In river food webs, the energetic coupling of nutrient-rich "fast" autochthonous (algal) and recalcitrant "slow" allochthonous (terrestrial) resources may confer stability. Additionally, microbial biofilms may enhance the nutritional quality of allochthonous resources for macroinvertebrate consumers, potentially facilitating trophic linkages within slow energy channels. We used 16S and 18S rRNA gene sequencing and essential amino acid (AAESS) carbon isotope (delta 13C) analysis to characterize microbial biofilms and quantify proportional contributions of AAESS derived from algae, terrestrial plants, and microbes (archaea, bacteria, fungi) to macroinvertebrates across a series of mesocosm and field leaf pack experiments in the middle Rio Grande of central New Mexico, USA. In our 12-week mesocosm experiment, leaves of native Rio Grande Cottonwood trees (Populus deltoides wislizeni) contributed greater mean estimated proportions of AAESS (0.41-0.81) to Chironomidae larvae than those of nonnative Russian Olive trees (Elaeagnus angustifolia; 0.18-0.50). Microbes were notable sources of AAESS to macroinvertebrates inhabiting native C4 grass (Spike Dropseed; Sporobolus contractus) leaf packs (0.08-0.26). Our field experiment demonstrated river sediment was the main source of microbes colonizing submerged Cottonwood leaves and that allochthonous resource use varied across macroinvertebrate taxa, such that Chironomidae assimilated the highest mean estimated proportions of AAESS from leaves (0.36-0.65), while Ephemeroptera and Trichoptera assimilated the highest mean estimated proportions of AAESS from algae (0.82-0.94). Our work indicates terrestrial tree leaves are important sources of AAESS to Chironomidae, while algae are the dominant source of AAESS to other macroinvertebrates in the middle Rio Grande.