-resistant but competitively inferior taxa. Thus, streams with higher levels of both micro- and macronutrients and unimpacted herbivorous fauna may represent biodiversity hotspots and targets for conservation. The relative importance of β_Nes and β_Gra increased with NAN, and at the functional dimension, β_Gra was the dominant component. Therefore, shorter environmental gradients may promote turnover, while longer gradients, colonization/extinction, the latter becoming particularly prominent at the functional dimension.