The invasive Asian clam (Corbicula fluminea) increases sediment denitrification and ammonium flux in 2 streams in the midwestern USA

The invasive Asian clam, Corbicula fluminea, can enhance conditions needed for denitrification, but the effects of C. fluminea on denitrification in streams have not been measured. In summer 2012, we incubated sand- and gravel-filled boxes in the North Branch of the Chicago River, Illinois (high nutrient), and Eagle Creek, Michigan, USA (low nutrient). One side of the box contained no clams (control), and the other contained C. fluminea. After 6 wk in situ, we measured inorganic N and dissolved-gas fluxes using a continuous-flow approach. At both sites, live C. fluminea significantly increased NH4+ and N-2 flux out of sediments and O-2 consumption (i.e., respiration) relative to control sediment. Clams did not affect NO3- flux. Clams probably increased NH4+ flux via direct NH4+ excretion or mineralization of clam wastes and increased N-2 flux through either increased coupled nitrification-denitrification or enhanced exchange of nutrients between water column and sediment via bioturbation (i.e., burrowing). We used benthic clam density in each stream to scale up effects of live C. fluminea on in situ N fluxes. In the low-nutrient stream, live clams increased NH4+ flux more than N-2 production. However, in the high-nutrient stream, live clams enhanced N-2 production more than NH4+ flux. The influence of dense assemblages of burrowing bivalves on denitrification may be an overlooked and potentially significant component of lotic N uptake in low- and high-N streams.