DETERMINE THE EFFECTS OF FOOD QUALITY ON JUVENILE UNIONID MUSSEL SURVIVAL AND GROWTH IN THE ST. CROIX NATIONAL SCENIC RIVERWAY: AN UPDATE

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

The St. Croix National Scenic Riverway (SACN) has been the subject of several nutrient and mussel investigations; however, additional information is needed to support current mussel preservation activities and nutrient management initiatives. Of particular interest are potential effects of the recent shifts in algal composition and on food quality for endangered mussels in the SACN. Our research objectives are to (1) characterize the current status of food resources available to native mussels in the river corridor and within the four basins of Lake St. Croix, (2) experimentally investigate the survival and growth of caged juvenile mussels positioned within the water column and on the sediment surface and (3) through quantitative analysis of fatty acids, provide baseline information on juvenile mussel diets, the nutritional quality of the consumed and assimilated food, and how this food is being affected by water quality among riverine and lacustrine reaches of the SACN.

Our accomplishments thus far include the deployment of 48 in situ exposure cages at eight locations (four riverine and four lacustrine), each containing two species of juvenile mussels (Lampsilis cardium and L. siliquoidea). Six cages (three positioned mid-water column and three on sediment surface), with two juveniles per cage (one of each species), were deployed for 28 d. Water quality monitors were deployed at six locations (upper and lower riverine reaches, and all four basins of Lake St. Croix) to continuously measure dissolved oxygen, pH, specific conductance, and temperature.

At all eight locations, four size fractions of seston (whole water, < 63, <32 and <10 microns) and three surficial sediment (top 1 cm) samples were collected to coincide with the presence of cyanobacteria. These samples will be analyzed for total lipid and fatty acid composition. Three additional sediment samples will be analyzed for carbon and nitrogen and pore water ammonia nitrogen. Additional depth-integrated whole water samples were collected and will be analyzed for chlorophyll a, total suspended and volatile solids, total phosphorus, soluble reactive phosphorus, total nitrogen, ammonia nitrogen, and nitrate-nitrite nitrogen, and carbon and nitrogen. To better characterize the composition of the algal community at each location, a whole water sample was collected and algae will be identified to the species level.

All cages were retrieved from the eight locations and all 96 juvenile mussels were recovered, with 95% survival. Prior to deployment and after retrieval, each individual mussel was measured for shell length, width, and height (mm) for analyses of growth. A subsample of foot tissue was collected before deployment and from each living mussel after deployment and will be analyzed for total lipid and fatty acid composition. A subsample of foot tissue from two juveniles from each position at each site (total of 32 samples) will be analyzed for bacterial fatty acid methyl esters to assess the importance of bacteria as a source of nutrition for juveniles.