In Mediterranean forested streams, the large input of particulate organic matter (POM) in autumn struc-tures the benthic microbial community into two interconnected habitats: that associated with the POM itself (leaves and branches) and that colonizing gravel and sandy substrata. Relationships were examined between microbial de-composition activities (β-glucosidase, β-xylosidase, cellobiohydrolase, phenoloxidase, peroxidase, peptidase and phosphatase) at different stream substrata (leaves, branches, sand and gravel) and total carbon (C), nitrogen (N) and phosphorus (P) content of attached microbial communities and stream water matter fractions (particulate and dissolved nutrient concentrations). Microbial communities associated to leaves and branches showed higher C:N and lower N:P molar ratios (averaging 21.7 ± 1.4 and 5.4 ± 2.1, respectively) and higher polysaccharide degrading activity (sum of β-glucosidase, β-xylosidase and cellobiohydrolase activities). Instead, biofi lms on sand and gravel, where algae accumulate and fi ne particulate material were more available, showed lower C:N and higher N:P molar ratios (averaging 10.3 ± 0.6 and 19.9 ± 3.7, respectively) and greater ligninolytic (sum of phenol oxidase and per-oxidase activities) and peptidase activities. These results suggest that enzyme activities of microbial attached com-munities are linked to their nutrient molar ratios and, at the same time, these might be modulated by the different nature of available organic matter (OM) in each substratum and the microbial groups accumulated (algae, bacteria, fungi, micro-and meiofauna). However, similarities (C:N) and divergences (N:P) between stream water and nutri-ent molar ratios in microbial communities may also affect nutrient demands and in consequence, the expression of extracellular enzymes. Our results show a relationship between function (extracellular enzyme activities) and nutrient molar ratios of attached microbial communities.