We examined community structure of benthic diatoms and their substratum preferences (epilithic vs. macroalgae) by season in 4 tropical mountain streams in central Mexico. Of the 74 species compiled, Cymbella silesiaca, Rhoicosphenia abbreviate, and Reimeria sinuata were the most abundant and frequent. About 14% of species were exclusively epilithic, 13% were associated with macroalgae, and 73% had no clear preference. Environmental factors most closely related to spatial and temporal distribution of epilithic diatoms were changes in stream discharge and water temperature. For diatoms associated with macroalgae, the most important factors were changes in current velocity and the coverage and richness of macroalgae. The similarities in species richness and diversity in the 4 study sites suggest that the diatom community develops under conditions of moderate disturbance. The few differences found in species richness between epilithic and macroalgae-associated diatoms suggest that these diatoms do not have substratum preferences. The abundance of macroalgae-associated diatoms in all sites suggests a positive effect (facilitation) of macroalgae on diatoms
