Population assessment and potential functional roles of native mussels in the Upper Mississippi River

Managers in the Upper Mississippi River (UMR) are reducing summer water levels in certain reaches to mimic historic conditions and rehabilitate habitats.  Concerns for the effects of these actions on mussel populations threatened to halt these multi-million dollar projects.  We conducted systematic surveys of mussels in three reaches of the UMR to estimate their number, spatial distribution, recruitment, and to evaluate their potential ecological role.  We sampled between 281 and 379 sites in each reach.  At each site, divers obtained two 0.25 m² total substrate samples.   Reach-wide population estimates (95% CL) were 190 (153, 227), 61 (46, 76), and 212 (169, 255) million mussels, resulting in overall densities of 4.3, 2.9, and 4.5 mussels/m² for the three reaches.  The number of live species ranged from 16 to 23.  Forty to sixty two percent of the reach-wide populations were young mussels < 5 yrs.  Spatial patterns varied with reach, but generally, more mussels were encountered in the impounded portion of each reach.  Total biomass ranged from 4.1-6.2 g dry tissue/m² and production ranged from 0.4-0.6 g C/m²/yr.   These data suggest that the UMR contains an abundant, diverse, and reproducing mussel community that likely serves important functional roles in this ecosystem.