Hornbach, D.J. 1994. The factors influencing the distribution of mussels in the lower St. Croix river. Final report submitted to the Minnesota Department of Natural Resources. 16+ pp.
Freshwater mussels (Family Unionidae) are widely distributed throughout the United States. There are 44 species of freshwater mussels currently on the federally endangered species list (Fish and Wildlife, 1991). Despite this fact, there is little known concerning the factors which control the distribution of these organisms, especially in flowing water systems. Certainly factors such as geology, stream size, water quality, substrate type, water flow, and food availability, among others, are important in the determination of the community structure and population dynamics of these organisms (Strayer, 1983; Holland Bartels, 1990).
Based on the river continuum concept (Vannote et al., 1980) large filter feeders (such as mussels) are significant components of larger river systems. They are one of the few organisms that are able to process the fine particulate mater which makes up a significant portion of the energy basis for these reaches of river systems. Without filter feeders, such as mussels, the nutrients (small particles) would be swept downstream. However, with the mussels removing these particles from the water column, the residence time for nutrients within the stream increases. This allows for a more efficient use of the available energy. Also, as the mussels die they contribute biomass for utilization by detritovores. It has also been suggested that the feces produced by mussels that is deposited in the sediment, helps to condition the sediment so that is more readily inhabited by other benthic organisms that are important in the aquatic food chain as energy sources for fish. No studies have been conducted to date which attempt to adequately quantify the role that mussels play in the nutrient processing in larger river systems, although a study be Cohen et al. (1984) has shown that the introduced asian clam Corbicula can influence phytoplankton concentrations in river systems. A significant amount of research has taken place in marine systems that have examined the role of mussels in removing nutrients in tidally influenced regions (similar to river systems). These studies indicate that mussels can have a tremendous impact on nutrient availability (Wildish and Kristmanson, 1984; Frechette et al., 1989). There is evidence that mussels "upstream" of others may influence the nutrients available to those downstream. Despite the fact that little is known about the role of unionids in flowing water systems there is evidence that they are important in nutrient cycling in lake systems (Lewandowski and Stanczykowska, 1975; Kasprzak, 1986; Nalepa et al. 1991).
Mussels are also well suited as indicators of pollution levels since they are long lived and relatively immobile (Fuller 1974; Davis, 1988). They are capable of removing heavy metals, pesticides, and other trace metals from the water and accumulating them in their tissue and shell (Fuller, 1974). Thus they are useful in situ monitors. In addition, there is reason to believe that, as filter feeding organisms, they may be important in controlling natural eutrophication (Officer et al., 1982).
In the past, there have been other studies on the unionids in the St. Croix River. Baker (1928) cited 15 species from the St. Croix River although he classifies some species as statewide. Dawley (1947) reported 29 species of unionids from the St. Croix River (in addition to 4 species found in tributaries to the St. Croix River proper). Fuller (1978) recorded 23 species of unionids from the St. Croix River at Hudson, Wl. Stern (1983) reported 14 species of unionids from a single site on the St. Croix River. Doolittle (1988) has conducted the most extensive study to date on the distribution of unionids in the St. Croix River. Thirty seven species of unionids (including two only represented by dead shells) were reported by Doolittle in the river proper. Quantitative studies by Hornbach (1992) at Franconia, MN have indicated densities of 12 mussels/m2 at that site. Semi quantitative estimates by Doolittle (1988) gave ranges of 0.1 to 16.3 mussels/m2 in established beds in the St. Croix River.
Of particular interest in the St. Croix River is the presence of two species of endangered mussels: Lampsilis higginsi and Quadrula fragosa . L. higginsi, while found in the St. Croix River, is also found throughout the Upper Mississippi River, albeit at low densities (Havlik, 1981; HollandBartels, 1990). The winged mapleleaf, Q. fragosa, previously distributed in eleven other states, is presently restricted to the St. Croix River (Fig. 1).
Due to the highly restricted nature of Q. fragosa, the primary focus of this research project was to investigate the factors which may influence the distribution and abundance of this species as well as to determine those factors influencing the general freshwater mussel community. In addition, efforts were made to characterize the mussels community associated with the presence and absence of Q. fragosa.