Freshwater mussels make important contributions to nutrient cycling and storage in rivers, but the overall magnitude of these contributions varies with mussel abundance, community composition, and species traits. We used an extensive dataset on mussel communities, their life history traits, and functional effect traits across eight rivers from a single biogeographic region, the Ouachita Mountains of the US Interior Highlands, to examine how differences in mussel communities and their functional traits affected overall functional diversity and dispersion and nutrient storage (carbon [C], nitrogen [N] and phosphorus [P]) and nutrient excretion rates (N and P). Our preliminary analyses indicate that there is high variation in richness, biomass, and consequentially excretion rates, nutrient storage, and overall stoichiometry. In general, larger stream reaches tended to harbor mussel beds with more species, greater community biomass, and subsequently higher excretion rates and overall storage. Most striking was how assemblage composition impacted overall excretion and nutrient storage stoichiometry. In particular storage N:P was negatively related to excretion N:P by the mussel communities. This general pattern was related to assemblage composition where communities with a greater proportion of lampsilines had higher excretion N:P and lower storage N:P. Our work highlights the diversity of traits within a single family of aquatic organisms and how that can result in high spatial variation in nutrient storage and cycling.