Lowland Neotropical streams have a chemically-diverse detrital resource base, where leaf quality has the potential to play a key role in mediating effects of eutrophication on stream food webs. I examined the hypothesis that the quality of leaf substrata (i.e. carbon quality and nutrient content) determines: (1) the rate at which microbes and invertebrates process leaf litter; and (2) the magnitude of the microbial and invertebrate response to high ambient nutrient levels in water and substratum. First, I studied the effects of leaf carbon-quality and nutrient content on leaf breakdown of eight common riparian species in a stream at La Selva Biological Station, Costa Rica. Results indicate that concentrations of structural compounds, not secondary compounds, are the most important predictors of breakdown. Results also suggest that microbes rapidly colonize and process high-quality leaves, while low-quality leaves serve primarily as substrata for attachment of invertebrates. Second, I examined if the same trends I observed with respect to carbon-quality and breakdown at La Selva also occurred at a temperate site. I used standardized analytical techniques to measure leaf litter chemistry from seven common riparian trees from Coweeta Hydrologic Laboratory, N.C. and compared results to leaf chemistry of species from La Selva. Structural compounds were strongly correlated with leaf breakdown rate at both sites. However, concentrations of condensed tannins were significantly greater (2.6 - fold) in Coweeta than in La Selva species and were negatively correlated to breakdown rate among Coweeta species, but not among La Selva species. Third, I examined if leaf carbon-quality mediates stimulation of leaf breakdown by elevated ambient phosphorus. I examined leaf breakdown rates of three species differing in carbon-quality across a natural landscape gradient in stream water P, and a whole-stream P-enrichment. Results suggest that high-P levels stimulate fungal biomass and microbial respiration to a greater extent on high than on low carbon-quality species. And finally, to avoid confounding effects of other leaf quality parameters, I manipulated both carbon-quality and phosphorus levels using a modified artificial (agar-diffusing) substratum technique. Results provide the first experimental demonstration supporting the hypothesis that carbon-quality of organic matter can determine the response of microbial respiration to P-enrichment of both substratum and water. My results indicate that carbon-quality has the potential to mediate the effects of elevated water nutrients on detrital-based food webs.