Animal-driven nutrient cycling processes can be important to ecosystem nutrient dynamics and are influenced by habitat size, ambient nutrient concentrations, and species-specific traits such as body size and nutrient composition. Nutrients released by animals contribute valuable ecosystem services and promote biological diversity by acting as sources of bioavailable energy and nutrient subsidies. Freshwater mussels (Order: Unionoida) are a diverse, threatened group of sedentary, filter-feeding bivalves that predominantly occur in dense multispecies aggregations and can create hotspots of biogeochemical activity. Although occurring within the same feeding guild, interspecific differences in body size and tissue stoichiometric ratios can result in interspecific differences in nutrients assimilated and released. Thus, differences in mussel assemblage structure should affect site level nutrient availability. We quantified nitrogen (N) and phosphorus (P) excretion, particulate organic matter (POM) egestion, and tissue carbon (C), N, and P for dominant species spanning nine sites in the Bogue Chitto Watershed, Louisiana and Mississippi, and scaled individual rates to the assemblage level. Egestion, excretion, and storage varied interspecifically, with the greatest differences occurring for P excretion rates and storage, which consequently led to interspecific differences in N:P ratios. High density sites composed of large-bodied individuals had the highest areal excretion, egestion, and stored nutrients. Differences in assemblage composition altered site level stoichiometric ratios, producing differences in assemblage scale N:P excretion. We estimated temporal changes in mussel contributions throughout the seasons with volumetric excretion, such that at lower stream discharge mussels have a greater contribution to ambient concentrations. This study provides novel information about how the distribution of mussel biomass and assemblage structure may influence nutrient cycling in a biodiverse coastal plain river.