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Lake food webs are primarily fueled by energy from pelagic, littoral–benthic, and terrestrial habitats. Aquatic consumers acquire C from across these habitats in varying proportions, either directly or by way of consumer–prey transfers along energy pathways. Several factors, including relative habitat availability, allochthonous inputs, and population density, influence consumer use of various basal resources. However, the extent to which these factors interact to control resource use is not well understood. We used mountain lakes in the Cascade Range of western North America to address the question of how different population sizes, along with the relative availability of illuminated benthic habitat and terrestrial influence, determine resource use by Rainbow Trout (Oncorhynchus mykiss Walbaum, 1792). We measured lake bathymetry, light attenuation, and fish catch/unit effort in 16 lakes and analyzed samples of Rainbow Trout muscle tissue, pelagic seston, littoral–benthic periphyton, and terrestrial vegetation for C and N stable isotope ratios. Stable isotope mixing models quantified proportional use of basal resources for each trout. Compositional regression analysis identified how interactions between relative habitat availability and population abundance influence Rainbow Trout use of basal resources. At low population abundance, we found low relative use of terrestrial derived resources with balanced relative use of pelagic and littoral–benthic derived resources. At high abundance, relative use of littoral–benthic derived resources was low, and relative use of terrestrial and pelagic derived resources varied according to habitat availability. Our findings highlight the importance of environmental and biological interactions when considering factors that influence relative resource use in lake ecosystems. [ABSTRACT FROM AUTHOR]