Understanding the causes and consequences of variation in across multiple levels of biological organization is a central goal in modern biology. Research integrating key ecological and evolutionary processes necessitates a framework from which phenomena at multiple levels can be tested. Ecological stoichiometry (ES) can serve as a useful integrative framework to both quantify variation at multiple levels and understand the mechanisms that underlie this variation. ES abstracts biological processes down to their constituent atoms of elements to understand ecological and evolutionary patterns. While much work in ES has focused on the causes and consequences of interspecific differences in stoichiometry, little is known about substantial intraspecific variation. Sexual dimorphism is perhaps the most striking example of intraspecific variation in many species, at least at and above the molecular level. Almost nothing is known about dimorphism at the elemental level, and how this dimorphism may drive sex-specific responses to key ecological parameters, such as environmental supply of biogenic elements. In this dissertation, I explore sexual dimorphism on the elemental level, the sex-specific physiological mechanisms organisms use to respond to changing resource availability, and the population-level effects of resource availability on patterns of sexual selection in Hyalella amphipods. To that effect, I first quantified sex-specific plasticity in phosphorus (P) content of Hyalella amphipods to changes in environmental P supply. I found that the sexes differ in their plasticity of body P content in response to P availability, with male composition behaving plastically and female composition remaining relatively canalized. Second, I used radiotracers to identify age- and sex-specific nutrient processing strategies in response to differences in dietary P. I found both age- and sex-specific patterns of acquisition and assimilation of C and P that are altered by dietary P, as well as preferential allocation to exaggerated male sexual traits relative to nonsexual traits. Third, I used microcosms to quantify population-level responses to environmental P supply and mating trials to explore the interaction between availability of P during development and during mating in influencing mating behavior. No evidence for effects of P on population dynamics were observed, yet mating behavior was influenced by P availability. Finally, understanding that organisms are composed of many elements beyond C, N, and P, I quantified sex-specific and trait-specific composition of 12 mineral and trace elements. Substantial differences between the sexes and traits in elemental composition in multiple dimensions were observed, suggesting potential importance of these elements in sexual dimorphism. Together, these studies highlight the importance of sex-specific responses to variation in the environmental supply of key elements, and the utility of ES in understanding the eco-evolutionary mechanisms that shape sexual dimorphism and selection.