In summer, streams in Portuguese eucalyptus forests frequently experience drought resulting in isolated pools, frequently saturated with leaf litter, in which the leaf leachates may generate toxic and hypoxic conditions. We assessed the ecological effects of Eucalyptus globulus leachate with and without aeration (i.e., low flow vs pool scenarios) on microbial decomposition of eucalyptus leaves and on toxicity to and survival, avoidance, and feeding behavior of Chironomus riparius Meigen (Diptera:Chironomidae), Echinogammarus meridionalis Pinkster (Amphipoda:Gammaridae), and Sericostoma vittatum Rambur (Trichoptera:Sericostomatidae). Eucalyptus globulus leaves immersed in a gradient of aerated or nonaerated eucalyptus leachate (100, 40.0, 16.0, 6.40, 2.50% volume/volume) were colonized by species-poor fungal assemblages. Leaf mass loss did not differ among treatments, but hypoxia suppressed conidia production and negatively affected fungal biomass. A concentration-dependent effect on fungal biomass was observed in aerated leaf extracts. A trade-off was found between the stimulatory effect of leachate nutrients and inhibitory effects of secondary compounds at leachate concentrations of 16 to 40%, and microbial respiration was depressed at concentrations >16% in nonaerated conditions. Only S. vittatum discriminated leaves conditioned in aerated water from leaves conditioned in nonaerated leachates. Leachates negatively affected all species, mostly in nonaerated conditions. Maximum leachate concentration caused mortality of 100% of E. meridionalis and 26 to 40% of C. riparius, regardless of aeration, and 70% of S. vittatum in nonaerated conditions. Sericostoma vittatum avoided the highest leachate concentrations, but C. riparius and E. meridionalis did not. Impoverished microbial communities and invertebrate assemblages with dissimilar tolerance to leachate may maintain functional properties and processes during drought disturbances in eucalyptus streams.