SECONDARY PRODUCTION OF HYALELLA AZTECA AS INFLUENCED BY DREISSENA POLYMORPHA, LARVAL FISH, AND NUTRIENTS. Kari Marley1,3, W.B. Richardson1,2, and M.B. Sandheinrich1 1Dept. of Biology and Microbiology, University of Wisconsin, La Crosse, La Crosse, WI 54601: 2Upper Mississippi Science Center, U. S. Geological Survey, La Crosse, WI 54601; 3Research supported by Wisconsin Seagrant. Beds of zebra mussels typically contain greater densities of benthic macroinvertebrates than adjacent areas, but little data exists on the effect of zebra mussels on benthic secondary production. We tested the hypotheses that (1) benthic production would increase where zebra mussels and elevated nutrients were present, but decrease in the presence of a vertebrate predator; and (2) the presence of both zebra mussels and elevated nutrients would offset the negative effects of fish on benthic production. In a 35-d mesocosm experiment we manipulated the presence or absence of (1) Dreissena (1500/m2), (2) larval fathead minnows (Pimephales promelas: 250/m3), and (3) nutrients at 2x supply water concentrations (0.44 mg/L N and 0.035 mg/L P). These factors were cross-classified, resulting in 8 treatment combinations; N=3. Benthic samples were collected weekly, and the amphipod Hyalella azteca removed from the samples to estimate secondary production (cohort growth increment summation method). We also measured chlorophyll concentration, zooplankton density, and fish diet and growth. Production of Hyalella increased in the presence of zebra mussels, nutrients, and larval fish in comparison to controls, however production decreased in the combined treatment of zebra mussels and larval fish. The results suggest that (1) secondary production is elevated in the presence of zebra mussels, and (2) because zebra mussels effectively outcompete the larval fish for zooplankton prey, the fish are forced to feed on the benthos more intensely than when zebra mussels were not present. Keywords: secondary production, Hyalella azteca, benthos, zebra mussels, mesocosm Kari Marley Department of Biology and Microbiology 3030 Cowley Hall University of Wisconsin, La Crosse La Crosse, Wisconsin 54601 Telephone: 785-6990 e-mail: Kari_L_Marley@uwlax.edu Preference for platform presentation, not willing to switch to poster. Paper is authored and to be presented by a student.