SURVIVAL AND GROWTH OF JUVENILE UNIONID MUSSELS IN THE ST. CROIX NATIONAL SCENIC RIVERWAY: EFFECTS OF FOOD QUALITY

Michelle Bartsch, U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, WI
William Richardson, U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, WI
Lynn Bartsch, U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, WI
Jon Vallazza, U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, WI
Brenda Moraska Lafrancois, National Park Service, St. Croix Watershed Research Station

Recent increases in nutrient and sediment loading have caused observable shifts in algal composition and have potentially altered the quality of mussel food in the St. Croix National Scenic Riverway. Juvenile Lampsilis cardium and L. siliquoidea were deployed in cages for 28 d at four riverine and four lacustrine sites. Mussel tissue and food resources (seston fractions: whole water, <63, <32 and <10 μm and surficial sediment) were analyzed for total lipids (TL) and quantitative fatty acid (FA) composition. Riverine sites were dominated by Chlorophyta, whereas Cyanophyta generally dominated lacustrine sites. Overall mussel survival was 95% and mean growth rates for both species were higher at riverine (3 and 23 μm/d) than lacustrine sites (-10 and 6 μm/d) for L. cardium and L. siliquiodea, respectively.

Total lipids were similar between mussel species and sites (averaged 10.5 ± 1.1%); however, seston TL ranged from 5.6 to 11.0% at riverine and 11.7 to 14.3% at lacustrine sites. Sediment TL averaged 0.031% at riverine and 0.047% at lacustrine sites. Mussel growth may be more dependent on dietary FA quality than quantity.