Little clams with big potential: nutrient release by invasive Corbicula fluminea can exceed co-occurring freshwater mussel (Unionidae) assemblages

Animal-mediated nutrient cycling research tends to emphasize either native or invasive fauna, yet communities comprising both groups are common, and biogeochemical control may shift from native to invasive species, altering local nutrient regimes. In North American rivers, co-occurring native mussels (Unionidae) and the invasive clam, Corbicula fluminea , have strong nutrient cycling effects through filter-feeding and bioturbation. When these two groups co-occur, the degree to which their nutrient cycling effects differ remains unclear. We quantified bivalve density, biomass, and nutrient excretion rates at four reaches in each of two rivers once during the same year to test whether differences in density and biomass led to different spatial and temporal nutrient cycling and stoichiometry patterns for co-occurring mussels and Corbicula . We hypothesized high densities, coupled with small body size would elevate Corbicula population-level nutrient cycling rates above those of less dense assemblages of larger-bodied mussels. Corbicula occurred at all mussel beds and their densities generally exceeded mussel densities, but Corbicula biomass was consistently lower. High densities and greater mass-specific excretion rates led to Corbicula population-level excretion rates that were greater than or equal to mussel aggregate rates at half the reaches. Abiotic conditions limited bivalve nutrient supply relative to ambient concentrations, but their contributions increased during low flows and are likely concentrated at finer spatial scales. Our results suggest spatial variation in invasive and native trait distribution associated with phylogenetic tribes influences the potential for animal-mediated nutrient cycling to shift from native to invasive species control. Overall, our study highlights the need for new management paradigms that account for nutrient cycling by invasive species.