The exposure effects of a commercially prepared spray dried powder (SDP) formulation of Pseudomonas fluorescens (strain CL145A) on the survival of seven species of unionid mussels endemic to the Great Lakes and Mississippi River basins was evaluated in this study. The study exposures were completed within replicated 350-liter test tanks contained within a mobile bioassay laboratory sited on the shores of the Black River near La Crosse, Wisconsin. The test tanks were supplied with flowing, filtered river water which was interrupted during the exposure period.

Two groups of seven species of mussels were used in equal proportions in the study. The first group was exposed to SDP for 8 hours, and the second group was exposed to SDP for 24 hours. Individually tagged mussels were randomly allocated to test tanks until all test tanks contained 8 to 10 mussels of each species (dependent upon the number available for testing).

The experimental unit for the trial was the individual test tank, and treatment group was assigned using a randomized block design. The treatment groups for each exposure duration consisted of (1) an untreated control group, (2) a group that received an application of 50 milligrams SDP per liter (mg SDP/L), and (3) a group that received an application of 100 mg SDP/L. All mussel species and both exposure duration groups were exposed concurrently (that is, one-half of the mussels were removed after 8 hours of SDP exposure and the remaining mussels were removed after 24 hours of SDP exposure). All treatment concentrations are reported as active ingredient.

After exposure, the mussels were consolidated into wire mesh cages and placed in the Black River for a 27-28 day postexposure period, after which time survival of mussels was assessed. Of the 1,170 mussels tested in the study, 3 were confirmed dead and 5 were not recovered and treated as mortalities in the analysis. The effect and interactions of species, SDP exposure concentration, and SDP exposure duration were analyzed and did not affect mussel survival (p > 0.98). The results from this study indicate that SDP exposure at the maximum approved open-water concentration of 100 mg/L for up to 3 times the maximum approved open-water exposure duration of 8 hours (in other words for 24 hours of exposure) is unlikely to reduce survival of subadult or adult mussels.