We attempt to define groups of functionally related benthic algal species or guilds to assess if the species richness of such guilds varies across experimentally manipulated nutrient gradients, and to determine the relative contribution of these guilds to total community diversity. Nutrient gradients were established using nutrient-releasing substrata; treatments consisted of Si, N + P, Si + N + P, and controls. Nutrient enrichment significantly altered the biovolume of 27 species (out of a total of 141). Results from one-way ANOVA tests coupled with multiple means range tests categorized these species into four major guilds: three guilds of species which achieved their highest abundance on either Si, N + P, or Si + N + P treatments, and a guild that grew best on controls. This pattern of structuring was corroborated by cluster analysis and principal components analysis. Total community diversity and the relative contribution of guilds to total community diversity was less on N + P and Si + N + P substrata compared with that on Si and the control substrata. This suggests that nutrient enrichment may narrow the conditions amenable to many algal species (nutrient generalists), creating a niche occupied by those taxa sufficiently equipped to benefit under such conditions (nutrient specialists).