Hornbach, D.J. 1991. Factors influencing the distribution of unionid mussels in the lower St. Croix river at Franconia, MN. Report submitted to the Minnesota Department of Natural Resources. 23+ pp.
The factors influencing the distribution of mussels in the St. Croix River at Franconia, MN were examined. This stretch of the river contained a diverse assemblage of mussels, with 33 species found in the two years of this study. The dominant members of the community were Truncilla truncata and Quadrula metanevra. In addition, the federally endangered species, Lampsilis higginsi and Quadrula fragosa were found at this site. The density of mussels at the Franconia site was relatively high (mean density over all sampling sites was approximately 12/m2). The sizes and density of mussels varied both along the length of the river and in relationship to the shore. Mussels size and density were found to be significantly correlated to sediment size, with larger and more numerous mussels found in coarser substrates. Generally mussel density and size was greater at inshore sites when compared to sites in the center of the river. The mussel density was lower in 1991 than in 1990 at comparable sites. The reason for this difference is not known but evidently was not related to sampling methodology. We also found that the total suspended solids varied over the bed. It appeared that resuspension of sediments over varying substrate type, rather than mussel filter-feeding, accounted for this variability. Mussels did, however, feed at relatively high rates (over 500 ml/hr for mean-sized Quadrula metanevra), and while they may not have significantly altered the total nutrient availability, it is possible that they influenced certain particle sizes. Despite the fact that the mussels may not have had a great influence on nutrient availability, the size and concentration of food particles significantly influenced mussel feeding rates. It was also apparent that, at least for the two species of mussels that we examined (Truncilla truncata and Quadrula mentanevra), the functional feeding responses differed between mussel species. It is possible that these differences, in part, allowed for a multitude of species to co-exist in one area of a river and thus may help to explain why there are so many species of mussels found in the St. Croix. Clearly, substrate type, food availability, (i.e. suspended solids), and water velocity all influenced the mussel assemblages in the river. There is a great interaction among these factors and it is not clear which of these are dominant in controlling mussel community structure.